Building Air Leakage

Reference Note

What Causes Air Leakage

■

Unbalanced Fan Systems

■

Wind

■

Chimney (Stack) Effect

How to Find Air Leaks
How to Estimate Air Leakage
Vapor Barriers and Air Leakage
Maintenance to Reduce Air Leakage

Air leakage through the skin of a building is a major

cause of energy waste in many buildings. This brief
Note explains the causes of air leakage, gives methods
for finding and estimating leakage, and lays the
groundwork for the Measures recommended in

Section 6

We use the term “building air leakage” to mean the

passage of air through the outer structural components
of the building, including the walls, windows, doors,
roof, and floors. The term “infiltration” is often used
for air leakage. Strictly speaking, infiltration is air
leakage into the building. The less common term
“exfiltration” is air leakage out of the building.

The total amount of air entering the building must

equal the total amount of air leaving the building, but
this does not mean that infiltration equals exfiltration.
The most important reason for the difference is the
operation of ventilation fans. For example, exhaust fans
create a negative pressure in the building that causes
infiltration, but no exfiltration. The air exhausted by
the fans is not “leakage,” as we define the term, although
it may carry away a large amount of energy.

What Causes Air Leakage

Air moves only in response to a pressure differential.

The following are the three major causes of pressure
differential across the building envelope. The amount
of air leakage is proportional to the aggregate effect of
these three causes. Air leakage is inversely proportional
to the resistance of the envelope to leakage.

■

Unbalanced Fan Systems

A pressure differential is created across the building

envelope if fans in one part of the building exhaust more
or less air than fans in another part of the system bring
into the building. Many air conditioning systems are
inherently unbalanced, usually because the system has
exhaust fans but no arrangements to force outside air
through the conditioning units. In some cases, pressure
imbalance in conditioning systems can be eliminated
with simple adjustments. In other cases, this would
require major modifications. See

Measure 4.2.1

about

balancing intake and exhaust in air handling systems.

A negative pressure is commonly created inside

laboratories, hospital rooms, and other isolated spaces

to prevent contamination of one part of the building by
another. This causes a certain amount of infiltration
through the outer envelope.

Conversely, a positive pressure is sometimes created

in buildings for the sake of comfort. The positive
pressure prevents entry of drafts. It also forces warm
air through the walls in cold weather, increasing the wall
temperature.

■

Wind

The pressure created by wind against a surface is

proportional to the square of the wind speed. For
example, a wind of 32 miles per hour has a “velocity
pressure” of about 0.5 inches water gauge. This means
that if all the energy of wind striking a wall is converted
to pressure, the pressure on the wall would be 0.5 inches.
This is similar in magnitude to the pressure differentials
created by the fans of air conditioning systems.

Wind pressure is positive on the upwind side of a

building, and is negative on the downwind side, so air
leakage caused by wind tends to flow through the
building. The magnitude of negative pressure on the
downwind side of the building is much less than the
magnitude of the positive pressure on the upwind side.
This is because the kinetic energy of the wind is
dissipated in turbulence on the downwind side.

Not much can be done about wind pressure on the

broad face of a building. However, individual outside
air intakes can be shielded from velocity pressure in a
fairly effective manner. Refer to

Measure 4.2.9

for

methods.

■

Chimney (Stack) Effect

If it is cold outside a building, the warm air inside

the building is less dense than the air outside, and hence
lighter. Therefore, the total weight of the atmosphere
acting at ground level creates more pressure outside the
building than inside the building. The difference in
pressure at the base of the building causes air to flow
into the building at its lower levels.

The increase in pressure at the bottom of the building

is transmitted to the upper levels of the building, which
become higher in pressure than the outside. Thus,
chimney effect causes air to leak into the bottom of a
building, and to leak out the top of the building. The

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11. REFERENCE NOTES

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situation is reversed in a building that is being cooled
during warm weather.

The force of chimney effect is proportional to the

difference between the inside and outside temperatures.
Unfortunately, this makes chimney effect worst when
the heating and cooling loads are highest.

Chimney effect is also proportional to the height of

the building. In tall buildings, chimney effect can be a
strong force, so special methods may be required to limit
air leakage. The most visible of these is the revolving
door, a hallmark of tall buildings. The revolving door
acts as an air lock at ground level.

There is an important fact to apply in minimizing

air leakage in tall buildings. If the upper portion of a
building is sealed so that air cannot leak out the top,
then the force of chimney effect will not induce
infiltration at the lower floors. Measures to prevent air
leakage from upper floors are recommended in

Subsection 6.4

. Personnel doors that lead to roofs and

penthouses are common paths for leakage. Measures
to reduce leakage through these penetrations are
recommended in

Subsection 6.1

In principle, chimney effect could be eliminated by

isolating each level of the building from the other levels.
However, the need for elevators and stairs makes this
practically impossible. Also, it might be possible to
counteract the negative pressure on lower flows by
pressurizing the elevator shafts and other vertical
penetrations. However, this approach requires energy
to move and condition the outside air that is used for
pressurization.

How to Find Air Leaks

Here are some methods of finding air leaks in the

building envelope:

• feel works well for localized inward leaks,

especially during cold weather. Feel is not sensitive
enough for outward leaks.

• small flames are sensitive to leaks in both

directions. Just don’t burn the place down. This is
how the Three Mile Island nuclear plant meltdown
got started. Candles are old fashioned. Use a
barbecue lighter.

• test smoke is the most effective visual method of

finding outward air leakage, but it is poor at
detecting inward leakage. Smoke testing has
become more practical with the introduction of
canisters that can release small puffs. (These are
sometimes called “smoke pencils.”) Get smoke
sources from air conditioning supply houses. Old-
fashioned smoke bombs produce far too much
smoke, frighten occupants, and set off some smoke
alarms.

• a lightweight cloth or a piece of tissue paper

dangled near a suspected leak is sensitive to both

inward and outward flow. This method’s only
disadvantage is lack of glamor.

• a lightweight ribbon attached to the end of a stick

works well for finding leaks deep in wall cavities
and other places where you cannot reach. Tape a
miniature flashlight to the same stick so you can
see into the dark hole.

• infrared thermography has the unique ability to

trace air leakage paths through envelope structures.
Scan inside surfaces for incoming leaks, and the
outside surfaces for outgoing leaks. Thermography
must be done during cold weather, the colder the
better. Scans should be done at night, several hours
after sunset, to avoid false images caused by solar
heating. Thermography is expensive, it cannot get
into tight places, and it requires special expertise.
For more, see

Reference Note 15

, Infrared Thermal

Scanning.

How to Estimate Air Leakage

It is not possible to estimate the amount of air

leakage with precision, even when the locations of leaks
are known. The amount of leakage depends on
imbalances in the conditioning systems, the average
chimney effect, and the average effect of wind, and other
factors that cannot be defined accurately. Go ahead and
minimize leaks that you find, and forget about making
detailed estimates of savings.

ASHRAE has developed formulas and tables for

estimating leakage around windows and doors. These
methods may be useful if it appears that most leakage
occurs at such components. You start by estimating the
total opening area of the leakage paths. For example,
air leakage around the sash of particular window types
can be estimated from ASHRAE tables by measuring
the total length of the joints where leakage can occur.
These methods are not accurate, but they give you some
basis for deciding whether to improve or replace leaky
windows or doors.

A significant amount of energy loss can occur from

leakage of outside air into the structure of walls and
ceilings, rather than into the occupied space. Such
leakage bypasses insulation, reducing the effective R-
value of the envelope. There is no practical method of
measuring the effect of such leakage in the field, but it
may be estimated from a thermographic survey. The
solution to such leakage is to seal air leakage at the
outside surface of the envelope wherever it is practical
to do so.

A “blower door” is a device used to test the overall

leakage rate in small buildings, such as residential
houses. It consists of a panel that fits tightly in a door,
window, or other exterior opening. A fan mounted in
the panel creates a pressure in the building. A
manometer mounted in the panel measures the pressure.
If the envelope is tight, the manometer registers a higher

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NOTE 40. BUILDING AIR LEAKAGE

pressure than if the building is leaky. The total amount
of leakage is derived from the characteristics of the fan.

Blower doors are difficult to use. In order to find

dispersed, small envelope leaks, it is necessary to seal
all the intentional openings in the building, including
furnace flues, hoods, fireplaces, etc. By the same token,
the blower door helps to highlight and quantify such
major leaks if the blower door test is repeated as each
leakage path is sealed. For example, the blower door
test may indicate that it is worthwhile to install a low-
leakage damper in a kitchen hood system.

Vapor Barriers and Air Leakage

See

Reference Note 43

, Vapor Barriers, for an

explanation of vapor barriers. In brief, a vapor barrier
is an impermeable membrane that blocks the flow of
atmospheric water vapor through the building envelope.
Vapor barriers can also be an effective means of

preventing air leakage through walls, ceilings, and other
large areas where they are installed.

Vapor barriers provide little benefit in the parts of

the building envelope that commonly have the worst air
leakage, such as windows and door frames. Vapor
barriers are broken where wiring must pass through the
surface. For example, this accounts for the drafts that
you may feel at electrical switches and receptacles that
are installed in outside walls.

Maintenance to Reduce Air Leakage

Leakage seals for doors, windows, and other

movable envelope components require occasional
replacement. Caulking materials used to seal fixed
components lose elasticity and shrink with time.
Therefore, maintenance is an essential part of deterring
air leakage. The effectiveness of sealing depends on
good workmanship and selecting appropriate sealing
materials and methods.

How Insulation Works

Reference Note

A Quick Review: What is “Heat”?
How Insulation Blocks Heat Flow

■

Preventing Heat Conduction

■

Blocking Heat Radiation

■

Preventing Heat Flow by Mass Transfer

■

Preventing Convection

External Factors that Affect Insulation

Performance

Conduction Lag and Heat Storage Effects
Heat-Reflecting “Insulation”

This Reference Note explains how insulation keeps

heat from moving. This is the “thermal” part of
insulation behavior. With this background as a
foundation,

Reference Notes 42

through

continue

with the practical aspects of selecting and installing
insulation.

A Quick Review: What is “Heat”?

Since the purpose of insulation is to control heat,

let’s review what “heat” is. Heat is a form of energy
that occurs in two entirely different forms. The first
form occurs in any material substance — solid, liquid,
or gas. This energy consists of the random vibration of
the atoms and molecules that make up the material. The
vibration energy increases with temperature.
(Conversely, temperature is a measure of the intensity
of heat energy. The exact definition of temperature is
very theoretical. You don’t need to know it for practical
work.)

The second form of heat is electromagnetic energy,

which travels in empty space. Heat radiation is the same
as light, X-rays, and radio waves. It originates from the
vibration of the electric charges in matter. Conversely,
any electromagnetic radiation that is absorbed by matter
is converted to heat in the material.

The energy of all electromagnetic energy can be

considered as heat. However, we usually consider heat
radiation as the energy emitted by a hot object, such as
the sun or a radiator. Heat is also radiated by objects
near room temperature, such as the walls of a building.
Most heat radiation falls within a broad range of
wavelengths that extends beyond the red end of the
visible light spectrum. This broad spectrum is called
“infrared.”

How Insulation Blocks Heat Flow

The purpose of insulation is to keep heat from

moving. To understand how insulation works, you need
to understand how heat moves. There are three
fundamental processes of heat movement: “conduction,”
“radiation,” and “mass transfer.” A fourth mode of heat
movement, “convection,” is driven by the first three. We
will summarize how insulation works in relation to each

of these four modes of heat movement. This will give
you enough background to interpret the thermal
performance specifications of any type of insulation.

■

Preventing Heat Conduction

Most heat loss through building insulation occurs

by conduction. Conductive heat transfer is an exchange
of kinetic energy between atoms and molecules as they
collide with each other. Therefore, conduction occurs
only in matter. You can keep heat from traveling by
conduction if you create an empty zone that has no atoms
and molecules. Stated differently, heat cannot move by
conduction across a vacuum. This is the insulating
principle used in thermos bottles. The reason why
vacuum is not used for building insulation is that no
one has yet devised a reliable, inexpensive vacuum
container for large surfaces.

Vacuum is not yet available as a practical form of

insulation on a large scale, so insulation uses the next
best approach. This is to approximate a vacuum by
minimizing the amount of matter through which the heat
can move. Gases have much lower density than solids,
so insulation works by eliminating as much solid
material as possible, and replacing it with a gas. In all
porous insulation, the surrounding air is the gas that is
used.

An empty air space does not work well as insulation,

because air is easily moved by convection and pressure
differences. Porous insulation holds the air in place by
using small quantities of sold material as a matrix to
hold the air still. Air adheres weakly to the solid material,
so the air is held stationary. (Even so, the insulation
must be enclosed effectively to keep air from moving.)
The solid material is distributed so it occupies as much
of the space as possible, so that each air molecule is
close to an attachment point.

The solid material used in porous insulation may

be in the form of fibers or granules. Insulation fiber is
usually made of glass or glassy slag. Granules can be
made of chopped waste paper (“cellulose”), various
minerals in expanded form, foam beads, or other
materials.

Reference Note 43

covers the common types

of insulation materials.

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11. REFERENCE NOTES

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Most heat conduction in porous insulation occurs

through the trapped air, much less is through the solid
material. However, heat conduction through the solid
material is significant. For this reason, manufacturers
try to make the solid component as thin as possible.

In the case of fiber insulation, conduction through

the fibers can be reduced by orienting them
perpendicular to the heat flow, so that heat cannot travel
along the fibers. Fiber batt and board insulation is made
this way. Loose fiber pouring insulation does not have
this advantage.

Many gases have better insulating properties than

air. Some of these are used in plastic foam insulation,
in less common types of insulation, and in windows.
Chlorofluorocarbons (CFC’s) have been the primary
gases used as alternatives to air. However, concern about
the effect of CFC’s on the atmosphere is motivating a
search for other gases to replace CFC’s.

All insulation that uses alternative gases requires

closed cells to keep the gas from escaping and being
replaced by air. This has proven to be a serious weakness
of this kind of insulation. Foam insulation, which
dominates the market because it is easy to manufacture,
holds the gases in tiny plastic bubbles. Unfortunately,
the insulating gas is able to slowly diffuse through the
plastic.

Manufacturers of foam insulation use various

methods of resisting this leakage, such as increasing the
thickness of the bubble walls and bonding metal foil to
the faces of insulation boards. The rate of leakage may
be low in good insulation, but all contemporary plastic
foam insulation will eventually lose its special gases.
This has spurred the use of other materials to hold the
gases, such as glass foam, but no alternative to plastic
foam is presently acceptable for wide application.

■

Blocking Heat Radiation

Radiation heat loss and heat gain is not, by itself, a

problem in the opaque parts of the building envelope.
All opaque construction materials stop heat radiation
completely. However, the energy of heat radiation must
go somewhere. Radiation that strikes an opaque material
is either absorbed or reflected. Absorbed radiation
increases the temperature of the material, and the added
heat is transferred by conduction. Since insulation
blocks heat conduction, it indirectly blocks heat radiation
also.

Radiation heat transfer occurs inside insulation

materials at a microscopic level, from one molecule to
another. However, this effect is small at normal
temperatures. For insulation of typical thickness, this
process accounts for less than 10% of heat transfer.

Heat radiation is important in transparent parts of

the envelope, namely, windows and skylights. In fact,
it is so important that

Section 8

is set up to deal with it

separately. Refer to there for the Measures that deal
with glazing.

■

Preventing Heat Flow by Mass Transfer

If you heat some material, then move it somewhere

else and let it cool, you have moved heat by mass
transfer. The mass is a carrier for the heat energy. Mass
transfer is a powerful method of moving heat. In air
conditioning systems, air is moved by fans and water is
pumped to transfer heat. Unfortunately, air is also
effective in moving heat through the building envelope.
If the envelope is leaky, loss of heat carried by air may
account for much more energy waste than conduction
or convection.

Air leakage in buildings is driven by wind, chimney

effect, and unbalanced operation of fans.

Reference Note

, Building Air Leakage, gives the details of these

processes.

You can eliminate air leakage by making the

envelope airtight. In most cases, the insulation itself
plays only a secondary role in minimizing leakage. Air
movement is stopped primarily by impermeable
structural components and by installing “vapor barriers,”
which are explained in

Reference Note 42

■

Preventing Convection

Convection is movement of air that occurs when

the air is heated or cooled. The air adjacent to a heated
surface is warmed by conduction, becomes less dense,
and rises. If the air encounters a colder surface, it
becomes more dense, and falls. In an enclosed space,
such as a wall cavity, this cycle continues as long as one
side of the cavity is warmer than the other. If the air
inside building envelope cavities is free to move,
convection is a powerful mode of heat loss.

Insulation prevents convection by holding the air in

a cavity still. To do this, the insulation must entirely fill
the space. Even a narrow void can seriously undermine
the effectiveness of insulation, especially if it runs
vertically. For this reason, the workmanship of the
installer has a significant effect on insulation
performance.

Reference Note 44

covers this aspect.

The density of insulation installed in a cavity is an

important factor. The optimum density is a compromise
between conduction and convection. In practice, the
density of commercial insulation is determined by
manufacturing limitations and economics. For example,
the mineral structure of vermiculite makes it too dense
to have a very good insulation value.

On the other hand, glass fiber blanket insulation

typically is less dense than optimum because the
manufacturer is minimizing material cost. You can
improve the performance of fibrous insulation by
packing somewhat more of it into the cavity than is
specified by the manufacturer. For example, use an R-
19 batt where an R-11 batt is specified. This will also
help to fill the cavity. However, you should still be
careful to fill the cavity as uniformly as possible.

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NOTE 41. HOW INSULATION WORKS

External Factors that Affect Insulation Performance

The insulation characteristics quoted by the

manufacturer are measured under a particular set of
conditions. If the insulation is used under a different
set of conditions, its performance may differ
significantly. Perhaps the most important external factor
is the temperature of the insulation. The thermal
resistance of all thermal insulation improves as the
temperature falls. This is a bonus for building insulation
in cold climates. Conversely, if insulation is used for
applications at much higher temperatures, such as
insulating steam pipe, the insulation value may be lower
than expected.

Another factor is the orientation of the insulation.

This may affect fibrous insulation, reducing the
insulation value somewhat if the fibers are oriented
vertically.

In most applications, these variations in

characteristics are minor. Usually, they will not create
any surprises if you select the type of insulation for the
particular application. However, if you use a generic
type of insulation, such as vermiculite, which may be
used in a wide variety of applications, be sure to obtain
its characteristics for the conditions under which you
plan to use it.

The thermal resistance of most types of insulation

is seriously degraded if moisture can condense inside
the insulation, or if air is allowed to move through it.
Design the installation and select the insulation to avoid
both problems. See

Reference Note 44

, Insulation

Integrity, for the details.

Conduction Lag and Heat Storage Effects

Insulation is inherently light in weight because it

must contain little mass. Although insulation obstructs
heat flow, it does not delay the heat flow that does occur.
However, if insulation is combined with the mass of a
heavy structural component, such as a masonry wall or
a concrete roof slab, the combination structure may have
a significant delaying effect on heat flow. The key factor
in the time delay is the heat storage capacity of the
massive material. All masonry materials have similar
specific heats, so their heat storage effect is proportional
to the mass of the insulated component.

The mass effect is important primarily with masonry

construction. Masonry buildings may have wall heat
conduction delays of several hours or longer. A lag in
conduction does not occur with metal structures, because
heat travels quickly in metal. Thermal lag is not
important in frame structures, because the heat storage
capacity of the structure is too low to develop much lag.

The lag effect is significant only if the mass of the

structure is exposed to outside conditions, i.e., if the
insulation is installed inside the structure. If the weather

alternates on a daily basis between too hot and too cold,
the lag effect is beneficial. This is because the lag effect
reduces the average temperature differential across the
insulation. However, if the insulation is installed outside
the mass, the mass is not subjected to large temperature
changes, so the lag effect is greatly reduced.

If the mass of the envelope is exposed to the inside

of the building, i.e., if the insulation is installed on the
outside, there is an adverse interaction with temperature
setback (which is described in

Measure 4.3.2

). What

happens is that a large amount of energy is required to
refill the masonry with heat at the end of the setback
period. On the other hand, using mass inside the
envelope is important with passive solar installations
(covered in

Subsection 8.4

Heat-Reflecting “Insulation”

The warm surfaces inside a building radiate heat

energy toward the envelope. Knowing this gave
someone the idea of installing reflective surfaces in the
building envelope to reflect heat back into the space. It
is convenient to piggyback this reflecting surface on batt
and board insulation, although their functions are
independent. (In the case of foam insulation, foil faces
are also intended to retard gas leakage, as we discussed
previously.)

Be careful when you read the specifications for

insulation with reflective backings. The stated R-value
is likely to include the effect of the reflective backing
when the insulation is installed in an ideal manner. The
benefit of the reflective backing increases strongly at
greater temperature differentials. Therefore,
manufacturers are likely to specify their insulation
performance at a large temperature differential,
exaggerating the R-value.

The reflective surface does not work unless there is

an air space in front of it. This space needs to be a large
fraction of an inch (about two centimeters), at least. For
example, if board insulation is attached to an inside wall
with battens (furring strips), an adequate gap is created
when the interior finish is applied over the battens.
Another opportunity to use reflective backing exists
where the insulation is exposed to the interior space.
This occurs in areas where appearance is not critical
and the insulation is not exposed to damage, as in ceiling
plenums and the exposed ceilings of industrial buildings.

If the design of the building does not include an air

gap that would allow you to use reflective insulation,
should you change the installation to create an air gap?
Probably not. You will probably get better overall
performance by filling the space with insulation.
Furthermore, creating a gap in an insulated assembly
invites serious convective heat loss.

Reflecting internal heat radiation is an important

technique for improving the poor thermal characteristics

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of glazing. So-called “low-emissivity” coatings are used
on glazing to reflect heat back into the building. See

Section 8

for details.

Heat-reflecting insulation can be valuable for

reducing heat loss from very hot surfaces, such as high-
pressure steam lines. See

Measure 1.11.1

for such

applications.

Vapor Barriers

Reference Note

What is a Vapor Barrier?
Purposes of Vapor Barriers
Where Should the Vapor Barrier be Installed?

Vapor Barrier Materials
How to Vent the Water Vapor
House Wraps are Not Vapor Barriers

What is a Vapor Barrier?

A vapor barrier is an impermeable membrane that

blocks the flow of air through the building envelope. A
vapor barrier is an essential part of the building envelope.
Because the purpose of a vapor barrier is not obvious,
this important component is often omitted or installed
incorrectly.

The main purpose of a vapor barrier is preventing

the passage of the water vapor that is contained in air.
Vapor barriers and the insulation affect each other. They
must both be installed so that they interact beneficially
rather than harmfully.

Purposes of Vapor Barriers

The specific functions of vapor barriers are:

• protecting the envelope structure and insulation

from condensation damage. Many wall materials
are permeable to the flow of water vapor from inside
to outside, or vice versa. As water vapor from the
inside of the building moves outward through a wall
on a cold day, it encounters progressively lower
temperatures. At the point in the wall where the
temperature of the air equals the dew point, the
vapor starts to condense, and it keeps condensing
from that point outward. Figure 1 illustrates this.
Condensation damages all types of envelope
structures. It rots wood structures, it rusts steel
structural members and steel masonry
reinforcements, and it causes freeze cracking of
masonry. Installing a vapor barrier on the warm
side of the envelope prevents water vapor from
traveling through the wall, and thereby prevents
condensation.
The protective function of vapor barriers is not
inherently related to insulation. Condensation can
occur inside the envelope structure whether it is
insulated or not. If water vapor condenses inside
insulation, the dampness reduces thermal resistance,
and may damage the insulation.

• preventing air leakage through the envelope. A

well-installed vapor barrier prevents or greatly
reduces air leakage through the envelope surfaces,
although it does not reduce air currents inside the
envelope structure itself. At the same time, the
vapor barrier reduces air flow through the

insulation, preserving the R-value. For more about
this function, see

Reference Note 40

, Building Air

Leakage. (As a matter of perspective, vapor barriers
do nothing to reduce air leakage through the major
envelope penetrations, such as doors, windows, roof
hatches, and fan openings. These penetrations

Fig. 1 Why building structures need vapor barriers This
is a cross section of an insulated stud wall with an outer brick
veneer. Below it is a graph of the temperature inside the wall.
It is cold outside and warm inside. It is also humid inside. If
water vapor can flow through the wall, it will reach a point at
which it condenses. From that point outward, the wall is damp.
A vapor barrier on the inner surface keeps the water vapor
from flowing through the wall.

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11. REFERENCE NOTES

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account for a majority of air leakage in most
buildings.)

• maintaining interior humidification. If

humidification is used, a vapor barrier reduces the
amount of energy and water required to maintain
the desired level of humidity.

Where Should the Vapor Barrier be Installed?

Vapor barriers must be installed on the warm side

of the insulation. This is because condensation occurs
as water vapor moves from the warm side of the wall to
the cold side. If a vapor barrier is installed on the cold
side, it traps moisture inside the envelope, making
moisture problems worse.

This poses a dilemma in climates where the weather

can be hot and humid in summer, but cold in winter.
The deciding factor is how cold it gets. Where the
winters are seriously cold, as in Minnesota, the best
compromise is to install the vapor barrier on the inside.
In humid climates where winter temperatures are mild,
as in Houston, the best compromise probably is not to
use a vapor barrier. If this decision is made, the envelope
should be made of materials, such as masonry, glass,
and aluminum, that withstand periodic dampening.

It might be tempting to solve this problem by

installing a vapor barrier on both sides of the envelope.
However, this is the worst approach. Vapor barriers on
both sides of the envelope would almost certainly trap
harmful amounts of moisture.

Vapor Barrier Materials

In principle, a vapor barrier can be any unbroken

surface that is impermeable to water vapor. For example,
a common vapor barrier material is polyethylene plastic
film, typically installed in thicknesses from .002" to
.008" (0.05 mm to 0.2 mm). This material is inexpensive,
transparent, easy to handle, and is available in wide
widths. It can be attached by stapling, mastic, and other
means. Figure 2 shows a properly installed vapor barrier
using this material.

Vapor barriers can be attached to permeable

insulation, such as glass fiber batts or blankets. The
vapor barrier is commonly in the form of impregnated
kraft paper, sometimes with a thin foil layer. This type
of vapor barrier is unreliable because there is no effective
way to close the gap between adjacent lengths of
insulation. Fold-over strips intended for overlapping
the vapor barrier of adjacent batts are generally ignored
by installers.

WESINC

Fig. 2 Perfectly installed insulation and vapor barrier This large room has wood stud walls and a wood
rafter cathedral ceiling. Glass fiber batt insulation has been inserted snugly into the stud and rafter spaces,
leaving no gaps. The tabs on the paper backing of the insulation are overlapped and closely stapled to the
edges of the studs and rafters. A vapor barrier of 0.008” thick polyethylene sheet is stapled over the insulation.
The vapor barrier is overlapped several feet at all joints. Plenty of excess plastic material is left in all corners.
This slack keeps the plastic from being torn when the wallboard is installed. The vapor barrier is stapled to the
window frames, preventing air leakage around the windows.

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NOTE 42. VAPOR BARRIERS

If the insulation material itself is very impermeable,

such as extruded foam board insulation, it may act as its
own vapor barrier. This characteristic is useful only if
adjacent sheets of insulation are joined to create an
unbroken surface. This requires special installation
techniques that are difficult to enforce on the job.

Some envelope construction materials, such as

asphalt roofing and sheetmetal walls, are impermeable.
Therefore, they act as a vapor barrier, whether this is
desirable or not. The critical question is whether these
impermeable materials are located on the warm side of
the insulation. If they are, they can serve as the vapor
barrier. If not, they create a moisture venting problem
that must be handled properly to prevent damage.

How to Vent the Water Vapor

When installing insulation, create a path for venting

water vapor from the insulated cavity. A vapor barrier
on the warm side of the envelope must be combined
with a venting path on the cold side of the insulation.
This is because no vapor barrier is perfect, and because
water may get into the structure, typically from rain. In
general, the better the vapor barrier and the drier the
conditions, the less venting is required.

Effective venting is a challenge with roofs, because

they are susceptible to leaks and have an impermeable
outer surface. In buildings with attics, a common
solution is to vent the attic to the outside. In cathedral
ceilings, leave an air space above the insulation to allow

water vapor to travel out to the vents, which should be
installed along the full lengths of the ridges and eaves.

Venting walls and soffits is just as important, and

the same principles apply. If there is an impermeable
surface on the cold side of the insulation, such as a
sheetmetal outer wall, leave a gap between the cold side
of the insulation and this surface. The gap acts as a
path for water vapor. In turn, vent this gap to the outside
of the cold surface. Walls that can be wetted by
precipitation require thorough venting.

At the other extreme, some wall materials are so

porous that moisture may vent directly through the wall.
Such material is especially vulnerable to rain soaking.
Keep insulation away from direct contact with the wall.
Generous roof overhangs are an excellent means of
keeping walls dry, if the walls are not too tall.

Portions of the building that are located over soil

have the problem of moisture migration into the building,
rather than outward. The usual solution is to vent the
crawl spaces to the outside, as with attics.

All insulated cavities where water may accumulate

should have drains, as discussed in

Reference Note 44

Insulation Integrity.

House Wraps are Not Vapor Barriers

“House wraps” are an item that goes through

episodes of popularity. House wraps tend to be confused
with vapor barriers, although their function is entirely

WESINC

Fig. 3 This is not a vapor barrier This is a house wrap. It must be made of appropriate permeable material.
Using vapor barrier material as a house wrap would cause moisture damage in the walls. The outer sheathing
of this house is plywood. If the plywood is well installed, the house wrap is superfluous.

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11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

different. Using vapor barrier material as a house wrap
can cause serious moisture damage to the structure.
Conversely, house wrap material will not work as a vapor
barrier.

The purpose of a house wrap is to prevent air

infiltration through the building structure. It is always
installed on the outer surface of the building. It plays
the same role as a wind breaker in human clothing. As
we said previously, one of the benefits of a vapor barrier
is preventing air leakage. However, in most climates,
vapor barriers are installed on the inner surface of the
structure. Therefore, they do not protect the structure,
or the insulation inside the structure, from heat loss that
is induced by wind.

House wrap material must be permeable.

Otherwise, it will act as a vapor barrier that is installed
on the wrong side of the surface, and cause moisture
damage. A common material used for house wrap is a
fiber-reinforced paper. The material is stapled to the
outer surface of the structure before installing the outer

weather surface (siding, brick, etc.). Figure 3 shows a
typical house wrap installation.

The fact that the material is permeable does not

significantly interfere with its wind protection. It is like
human clothing that protects from wind while allowing
moisture to vent from the body.

We should ask whether house wraps are really

needed. If a building is constructed properly, house wrap
is superfluous. If the exterior sheathing is installed with
sufficient care, it will shield the wall structure from wind.
Furthermore, a building should not depend on a
structural component that has a reliable life that is less
than the life of the building. House wraps are fragile,
compared to other structural materials. It is unlikely
that they will survive for the life of the building,
especially if the exterior surface that protects the house
wrap will be replaced during the life of the building.

House wraps are not snake oil, but they have a

limited range of useful application. They are most
valuable when renewing the exterior of a house with
leaky walls.

Insulation Selection

Reference Note

U-Value and R-Value
Other Insulation Selection Characteristics

■

Fire Characteristics

■

Material Cost

■

Ease of Installation

■

Care Required in Installation

■

Ability to Fill Voids

■

Tendency to Settle

■

Resistance to Moisture Damage

■

Aging Behavior

■

Resistance to Physical Damage

■

Temperature Range

■

Vermin Resistance

■

Growth of Microorganisms

■

Moisture Permeability

■

Emission of Noxious Chemicals

Contemporary Types of Envelope Insulation

■

Glass Fiber

■

Mineral Fiber

■

Plastic Foam Boards

■

Plastic Foam Beads

■

Sprayed Plastic Foam

■

Loose Dry Cellulose

■

Wet Sprayed Cellulose

■

Vermiculite

■

Perlite

■

Lightweight Concrete

■

Soil

This Note provides specific information about the

types of building insulation that are presently available,
along with the factors that you should consider in
selecting them.

U-Value and R-Value

The thermal effectiveness of insulation is measured

by “U-value” and “R-value,” which are two ways of
expressing the same information. The formal name for
U-value is “thermal conductivity.” In English units,
U-value is the number of BTU’s per hour that pass
through one square foot of the material for each degree
Fahrenheit of temperature differential across the
material.

U-value is used in the well known formula for heat

transfer by conduction. If Q is the rate of heat flow,
expressed in BTU’s per hour, then:

Q = U x (surface area) x (temperature differential)

Thermal conductivity is also expressed in some

tables as “k,” “c,” or “C.” These symbols are not used
consistently. “C” usually is the thermal conductivity
for a particular thickness that may be specified
arbitrarily. “k” is commonly used in two ways, for a
thickness of either one inch or one foot. Metric tables
use these same symbols with definitions that are
expressed in metric units.

The “thermal resistance” of a material is the

reciprocal of its thermal conductivity. In English units,
thermal resistance is expressed as “R-value.”
Mathematically, R = 1/U.

R-value is handy because it allows you to calculate

the total R-value of an assembly, such as a wall or roof,
simply by adding the R-values of all its components.

For example, to calculate the total R-value of a stud
wall that is filled with insulation, add the R-values of
the outer air layer attached to the exterior siding, the
exterior siding itself, the air layer between the siding
and the outer wall sheathing, the wall sheathing itself,
the insulation inside the stud space, the interior
sheathing, and the air layer inside the space.

U-values cannot be added. To calculate the heat

transfer through an assembly, first calculate the total
thermal resistance by adding the R-values. Then, take
the reciprocal of the total R-value and use it in the
formula above.

The R-values of most insulation materials are fairly

independent of external factors. The R-value of
insulation increases at lower temperatures, but this effect
is not large enough in envelope insulation to favor one
type of insulation over another. (In contrast, the R-values
of glazing units are strongly influenced by orientation,
temperature, internal drafts, and wind. These insulating
characteristics of glazing are covered in

Section 8

Table 1 lists the typical range of R-values for the

common types of building insulation. The R-values of
some common construction materials are included for
comparison.

Be aware that R-value does not imply resistance to

air leakage. It is a common mistake to use insulation as
caulking material to plug envelope leaks. All types of
fibrous and porous insulation are poor at blocking air
flow.

Other Insulation Selection Characteristics

The R-value of insulation is by no means the only

important selection consideration. There are many types
of insulation, each having advantages and disadvantages
as a construction material. You need to consider a

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11. REFERENCE NOTES

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surprisingly large number of characteristics for each
application. If insulation is mismatched to the
application, you may have serious problems. Make your
selection using reliable product data.

The following characteristics are important for

typical building insulation applications. In specialized
applications, there may be other insulation
characteristics that are important, such as adhesion or
the ability to mold the insulation into shapes. The main
point is to recognize all the characteristics that are
important for each application.

■

Fire Characteristics

The fire characteristics of construction materials are

expressed in a variety of ways. Three of the most
common are:

• fuel value. If the insulation has no fuel value, it

cannot burn. In fact, its presence inhibits the spread
of fire. However, be aware that noncombustible
insulation materials may be combined with
materials that have fuel value, such as binders and
backing paper. An example is glass fiber batt
insulation, which uses an organic binder, and which
may have a flammable backing sheet.

• flame spread, roughly speaking, is the rate at which

fire moves along the material. This factor is

significant if the insulation itself is the primary fuel
for the fire. It is irrelevant if fire spreads outside
the insulation.

• smoke production indicates the amount of smoke

that the insulation material produces when it burns.
Unfortunately, this rating does not address the
critical issue of the smoke’s toxicity.

Fire safety classifications still fail to adequately

define another critical factor, which is ease of ignition.
This factor may never be defined satisfactorily, because
the ignition of material in a fire depends on many factors,
such as the manner of installation, the way the fire
surrounds the material, changes in the properties of the
insulation before it ignites, etc.

The fire resistance of insulation may be improved

considerably by adding retardants, which keep the
insulation from catching fire. The performance of
retardants varies considerably depending on test
conditions. The fire retardant rating assigned to a given
insulation material may differ radically from one country
to another. The basic problem is that fire retardants cease
to function effectively if the fire exceeds a certain
temperature and duration. Any material that has fuel
value will eventually burn.

The importance of fire characteristics depends on

where the insulation is installed. For example, fire
behavior is less important for insulation installed on a
concrete roof deck than for insulation installed under
interior paneling.

■

Material Cost

There are large cost differences among different

types of insulation. The main reason that more expensive
types of insulation are on the market is that they are
easier to install.

■

Ease of Installation

Labor is a large component of insulating cost, and

labor requirements vary widely with different types of
insulation. For example, insulation that can be installed
by pouring or blowing requires less labor than insulation
that is fitted into place by hand. Using foam board
insulation is a very quick way of insulating vertical
surfaces.

■

Care Required in Installation

This is not the same as ease of installation. The

most common example is fiber batt insulation. Even
though batts are easy to install, they are commonly
installed in a manner that leaves voids and reduces their
effectiveness. Similarly, foam board insulation is
exceptionally easy to install, but failing to seal the boards
properly at the edges can waste most of their insulating
value.

■

Ability to Fill Voids

Voids transfer heat because they allow convection.

Aside from open areas that are needed for venting, the
entire cavity in the envelope structure should be filled.

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NOTE 43. INSULATION SELECTION

Most types of insulation have some tendency to leave
voids. Each type of insulation requires its own
techniques to eliminate or minimize voids.

■

Tendency to Settle

Loose fill insulation tends to settle, which reduces

its R-value. If a flat surface is being insulated, the
solution is simply to add more material to compensate.
However, if the insulation is used to fill a vertical cavity,
such as a stud wall, settling causes the upper portion of
the cavity to become completely uninsulated. Cellulose
insulation has the worst settling among the common
insulation types.

Fiber batt and blanket insulation does not settle if

laid flat. If installed vertically, it must be installed snugly
in cavity, and it must be supported by a backing sheet.
If batt insulation is suspended by a backing sheet alone,
it will eventually fall away.

■

Resistance to Moisture Damage

Insulation may be wetted occasionally in ways that

are not obvious. Insulation installed in a masonry wall
is wetted by rain soaking through the wall. Attic
insulation is wetted by occasional roof leaks. Lack of
an effective vapor barrier causes permeable insulation
to be wetted by condensation during cold weather. Most
types of insulation tolerate occasional light moisture,
but some do not. Some applications, such as exterior
foundation insulation, subject the insulation to
continuous soaking. This destroys the R-value of most
insulation sooner or later, unless the insulation material
is exceptionally resistant to moisture.

■

Aging Characteristics

Inorganic insulation generally does not deteriorate

with time. At the other extreme, plastic foam insulation
suffers a rapid initial lowering of R-value, followed by
a slower long-term degradation of R-value. Cellulose
insulation is reported to deteriorate somewhat with time.

■

Resistance to Physical Damage

Insulation may encounter various types of hazards.

It can be protected by selecting a type that resists the
hazard, or by external protection, or by a combination
of both. For example, roof insulation must resist people
walking on it. The protection can be provided by
selecting a plastic foam insulation that is dense enough
to resist the pressure, or by distributing the pressure with
a covering of ballast, or by installing a walkway over
the insulation. Plastic foam insulation is destroyed
rapidly by sunlight, so it must be protected with an
opaque surface. Pipe insulation in a machinery room is
subject to local impact damage, which can be prevented
with suitable guards. And so forth.

■

Temperature Range

Temperature tolerance is not often a factor in

envelope insulation, but recognize when it may be. For
example, do not use polystyrene foam where it may be
exposed to temperatures of 170°F or higher. Such

temperatures may occur near flues, ceiling heaters, and
incandescent lighting fixtures.

■

Vermin Resistance

Anyone who reads H. P. Lovecraft knows about rats

in the walls. Vermin eat cellulose, and they may use
other types of loose fill insulation as nesting material.
The problem is not primarily loss of insulation value,
but proliferation of the vermin.

■

Growth of Microorganisms

Some types of insulation may act as a growth

medium for microorganisms, such as mold or fungus.
Microorganisms may grow in insulation under some
conditions even if the material itself is not organic. These
microorganisms may contribute to “sick building
syndrome.” This is primarily a problem in duct
insulation, not in envelope insulation, but envelope
insulation may become a problem if it is wet.

■

Moisture Permeability

Preventing moisture migration through insulated

surfaces is important in most applications. If the
insulation is impermeable, it may serve as a vapor
barrier, but only if it is installed in a manner that leaves
no gaps. For more about vapor barriers, see

Reference

Note 42

Permeability is not an all-or-nothing characteristic.

For example, some people assume that plastic insulation
is an absolute vapor barrier. This is not true. All plastic
insulation is permeable to some degree, and some types
of plastic insulation are much more permeable than
others.

■

Emission of Noxious Chemicals

Experience with urea formaldehyde wall insulation

during the 1970’s and 1980’s demonstrated that some
types of insulation may emit dangerous chemicals. No
similar problem is known with other common types of
insulation, but the lesson should be remembered. In
particular, be cautious about using insulation that
requires mixing noxious components on site, or that
emits noxious chemical products.

Contemporary Types of Envelope Insulation

The diversity of insulation applications keeps a

variety of insulation types on the market. Each
application usually has no more than one or two types
of insulation that are best suited to it. The following
listing includes the major types worth considering for
conventional applications. It omits materials that are
now obsolete or unacceptable for building insulation,
such as cork, asbestos, and urea formaldehyde.

■

Glass Fiber

Glass fiber insulation has good R-value because the

fibers are oriented perpendicular to the direction of heat
flow, and have little contact with each other. It is

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11. REFERENCE NOTES

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available in the form of batts, blankets, semi-rigid
boards, and loose fill.

Batts are used for wall cavities. They have a stiff

paper backing sheet for attachment and handling, and
to act as a vapor barrier. Board insulation is attached to
the inside surfaces of walls and roofs. Batts, blankets,
and loose fill are used for attic insulation. Batts,
blankets, and boards use a binder to hold the fibers
together. Loose glass fiber insulation can be installed
by blowing.

Contrary to what you might expect, the R-value of

fiber insulation is higher if the material is compressed
more densely than it is supplied, without going to
extremes. The cost of material leads manufacturers to
produce batts that have less than optimal density. For
this reason, when filling a cavity with blankets or batts,
consider using thicker insulation to achieve greater
density. This also tends to reduce voids. (But, do not
do this if it intrudes on an air space needed for moisture
venting.) Semi-rigid fiber boards have better R-value
than the fluffy blankets and batts, even though they
contain more solid matter.

The kraft paper backing sheets of batt insulation is

a mediocre vapor barrier at best. To achieve reliable
blocking of water vapor, install a separate vapor barrier,
such as large sheets of plastic film, over the insulation.

Glass fiber is inert, which makes it generally safe

for occupants of the structure. However, the fibers are
a safety hazard during installation. They can penetrate
eyes and skin, and there is suspicion that they may cause
lung disease. Glass fiber may be a hazard to occupants
if it can break loose, as from duct linings,

■

Mineral Fiber

Mineral fiber has essentially the same physical

properties as glass fiber and it is used the same way. It
is made from dirty raw materials, such as boiler slag, so
it is not as pretty as glass fiber. It has shorter fibers, so
it is more likely to be used in loose form, as “mineral
wool,” than in blankets and batts.

■

Plastic Foam Boards

Plastic foam insulation created a revolution in the

construction industry because of its high R-value, low
moisture permeability, and ease of installation. Most
plastic foam is installed in the form of boards, which
are used for wall, roof, and subsurface insulation.

Plastic foam has high R-value because the gas is

perfectly encapsulated, which prevents gas movement
by convection or external pressure differences. The walls
of the foam bubbles are thin, which limits conduction
through the solid material. The plastic is expanded into
a foam by gas injection. CFC’s were originally used as
the blowing gas, but these are being replaced by other
gases because of concern about ozone depletion in the
atmosphere.

Unfortunately, the R-value diminishes with age

because the insulating gas leaks out and is replaced by

air. Even aged foam insulation has a better R-value than
other types of insulation, but the advantage may be
substantially reduced.

Foam boards are easy and safe to handle. They have

almost no weight, they can be cut easily with a box knife,
and they do not irritate eyes or skin. The foam can be
bonded to structural and finish materials, such as
plywood, chipboard, and gypsum board.

The major disadvantage of all plastic foam

insulation is its behavior in a fire. Plastic is made from
petroleum, and it burns if it is ignited adequately. A
great deal of effort has gone into making plastic
insulation fire retardant, and some types can resist small
fires. However, once ignited by a surrounding fire,
plastics produce large amounts of toxic smoke that kills
occupants quickly.

For this reason, it is prudent to avoid using plastic

insulation in interior applications. Sheathing plastic
insulation in non-flammable material, such as gypsum
board, may buy time to evacuate a burning space, but
this practice cannot keep the plastic from igniting. In a
big fire, plastic insulation melts, runs out into the fire,
and then ignites.

There are several types of plastic foam used in board

insulation. At present, the main types are:

• extruded polystyrene foam. Polystyrene is the

cheapest of the foam insulation materials, but it is
still about twice as expensive as glass fiber for a
given surface area. It is limited to temperatures
below about 170°F (77°C).

• polystyrene beadboard. Polystyrene foam beads

can be fused together to form insulating boards. In
fact, the beads can be molded into any shape, such
as coffee cups, but boards are the most common
shape for insulation purposes. If kept dry,
beadboard has about the same thermal properties
as extruded polystyrene. It is less resistant to long-
term moisture penetration. This is important, for
example, in insulation that is installed below grade.

• extruded polyurethane foam. Polyurethane foam

has better R-value than polystyrene. It is more
expensive. It tolerates somewhat higher
temperatures, around 250°F. It is less resistant than
expanded polystyrene to long-term moisture
penetration in wet applications, such as foundation
insulation below grade.

• extruded polyisocyanurate foam. Polyurethane is

combined with isocyanurate plastics to create
insulation materials that are usable over a wider
range of temperatures, from about -290°F to about
+300°F.

■

Plastic Foam Beads

Loose plastic foam beads are used for pouring

applications, such as the cavities of masonry walls.
Polystyrene is the most common material for making

1401

NOTE 43. INSULATION SELECTION

foam beads. The fire characteristics discussed for foam
boards also apply to foam beads. Foam beads are
replacing perlite and vermiculite in applications where
fire resistance is not important. They have become
popular as an aggregate for lightweight concrete.

■

Sprayed Plastic Foam

Sprayed plastic foam typically is applied to flat roof

decks. It requires specialized equipment to create the
foam on site. It produces a lumpy surface that can puddle
water if it is installed on a flat surface. Foam is destroyed
rapidly by sunlight, so exposed surfaces need a durable
opaque coating. Since the surface is irregular, it is not
practical to apply flat sheathing to protect the foam, and
the foam is not quite strong enough to support people
walking on it. Therefore, you need separate walkways
to protect the foam.

■

Loose Dry Cellulose

“Cellulose” insulation consists of shredded old

newspapers treated with chemicals. For example, borax
may be added to retard fire, and other chemicals may
be added to improve vermin resistance. Cellulose
insulation was advocated heavily during the 1970’s as a
useful way of disposing of waste paper. Its thermal
properties are good. Its major weaknesses are settling,
fire hazard, degradation if wetted, and the fact that it
makes food and nesting material for vermin. Its
properties are largely dependent on additives, so quality
varies widely among different manufacturers.

Its main advantages are low cost and ease of

installation. It typically is blown into place using
specialized pneumatic equipment. It is widely used in
attics. It can also be blown into walls, but this application
is riskier because of its tendency to settle, difficulty in
filling voids, vulnerability to moisture, and vulnerability
to the skill and honesty of the installer.

Paper burns, so it is important to isolate cellulose

insulation from any heat sources, such as flues and
incandescent light fixtures.

■

Wet Sprayed Cellulose

Cellulose insulation can also be applied as a slurry

of cellulose material mixed with a glue. It can be applied
to any orientation, including overhead, although the
thickness of each application is limited. When the slush
dries, it becomes like papier mache, and it is
dimensionally stable.

■

Vermiculite

Vermiculite is a class of minerals consisting

primarily of silicates of magnesium, aluminum, and iron
that are bound with water. Vermiculite ore has a flat,
layered structure that looks like mica, and it has a
significant water content. It is converted to insulation
by a process called “exfoliation,” which is similar to
making popcorn. When exposed quickly to high
temperature, the water in the ore bursts into steam,

expanding the layers and creating hollow granules that
look like tiny accordions.

The R-value of vermiculite is lower than that of other

insulation materials, and it varies depending on the
source of the raw material, the particle size, and the
manufacturing process. The main advantages of
vermiculite are lack of flammability, ability to withstand
high temperatures, benign handling characteristics, and
easy pouring. Vermiculite is a good aggregate for
lightweight concrete. It is also used as a component of
sprayed fire retardant insulation.

■

Perlite

Perlite is made from a certain type of volcanic rock

that is high in silica. The material contains a small
percentage of water. The rock is first crushed. Then, it
is heated quickly to a temperature above 1,600°F
(870°C). As the rock melts, the water creates a froth of
tiny bubbles, expanding the material by a factor of four
to twenty. The cooled material retains the sealed
bubbles, making it relatively light and providing
significant thermal resistance. The individual particles
are physically strong, making the material an effective
aggregate for lightweight concrete. It can also be used
in loose form, typically to fill the holes in concrete block
walls.

■

Lightweight Concrete

Concrete can be made with virtually any material

as an aggregate. If a lightweight material is used instead
of sand or gravel, both the weight and the thermal
conductivity of the concrete may be greatly reduced.
Lightweight concrete does not have the high R-value of
true insulation, but it insulates much better than
conventional concrete.

Plastic foam beads make a concrete that is incredibly

light. Perlite and vermiculite produce a concrete that is
somewhat heavier and stronger. There is a compromise
between R-value and strength. The compromise can be
adjusted over a wide range by selecting the type and
density of the aggregate material.

■

Soil

In the frenzied days of the 1970’s energy crises,

“earth sheltered” construction captured the fancy of
many, perhaps because the idea of dirt was appealing in
its own right. Soil can be considered a very poor grade
of expanded mineral insulation. Its only advantage is
that it is dirt cheap. However, it costs so much money
and energy to modify a building’s structure to
accommodate soil that using it for insulation is self-
defeating. Using soil adds serious problems, including
high pressure on the retaining structure, roof weight,
rain and ground water leakage, radon entry, visibility
limitations, and the esthetics of a prehistoric burial
mound.

Insulation Integrity

Reference Note

Minimize Thermal Conduction Paths
Eliminate Convective Paths
Provide Water Drainage, if Necessary
Install the Vapor Barrier Properly

The effectiveness of insulation depends on the way

it is installed. This Note covers the main points of
installing insulation in a way that maintains its
effectiveness.

Minimize Thermal Conduction Paths

Any solid material that bridges the space between

the inner and outer surfaces of the envelope acts as a
short circuit for heat. The amount of heat loss is
proportional to the R-value and thickness of the
conducting material. Wall studs, sill plates, sole plates,
window and door framing, floor slabs that extend to the
exterior structure, and the edges and supporting
structures of curtain wall panels are all conductive short
circuits. Metal structural elements have high thermal

conductivity, and their effect is especially severe if they
have wide flanges at the inner and outer envelope
surfaces.

In the worst cases, conductive short circuits can

severely limit the benefit of adding insulation. An
example is insulating a concrete block wall by pouring
insulation into the holes in the blocks. This provides
some benefit, but a large amount of heat will continue
to be conducted through the webs in the blocks.

To avoid conduction short circuits, try to use an

insulation method that provides an unbroken layer of
insulation. For example, covering the interior or exterior
surface of a concrete block wall with insulation board
is much more effective than filling the holes in the
concrete blocks with insulation.

WESINC

Fig. 1 What is wrong in this picture? The batt thickness is appropriate for the stud widths. However, the
insulation is stuffed far inside the cavity, reducing its thickness and creating a large convection path in front.
The kraft paper vapor barrier is not effective, because the paper is not joined at the edges. The method of
installing the electrical wiring force the insulation installer to stuff the insulation behind the wiring. Fiber insulation
is used instead of proper caulking around the window, and under the sole plate. Anything else?

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Eliminate Convection Paths

Convection carries heat through voids in the

envelope structure. Convection is a powerful mode of
heat transfer, so voids should be minimized when
installing insulation. This is largely a matter of
workmanship and selecting the appropriate type of
insulation for the job.

Everyone has seen batt insulation that is stuffed

between stud spaces in a manner that leaves large gaps.
See Figure 1. A good way to insert tight fitting batt
insulation between studs is to use a trowel to tease the
edges of the batts past the studs. If electrical wiring is
in the way, a large void may be left behind the wiring
by sloppy installation. Cut notches or gashes in batts to
fit around wiring.

When inserting loose fill insulation into existing

walls, there is a risk that the material will snag on
obstructions, leaving voids. Preventing voids requires
installation methods that are vigorous enough to move
the insulation past obstructions. Air blowing, rodding,
and vibration are the methods used. Good pouring
characteristics may be a major factor in selecting the
insulation material.

A major problem with pouring insulation into

existing walls is the presence of obstructions that create
voids in the insulation. Frame walls may have fire stops,
which are horizontal pieces that block the space between
studs to retard the spread of fire by convection in the
stud space. The gap between the brick and block courses
of masonry walls is filled with dried mortar and wall
ties. With any wall, it is difficult to get the insulation to
fill the space all the way to the top.

If you want to use rigid insulation in a wall cavity,

you will have to take special steps to prevent convection.
If rigid insulation is placed loosely in a cavity, convection
provides a way for heat to bypass the insulation, making

the insulation almost useless. There is little hope of
blocking convection unless the insulation boards are
firmly attached to one of the wall surfaces and are sealed
at the edges.

There are two conditions that make it important to

leave a space between the insulation and the outer skin
of the building to vent water vapor. One is that the outer
skin is impermeable. The other is that the outer skin is
permeable and can get wet. (This is discussed further
in

Reference Note 42

, Vapor Barriers.) The vented space

allows strong convection. From the standpoint of
insulation value, the vent space is effectively outside
the insulated portion of the envelope.

Provide Water Drainage, if Needed

Anticipate the possibility that water will

occasionally get into the envelope structure. The
moisture may come from rain, condensation, piping
leaks, etc. If the structure is well vented and is not
vulnerable to wetting, it may suffice to allow the water
to escape by evaporation through the vents.

If water may accumulate in a cavity, install drain

holes. Design the drain holes so they will not be plugged
by the insulation. This is a problem especially with
insulation materials that can flow, such as foam beads,
cellulose, and vermiculite.

Install the Vapor Barrier Properly

Installing the vapor barrier is an integral part of the

insulation job. Failing to install the vapor barrier
properly can reduce the effectiveness of the insulation
and can cause structural damage. For example, in
Figure 1, the vapor barrier material that is attached to
the insulation has been rendered almost useless by bad
installation practice. See

Reference Note 42

, Vapor

Barriers, about installing vapor barriers.

Insulation Economics

Reference Note

This brief Note is intended to clarify the economic

aspects of installing insulation. The points are illustrated
with a typical example.

Unlike most energy conservation measures, adding

insulation is a matter of degree. Furthermore, it is not
possible to clearly define a “best” amount of insulation.
From the standpoint of energy savings alone, most
applications would benefit from installing as much
insulation as possible. The total lifetime saving
continues to increase as the amount of insulation is
increased. However, more is not necessarily better.
Beyond a certain amount of insulation, the payback
period gets longer.

The economics of adding insulation to an existing

building are heavily influenced by the original thermal
resistance of the structure. For example, if an attic
already has a certain amount of insulation, adding more
insulation has a relatively long payback period.

The economically optimum amount of insulation is

also affected by the installer’s overhead and labor costs.
Both of these cost factors tend to be independent of the
R-value of the insulation installed. Therefore, high
overhead and labor costs argue in favor of adding more
insulation.

These points are illustrated in the following example

of adding insulation to an uninsulated masonry wall.
These are the conditions:

• the bare, uninsulated wall has an R-value of 3.0
• the annual average temperature differential is 30°F
• energy loss costs $10 per million BTU
• overhead cost is fixed at $0.12 per square foot
• base labor cost is $0.20 per square foot
• additional labor cost is $0.01 per R-unit per square

foot

• material cost is $0.09 per R-unit per square foot.

From these numbers, the table gives the savings per

square foot and the economic return that result from
installing different amounts of insulation. The economic
return is expressed in two ways, as payback period and
as savings-investment ratio. The latter is the inverse of
payback period, if no discount factor is included.

In this example, the shortest payback period occurs

with an R-value of 6. A wide range of R-values provide
payback periods in the same range. However, total
savings always increase with additional R-value. An

accountant might select the R-value of 6 for best rate of
return, whereas a conservation enthusiast might select
an R-value of about 20 to maximize long-term savings.
The prudent building owner might select a value
somewhere between these two. The range in this
example is typical of many insulation jobs.

In many cases, such as insulating wall cavities, there

is no need to perform such a detailed analysis because
space limitations make it impossible to add enough
insulation to achieve the shortest payback. In any case,
it is difficult to go seriously wrong by adding too much
insulation.

This example does not take the time value of money

into consideration, nor does it consider changing energy
costs. The time value of money is an important
consideration from an economic standpoint, but strictly
speaking, it is not relevant to energy conservation. In
essence, the time value of money is largely the crux of
the argument between energy conservation enthusiasts
and those who make choices on a strictly economic basis.

It is tempting to ignore the time value of money

and to ignore the possibility of cost changes because
these factors are unpredictable over the long term,
whereas energy consumption is fairly predictable.
However, the fact that the economic factors are not
predictable does not mean that they are unimportant.

Daylighting Design

Reference Note

How to Visualize the Performance

of a Daylighting Installation

The Key Aspects of Daylighting Design

■

Visual Quality

■

Daylight Penetration

■

Avoid Visual Desensitization

■

Control Electric Lighting Efficiently

■

Understand Solar Cooling Load

■

Exploit Passive Solar Heating

A Daylighting Wish List

The amount of sunlight falling on the surface of

almost any building, even on a cloudy day, theoretically
is adequate to provide all lighting requirements
throughout the building. However, methods of
distributing sunlight deep within buildings have not
progressed much beyond the conceptual stage.

Most of the successful daylighting installations that

you can find today use skylights or translucent roof
panels. Figures 1, 2, and 3 are examples. Skylights
and translucent roofs can provide daylighting throughout
a large space, but they are effective only for the floor
level immediately below them.

Just making glazed holes in the roof does not provide

satisfactory illumination. Failure of skylight installations
is common, as illustrated by Figure 4.

Daylighting through windows has been far less

successful. With current techniques, daylighting by
windows typically cannot extend into the space much

farther than the height of the window above the floor.
However, even this limited benefit is rarely achieved.
Ordinary windows do not convert sunlight into useful
illumination. Instead, bare windows primarily cause
discomfort. Occupants respond to the discomfort they
cause by excluding sunlight from the building. Aside
from wasting the energy saving of daylighting, this also
forfeits the view and ambiance that windows are
supposed to provide. Figure 5 illustrates this failure of
contemporary architecture.

There is probably no area of energy conservation in

which so many fundamental problems have been ignored
as in daylighting. Unsuccessful attempts at daylighting
not only fail to provide satisfactory illumination, but
they typically increase energy consumption for lighting,
heating, and cooling. In other words, most daylighting
installations waste more energy than they save. In
addition, they often create heating and cooling comfort
problems.

Kalwall Corporation

Fig. 1 Daylighting for a school cafeteria The entire roof
over this space is translucent, so the roof material must have
very low light transmission to avoid glare.

Kalwall Corporation

Fig. 2 Daylighting for a swimming pool This is a classic
application for daylighting because the illumination levels and
lighting geometry are not critical. It also benefits from a large
amount of passive solar heating. The surfaces of the
translucent panels are warmed by absorption, which keeps
them dry in this humid environment.

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Despite this dismal record, there is hope for tapping

the potential of daylighting. The key insight is that
daylighting is complex. To achieve success, you must
know where the pitfalls are hidden. Therefore, this brief
introduction to daylighting will show you various
examples of daylighting failures, and explain why they
occurred.

To create effective daylighting, you must master

three subject areas. The first is the human factors of
illumination requirements and visual comfort. The
second is integrating daylighting with artificial lighting.
The third is integrating daylighting with the heating and
cooling of the building. None of the individual
specialties involved in the design of buildings deals with
all these factors. Therefore, you will have to go outside
your own professional specialty to learn all you need.

How to Visualize the Performance of a
Daylighting Installation

Daylighting design faces an especially difficult

problem, which is visualizing the conditions that will
exist in the finished project, including visual quality,
heating and cooling comfort, and ambiance.

Architects’ models are a trap for the unwary.

Viewing a model from the outside utterly fails to convey
how you would feel on the inside of the real space.

The best school for learning daylighting consists of

finding good examples of daylighting, and studying them
under all operating conditions. At present, you will have
to do a lot of searching to find good examples, unless
you are lucky. But, there is no other way. Designers
need to spend more time outside their offices, and to
develop the skills of critical observation in the field.

Once you have developed this base of experience,

you may be able to benefit from various computer
models and other computational tools. However, using
these tools without extensive observation of real,
successful installations will only mislead you.

The Key Aspects of Daylighting Design

Daylighting design is demanding because it requires

dealing with a large number of factors, some of which
are in conflict. These factors can be grouped in the
following subject areas.

■

Visual Quality

To make daylighting satisfactory from a visual

standpoint, it must have these characteristics:

• the right intensity, neither too dim nor too bright.

Nightfall, clouds, and other things interfere with
sunlight, as illustrated by Figures 6 and 7. Electric
lighting must be available, and it must be controlled
to provide just the right amount of supplemental
lighting when sunlight is inadequate. On the other
hand, daylighting can never be allowed to make
any area of the visual field too bright, or occupants
will block out the daylighting permanently.

• absence of excessively bright areas within the

visual field, which causes visual desensitization and
discomfort. This problem is called “glare.” (The
term has other meanings as well.) Bright areas may
be acceptable if they are behind the viewer or are
well above the horizontal. Figure 8 shows an
example where daylighting through windows was
abandoned because it failed this criterion.

• the right direction, which basically means

eliminating veiling reflections. This is a seriously
neglected aspect of lighting design. In general,
veiling reflections are avoided by not having
lighting originate in the direction that the viewer is
facing. This involves space layout as well as
lighting.

See

Reference Note 51

, Factors in Lighting Quality,

for a more complete discussion about each of these
factors.

■

Daylight Penetration

If daylighting is to be an important energy

conservation measure, it must be able to serve as the
primary source of lighting for a large area. Not only
must the sunlight reach the activity area, but it must
arrive from a direction that is visually effective, as
discussed above.

Vistawall Architectural Products

Fig. 3 Daylighting for an atrium This kind of grandiose
application is often used as an element of ambiance. It can
save a significant amount of lighting energy, but only if the
architect cooperates with the lighting engineer.

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NOTE 46. DAYLIGHTING DESIGN

WESINC

Fig. 4 Skylights are not the same as daylighting Just making glazed holes in the roof does not create
satisfactory daylighting, as this abandoned and covered skylight on a classroom building testifies. The problem
was excessive glare.

WESINC

Fig. 5 Windows are not the same as daylighting If direct sunlight can enter a space at any time, the
occupants will shut out sunlight entirely, as we see here. The situation is not helped by the vertical blinds, which
are an unsatisfactory method of modulating sunlight. There is a nice view, but the occupants can’t enjoy it,
except on the top floor, which is shaded by a deep overhang. This vast amount of glazing serves little useful
purpose, and it increases heating and cooling costs considerably.

1410

11. REFERENCE NOTES

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WESINC

Fig. 6 Accommodate clouds A thick cloud passing in front of the sun may reduce the available sunlight by
a factor of five. Your daylighting application must be able to tolerate this large, abrupt change. Or, you must
install devices to regulate the amount of sunlight that enters the space. And, the electric lighting controls must
be able to provide just the right amount of supplemental lighting.

WESINC

Fig. 7 Accommodate shading by external features Portions of this large hotel are shaded at different times
of day by taller adjacent buildings. Plans for daylighting should expect this, including the possibility of later
construction nearby.

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NOTE 46. DAYLIGHTING DESIGN

WESINC

Fig. 9 Daylighting abandoned because of visual desensitization This classroom has continuous rows of
tall windows at the sides and rear. The room is used to train second lieutenants of the U.S. Marine Corps. Even
these perfect physical specimens cannot accommodate the large differences in brightness between the windows
and the inside of the space. The windows have been covered with draperies, which are too cumbersome to
adapt to changes in daylighting and to the changing lighting needs inside the space. The room will remain
dependent on electric lighting.

WESINC

Fig. 8 Daylighting abandoned because of glare Large windows were installed high in the walls of this
gymnasium. They have been blocked completely. Put yourself in the position of a basketball player attempting
to follow the ball with direct sunlight coming through the windows. It is difficult to imagine any usage of this
space for which the windows would provide acceptable daylighting. On the other hand, well designed skylights
might have worked well. This space will remain dependent on electric lighting.

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11. REFERENCE NOTES

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There are many conceptual methods that have been

devised for increasing the penetration of sunlight into a
space. They all use some combination of reflection,
refraction, and diffusion. The Measures in

Subsection

8.3

recommend the techniques that are most practical

at the present time.

■

Avoid Visual Desensitization

The human eye is able to adapt to an enormous range

of illumination levels, over a ratio of approximately a
million to one. However, at any given moment, the eye
adapts to the brightest illumination levels within the
visual field, and cannot see well within the darker areas
of the visual field.

The eye adapts to different light levels by three

processes: (1) changes in the size of the pupils, (2)
changes in the response of the nerves in the visual
system, and (3) changes in the amounts of certain
chemicals (photopigments) in the light sensing cells of

the retina. At normal indoor brightness levels, only the
first two of these processes occur, and both of them
respond in a time frame of about one second. At normal
indoor lighting levels, the delay in adapting is barely
noticeable when looking from a well lighted part of a
room to a darker part, especially if the range of
brightness falls within a ratio of less than 10:1. (This
ratio declines with age.)

Electric lighting designed to contemporary standards

keeps illumination levels within a narrow range, which
allows people to see with little effort of adaptation. In
contrast, even the most cleverly designed daylighting
applications produce large variations of illumination
levels within the space. If daylighting is not well
planned, the range of illumination levels within the visual
field can far exceed the ability of viewers to adapt to the
differences in brightness. Figure 9 shows an example
where valuable daylighting was abandoned because
windows did not have effective methods of modulating
the brightness of daylighting.

People are not forgiving about large variations of

brightness within a space. If daylighting causes strong
contrasts in illumination, occupants invariably turn on
all the electric lights in an attempt to match the
illumination levels provided by the daylighting. (The
term “fill-in lighting” has been coined by lighting
designers to certify this practice, in defiance of the logic
of daylighting.) Figure 10 shows an example.

Even worse, unless daylighting is executed very

cleverly, the brightness of windows or skylights activates
the third adaptation process, depleting the
photopigments in the retinal cells, which greatly reduces
visual sensitivity. Recovery from this chemical change
is slow. Therefore, the eye remains desensitized as long
as daylighting is present. This increases the illumination
levels needed inside the space, typically to a level several
times higher than needed by people whose eyes are
adapted to lower lighting levels. This may be acceptable
if daylighting is the only source of lighting in the space.
However, if a small daylighted perimeter area reduces
the visual sensitivity of people farther inside the space,
they are likely to turn on all the available lighting
equipment.

■

Control Electric Lighting Efficiently

Daylighting saves energy only if it provides a

reduction in artificial lighting. Daylighting is useless
unless the controls of the lighting fixtures are
sophisticated enough and reliable enough to respond to
the amount of daylighting that is available from moment
to moment. Lack of appropriate lighting controls is a
common failing of daylighting installations, as illustrated
in Figure 11.

In addition, the electric lighting controls must

respond to other variables, such as the occupancy of the
spaces, the types of activities being conducted, and
individual needs and preferences. For example, the

WESINC

Fig. 10 An expensive daylighting installation that saves
no energy A lot of useful space in this office building was
given up to create an atrium illuminated by a large skylight.
However, note the electric lights that are turned on everywhere
around the perimeter of the atrium. These attempt to equalize
the illumination levels of the shaded areas with the sunlighted
areas. Someone probably won a design award for this.

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NOTE 46. DAYLIGHTING DESIGN

WESINC

Fig. 12 The inevitable fate of greenhouse additions to restaurants The restaurant wants a cheap addition
that offers daylighting and ambiance. But people are not African violets. The owner must eventually address
the discomfort during both hot and cold weather, and covers the greenhouse. This one uses a canvas tarpaulin,
which needs replacement every few years. Still, it is better than reflecting films, which are ugly and do not
reduce the heat gain and glare sufficiently. Why do architects continue to make this common mistake?

WESINC

Fig. 11 What’s wrong in this picture? This common area of a shopping mall is illuminated effectively by the
available daylighting from the skylights. The electric lights, which are all turned on, add virtually nothing on this
bright day. There may be a legitimate concern that the HID lighting cannot respond quickly enough to large
changes in daylighting caused by clouds. Installing fluorescent lighting instead of HID lighting would have
avoided this problem.

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11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

electric lighting in a well designed space may have
photoelectric controls for some of the fixtures, time
controls for all the fixtures, separate controls for different
numbers of lamps within each fixture, override switches
for late cleaning crews, and so forth.

People tend to turn lights on, but not to turn them

off. This behavior is generally beyond the designer’s
control. Hoping that occupants will keep unnecessary
electric lights turned off is not an acceptable design
approach. For specific methods of controlling electric
lights to exploit daylighting, see

■

Understand Solar Cooling Load

It appears that people who design daylighting

installation commonly have no understanding of the
amount of solar heat gain that comes along with
daylighting. Sooner or later, the cooling load becomes
unbearable, in terms of both comfort and energy cost.
The result is that all or most of the daylighting
installation is abandoned. Figure 12 shows an example.

Some daylighting advocates claim that daylighting

reduces cooling load because sunlight is a cooler light
source than electric lamps. The basis of this claim is
that sunlight has a light-to-heat ratio of about 110 lumens
per watt, if the entire solar spectrum enters the space in
equal proportions. If all but the visible portion of the
sunlight were filtered out, the efficacy exceeds 300
lumens per watt. Currently available glazing can provide
daylight that is between these two values. By
comparison, the light-to-heat ratio of conventional
fluorescent lighting is about 70 lumens per watt.

This theoretical advantage is undermined by the fact

that daylighting cannot be distributed as effectively as
artificial lighting in real applications. Therefore, an
excess of sunlight must be admitted to provide adequate
illumination. In a sloppy design, so much extra sunlight
may be admitted that the added cooling cost may cancel
the saving in lighting cost.

Reflective and absorptive window coatings are

widely used as a means of reducing solar cooling load.
(These are recommended by

Measures 8.1.3

and

8.1.4

for retrofit applications.) These methods have also been
used in the vain hope of taming sunlight for illumination
purposes. This cannot work, because a direct beam of
sunlight is unsatisfactory for illumination, no matter how
much it is attenuated. If tinted glass were dark enough
to reduce the intensity of a direct beam of sunlight to
acceptable levels, it would have to be almost opaque.
The illumination provided by direct sunlight is about
6,000 footcandles. Dark tinted glass provides an
attenuation of about 80 percent, which still leaves an
illumination level of about 1,200 footcandles. Even if
the windows are darkly tinted or highly reflective, the
response of building occupants is still to close the
curtains over all windows that allow direct sunlight.

For maximum efficiency, daylighting should use as

little glazing area as possible, which means that the
glazing should be clear or translucent. Daylighting
forces a compromise between the positive value of free
illumination and the penalties of conductive heat loss
(while heating) and solar heat gain (while cooling). Any
form of window treatment that attenuates the passage
of light through the glazing increases the glazed area,
along with its undesirable consequences.

Design is complicated by the fact that most climates

require both heating and cooling. In such climates, one
approach is to base the amount of glazing on the
daylighting requirement, and not provide any extra for
heating. Another approach is to use shading that can be
adjusted continually to provide the best compromise
between lighting costs and cooling costs during warm
weather. External shading (

Measure 8.1.1

) is most

effective, while interior shading (

Measure 8.1.2

) may

be marginally useful for daylighting.

■

Exploit Passive Solar Heating

Once you have a good understanding of the amount

of heating energy in sunlight, try to combine daylighting
and passive heating wherever possible. Both involve
using solar energy through glazing in a controlled
manner. However, there are major differences between
daylighting and passive heating. Daylighting can be
accomplished with relatively small amounts of glazing,
whereas passive heating requires large areas of glazing.
Daylighting is desirable any time that sunlight is
available, but passive heating is desirable only when
there is a heating load in the space.

During the dark hours of cold weather, the glazing

used for daylighting is a path for considerable heat loss.
Furthermore, cold weather brings many more hours of
darkness than of sunlight. It has been argued that
continuously exposed glazing collects more energy
during the day than it loses at night. In northern
locations, this is true only if the glazing is oriented
toward the sun and has exceptionally high thermal
resistance.

See

Reference Note 47

, Passive Solar Heating

Design, for details.

A Daylighting Wish List

We can make a “wish list” of developments that

would make daylighting a powerful and widely
applicable energy saving technique. These
developments would create a breakthrough, if they were
economical and free of serious practical problems:

• light pipes and other devices to move light into the

interior of the building in a visually effective
manner

• glazing materials with high thermal resistance, for

applications where heating is a major cost

1415

NOTE 46. DAYLIGHTING DESIGN

• glazing materials with opacity that is variable over

a wide range, for applications where daylighting is
combined with passive solar

• glazing that filters out the infrared component of

sunlight in a non-absorptive manner, for
applications in warm climates.

Development continues in all these areas. Check

their current status when you have an opportunity to
use daylighting.

Passive Solar Heating Design

Reference Note

System Elements
Comparison between Active and Passive Solar
Where to Consider Passive Solar Heating
Energy Saving Potential

■

Heating

■

Lighting

Primary Design Issues

■

Glazing Area

■

Glazing Location

■

Absorption, Distribution, and Control of Sunlight
Within the Building

■

Regulating Heat Input

■

Limiting Heat Loss

■

Amount of Heat Storage Mass

■

Mechanical Properties of the Storage Medium

■

Location of Heat Storage Mass

■

Control of Heat Storage Input and Output

■

Coordinate with Daylighting

■

Coordinate with Electric Lighting

■

Coordinate with Heating and Cooling Equipment

■

Longevity of Materials and Installation Methods

■

Water Leakage

■

Wind

■

Snow

■

Maintenance

■

Esthetics

■

Cost Efficiency

Passive solar heating is the direct use of sunlight

for space heating. The concept is simple, but creating a
successful installation may be complex. Passive solar
heating is not a concept for casual experimentation,
because failure is almost certain to leave a big mess. In
general, the larger the fraction of the building’s heating
that is provided by passive solar, the more complex the
design must be to avoid adverse effects.

Passive heating should include daylighting wherever

possible, since both involve the controlled intake of solar
energy through glazing. However, combining the two
is not easy.

Passive solar heating is a broad concept. The

following discussion presents the general principles. Use
them as a basis for developing specific applications.

System Components

Figure 1 shows the basic conceptual scheme of a

passive solar installation, which includes these
components:

• large glazing units, to collect the thinly concentrated

energy of sunlight

• variable shading devices, to control solar input
• removable glazing insulation, to limit heat loss

during darkness

• devices to absorb sunlight and emit heat at desired

locations within the space

• thermal storage mass, to provide heating during

periods of darkness

• shading devices to control the flow of sunlight into

the storage mass

• adjustable insulation, to control the flow of heat

from the storage mass

• light diffusion and distribution devices, to make the

incoming sunlight suitable for illumination as well
as for heating.

This description is conceptual. Probably no real

system would have all these elements. For example, it
is simpler to control the release of heat from the thermal
storage mass by exploiting the inherent time lag of the
material, rather than by using adjustable insulation.

The term “passive” implies the absence of moving

parts. However, Figure 1 shows that a passive system
may require moving parts. These are potentially the
most troublesome part of the system. These components
are not presently available as standard equipment, and
it may be difficult to fabricate them for many
installations. The future evolution of passive solar
depends largely on eliminating custom components.
This will reduce cost, simplify design, and improve
reliability.

Figure 2 shows a rationally designed passive solar

installation for a house. It makes an interesting contrast
with the comprehensive system depicted in Figure 1.

Comparison between Active and Passive Solar

Active and passive solar systems have almost

nothing in common, except for the advantage of
collecting free solar energy. Active solar systems are
primarily mechanical systems, which have architectural
ramifications. Passive solar is primarily an architectural
feature, which must be tightly integrated with the
building’s mechanical systems. Table 1 summarizes the
main areas of difference between the two.

Where to Consider Passive Solar Heating

In terms of geography, there is sufficient sunlight

for passive solar heating throughout the middle latitudes.
The coldest weather in these latitudes is associated with
the passage of cold fronts, which are followed by clear
skies. So, passive heating is often available when it is
needed the most. The value of passive solar is greatly

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11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

reduced in locations that tend to be cloudy or foggy in
winter.

Of course, solar energy is available only during the

daytime. Winter days become shorter at higher latitudes.
More northerly latitudes tend to be ineligible because
they have few hours of sunlight in winter, and because
their lower average temperatures cause greater heat loss
through the glazing.

In terms of building configuration, passive heating

requires exposure to the sun. Therefore, passive solar
requires an orientation that is generally toward the south,
or facing upward. The acceptable range of orientation
is fairly narrow. In winter, the sun rises well to the south
of due east, and sets well to the south of due west. Also,
the sun remains low in the sky all day. (

Reference

Note

, Characteristics of Sunlight, covers the geometry of

solar motion in greater detail.)

Reference Note 46

, Daylighting Design, points out

that it is difficult to get sunlight to penetrate far into the
building interior. This is generally not a limitation for
passive heating, because building heat loss occurs
through the envelope. Passive heating can be quite
effective as a perimeter heating system. However,
passive heating is limited to sides of the building
(including the roof) that face the sun.

In a building that consists of tall, open space, such

as a warehouse, the geometry of the space may allow
passive solar to heat the entire building. If sunlight can
be delivered from overhead, it is possible to provide
widespread daylighting along with heating.

Advocates of passive solar heating tend to promote

it for residential and small commercial applications.
However, passive solar heating is especially well adapted
to many industrial activities, for these reasons:

Fig. 1 Complete passive solar heating system This conceptual drawing symbolizes all the functions
that any passive heating system should have. Bear in mind that any passive heating system is also a
daylighting system, and it must perform well in both roles. In real installations, the clever designer will
combine these functions wherever possible, and will exploit the inherent features of the building, such
as heat storage mass.

1419

NOTE 47. PASSIVE SOLAR HEATING DESIGN

• they are less sensitive to passive solar’s lack of

precise control of heat gain and illumination.
Industrial work involves physical exertion, which
makes people less sensitive to small temperature
changes that would annoy sedentary workers.

• most industrial tasks are tolerant of a greater range

of illumination levels than is office work

• in industrial-type structures, it is often practical to

substitute glazing elements for conventional roof
and wall materials

• unconventional sunlight control devices are less

likely to create an appearance problem in the
industrial environment

• industrial facilities tend to have high inherent

thermal storage because of the mass of equipment
and exposed floor slabs

• availability of skilled maintenance personnel is an

important advantage for passive solar installations
that have unusual mechanisms.

The need to integrate passive solar heating with both

the external and internal design of the structure tends to
limit passive solar heating to new buildings, where

passive heating is an integral part of the design. This
being said, do not overlook the possibility of exploiting
passive solar in existing buildings, especially if
conditions, such as climate, glazing exposure, and
internal layout, are favorable.

Energy Saving Potential

■

Heating

The heating capability of sunlight is weak in

comparison with the heat losses that can occur from a
poorly insulated building in cold weather. (See

Reference Note, 24

, Characteristics of Sunlight, for the

heat content of sunlight.) Therefore, the effectiveness
of solar heating is dominated by the quality of the
building envelope. If a building has good envelope
insulation and little air leakage, passive solar may
provide over half of the heating requirement in most
eligible climates. This assumes that the building does
not have a large ventilation requirement.

Passive heating does not necessarily have to provide

a large part of total heat input to be worthwhile. In fact,
passive solar is most economical as a supplemental heat

WESINC

Fig. 2 Rationally designed passive solar installation Two large sunlight collectors are installed in the
cathedral ceiling of the living room of this house. Each has an insulated cover, one shown fully open, and the
other fully closed. Actuators to control the position of the covers are not yet installed. Before the covers were
installed, the surface area proved adequate to maintain the temperature of the space during most winter
weather, without other heat. Satisfactory temperature was maintained throughout the night, probably because
of heat absorption in the gypsum wallboard. However, the uninsulated glazing sweated profusely at night,
causing damage to the frame and floor. The lighting level is bright, but not oppressive, except when reading.
Placing the reading chairs in the shaded portion of the room solved this problem. During warm weather,
holding the covers in a slightly open position provides very pleasant daylighting with minimal cooling load. The
slope of the roof faces southeast, which is not optimum for collecting sunlight in winter. Therefore, the covers
are hinged on the right side. They are intended to track the sun so that sunlight will reflect into the space from
the white underside of the covers. The actuators, not yet installed here, are the only serious challenge. They
must hold the covers rigidly against strong wind in both directions, and they must not be too ugly for the
neighborhood. Sliding covers would have been a much easier solution if the roof had faced toward the south.

1420

11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

1421

NOTE 47. PASSIVE SOLAR HEATING DESIGN

source. Probably the best approach is to add as much
passive solar capacity as possible without requiring
elaborate and expensive features.

■

Lighting

See

Reference Note 46

about the energy saving

potential of daylighting. If cleverly designed, a passive
heating system can provide as much illumination as a
system designed primarily for daylighting.

Primary Design Issues

Success with passive solar requires attention to an

array of considerations that initially appears bewildering.
As with any complex matter, the best approach is to
identify all the elements, attack them individually, and
then figure out a clever way to combine them. The
following are the major issues of passive solar design.

■

Glazing Area

Because of the low energy density of sunlight,

passive heating requires large glazing areas, unless the
climate is mild. There is a trend of diminishing returns
as glazing area is increased. That is, adding more glazing
results in a greater number of hours per year when
heating capacity exceeds need.

Daylighting requires much less glazing area than

passive heating. To avoid excessive brightness, convert
most of the sunlight to heat before it is seen by the
occupants.

■

Glazing Location

Heat is released into the space at points where the

sunlight is absorbed, rather than where it enters.
Therefore, consider the location of glazing in relation
to the locations of the heat absorbers and the heat storage
masses. You may have a great deal of flexibility in
making these arrangements. Furthermore, as with
conventional heaters, convection can be exploited to
distribute heat through the space. You need to tailor the
location of glazing more carefully for daylighting than
for heating.

Sunlight absorbers, which may consist of nothing

more than dark pieces of cloth, can easily be moved to
accommodate the glazing. However, the geometrical
relationship between the glazing and the heat storage
masses is fixed. For example, if you use a masonry
wall as a heat storage element, locate the glazing so that
sunlight falls primarily on the wall.

■

Absorption, Distribution, and Control of Sunlight
Within the Building

Many attempts at passive solar failed because they

simply dumped sunlight into the space without regard
to consequences. Each location in the space where
sunlight is absorbed acts as a heating unit, so these
locations must be planned. For example, painting a sunlit
wall in a dark color causes heat and light to be absorbed
there, whereas painting the wall in a light color causes
heat and light to reflect throughout the space.

People cannot work at most indoor tasks in full

sunlight. Also, strong sunlight eventually destroys
organic materials, such as upholstery, wallpaper, etc.
Therefore, provide shading for sensitive areas. For
example, install diffusing screens over individual work
areas.

Activities within the building may move repeatedly,

so design the passive heating system to adapt easily to
changing activities.

■

Regulating Heat Input

One of the crippling flaws of passive solar has been

failure to provide effective methods of blocking excess
sunlight. The most efficient approach is to stop the
sunlight outside the building with exterior shading. This
may be difficult. External shading devices are large,
they must be monumentally strong to resist wind forces
and snow loads, and they require mechanisms that are
cumbersome and unattractive. In addition to being an
engineering challenge, they radically change the
appearance of the building.

A less efficient method of controlling excess heat

gain is to vent warm air from the space. This method
generally is limited to spaces with tall or sloped ceilings.
Relying on vents poses the risk of serious heat loss
through convective leakage. Vent dampers are the kind
of equipment that tends to be forgotten, so they are not
operated or maintained properly.

Glazing with controllable opacity may come to

market at an acceptable price. This would be a major
advance for passive solar, because it would allow control
of heat input without the external apparatus.

■

Limiting Heat Loss

Another major problem with passive heating is the

high conductive heat loss of glazing, especially during
hours of darkness, when there is no heat input and
outside temperatures are lowest.

With skylights and other glazing that is slanted, the

problem is especially severe. ASHRAE data indicate
that multiple-glazed units have two to three times more
conductive heat loss when they are installed in a heavily
slanted orientation than when they are installed in a
vertical orientation. In addition, slanted glazing requires
more frame structure and stiffeners to resist sagging,
and these components conduct heat.

The low thermal resistance of glazing, especially

of skylights, allows the inside surfaces to get cold at
night, causing condensation problems. In facilities that
have humidification or other moisture sources, skylights
may sweat prodigiously. The sweating is unsightly, it
grows mildew, and it destroys wooden or steel framing.
The condensation may be copious enough to drip on
the space below, causing moisture damage inside the
space. Condensation usually is not serious during
daylight hours, because enough solar heat is absorbed

1422

11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

by the glazing to keep it above the dew point of the
inside air.

The large, cold surfaces of bare glazing may create

discomfort at night, especially if people are close to the
glazing.

The heat loss problem was recognized early in the

history of passive solar, although it probably was
underestimated. Many concepts were devised to add
insulation to the glazing during hours of darkness. This
conceptual class of insulation has been given a variety
of names, including “movable insulation” and “thermal
shutters.” Unfortunately, all the methods that have been
popularized so far have serious practical problems.

At first glance, the easy approach seems to be

installing movable insulation inside the glazing. In fact,
many types of internal insulation have been tried. These
included various types of quilted shades, movable panels
of various designs, and insulating shutters.
Unfortunately, all these methods fall afoul of
condensation problems. Interior insulation keeps the
glazing at outside temperature. Moisture infiltrates past
the insulation and condenses on the glazing. The
insulation traps the moisture against the glazing and its
surrounding structure, promoting mildew, rot, and rust.

Another approach is installing movable insulation

on the outside of the building. This approach avoids
condensation problems. It also protects the glazing from
hail, snow, etc. However, the insulating panels must be
as large as the glazing. Large external movable panels
are an engineering headache. They must be able to
withstand wind, they must be designed to prevent air
leakage between the panels and the glazing, and they
must not look too bizarre for the neighborhood.

A third approach is installing movable insulation

inside the glazing, between the panes. For example,
one briefly popular concept was blowing foam beads
into the space between the panes overnight. This method
fell out of favor because the beads stick to the glazing
from electrostatic attraction. It is a pity that this did not
work. Installing movable insulation between the panes
avoids condensation problems, provided that the space
is vented to the outside. Adjustable insulation inside
glazing units merits more development effort.

Since you need adjustable shading to control heat

gains, try to design the exterior movable insulation to
act as an adjustable shading device. Needless to say,
this involves additional complications.

The oppressive need for movable insulation would

disappear entirely if glazing were available for skylights
that has a high thermal resistance. Depending on
climate, a minimum R-value between 6 and 15 would
be sufficient to eliminate the need for movable
insulation. For passive heating and daylighting, the
glazing would not have to be transparent, only
translucent, with a reasonably high light transmission.
Such glazing may become available within the
foreseeable future.

■

Amount of Heat Storage Mass

Heat storage is a necessary part of almost any

passive heating system, because the sun does not shine
continuously. With passive systems, storage occurs by
the absorption of sunlight in mass. Storage capacity is
determined primarily by the amount of mass that is
exposed to direct sunlight. Ideally, enough solar energy
should be absorbed in the storage mass during the
daytime to carry through the hours of darkness.

Buildings are heavy, so a large heat storage potential

exists in the building structure. For example, a typical
office space may contain several tons of gypsum board
that can serve as an effective thermal storage medium if
it receives sufficient exposure to sunlight. Concrete
floors and masonry walls often have enough mass to
provide all the thermal storage mass that may be desired,
provided that it is exposed to sunlight. Heavy machinery
has a significant amount of heat storage capacity. The
clever designer will exploit the mass of the building and
its contents as much as possible.

The storage effectiveness of mass is reduced if it is

covered by insulating materials, such as carpets, and
wall finishes installed over furring strips.

In new construction, the heat absorbing capacity of

massive components, such as floor slabs, often can be
increased inexpensively by adding more material.

The usual candidate material for thermal storage

mass is some type of masonry, such as concrete, brick,
stone, tile, etc. Water also has been used. The weight
of material required for storage is depends on the
material’s specific heat. (Specific heat is the heat
capacity per unit of weight, in comparison with the heat
capacity of water.) The specific heat of water is 1.0, of
concrete and most masonry products is between 0.20
and 0.27, of steel is about 0.12.

The volume of the thermal storage mass depends

on both the specific heat and the specific gravity of the
material. (Specific gravity is the density of a material
in comparison with the density of water.) The specific
gravity of water is 1.0, of concrete and most construction
stone is between 2.0 and 3.0, of steel is about 7.7.

If you multiply the specific gravity of each of these

materials by its specific heat, you get the heat storage
capacity per unit of volume. By coincidence, water and
most bulk construction materials have about the same
heat storage capacity on a volumetric basis.

■

Mechanical Properties of the Storage Medium

The heat storage mass is subject to thermal

expansion and contraction. Centuries of experience have
taught designers how to deal with this in common
structural materials.

Masonry tolerates expansion well, but it must be

kept loaded in compression, like the bricks in a wall.
Tiles cemented to a surface are likely to break loose
because of differences in thermal expansion between
the tile and the masonry behind it.

1423

NOTE 47. PASSIVE SOLAR HEATING DESIGN

Water is cheap. It has exceptionally high specific

heat, which reduces structural loading, but it cannot serve
as a structural element itself. It has no thermal lag.
Convective currents in water prevent thermal lag, and
also cause vertical temperature stratification. If water
is stored in a transparent container, a dye should be added
to it to absorb sunlight.

■

Location of Heat Storage Mass

When masses are releasing heat, they act as huge,

low-temperature radiators, which provide comfortable
heating throughout a large area. Their location tends
not to be critical.

As with any heating system, it is desirable to release

the heat near the envelope, to offset the envelope heat
losses. From this standpoint, for example, it is better to
absorb sunlight in a floor slab close to the exterior wall
than to absorb sunlight in an interior wall.

■

Control of Heat Storage Input and Output

Using heat storage efficiently involves three factors:

the rate of heat absorption, the rate of heat release, and
the timing of heat storage and release. Fortunately, in
keeping with the concept of a “passive” system, it is
often possible to design these factors into the system
without resorting to devices that need to be controlled.

The rate of heat absorption is determined by the

surface area exposed to sunlight and by the absorptance
of the surface. Mass is effective for heat storage only if
it is directly illuminated by sunlight. Therefore, the
relative placement of the glazing and storage mass is
critical. The motion of the sun causes different parts of
the interior to be illuminated throughout the day, which
may be advantageous. Warming the space in the
morning can be accelerated, at the expense of delaying
heat storage, by using internal heat absorbing screens
to shade the mass.

The absorptance of the storage mass is determined

entirely by its surface. In general, dark colors absorb
the most sunlight. “Color” indicates absorptance only
in the visible portion of the solar spectrum, which
accounts for only about 35% of total solar energy.
Absorptance in the infrared portion of sunlight is more
difficult to determine. Refer to

Measure 8.2.2

for

methods of determining the absorptance of particular
materials.

The rate of heat output from the storage mass is

determined by the surface area and by the emittance of
the surface. The emittance of most solid materials used
for heat storage is about 0.8, which is satisfactory for
the purpose. There is generally no need to tinker with
emittance. (Refer to

Measure 8.2.2

if you want to learn

more about it.)

The timing of heat release in most passive solar

systems depends on the thermal lag of the storage mass.
Thermal lag is a delay in the release of heat from a mass
after the heat has been absorbed. Using thermal lag to

control the timing of heat release is usually the preferred
method. It is not precise, but it minimizes the need for
maintenance or active control.

Thermal lag in a material results from the interplay

of its thermal conductivity, heat capacity, and geometry.
As heat is absorbed by the sunlit surface of a material,
the temperature of the surface is raised, and the rise in
temperature forces the heat deeper into the material.
When the surface is no longer illuminated, the process
reverses. The surface emits heat and becomes cooler,
which creates a flow of heat toward the surface. Of
course, this is not an on-and-off process. Heat is
continuously emitted from the surface, whether it is
sunlit or not. As the space cools at night, the increased
temperature differential across the surface draws more
heat out of the mass.

The thermal lag is much longer and more distinct if

the storage mass is heated by the sun on one side and
the space being heated is located on the other side. In
such cases, there is a distinct peak in heat emission into
the space that may occur many hours after sunset.

Calculating thermal lag is somewhat complex. It

can be done using some of the more sophisticated energy
analysis computer programs. The U.S. National Institute
of Standards and Technology has done much of the work
in calculating thermal lag in buildings. They offer
guidance in this subject.

Water has virtually no thermal lag, because

convection keeps transferring heat to the outer surface
of the container. The thermal lag of metals is minimal
because of their high thermal conductivity.

In some cases, it may be desirable to use adjustable

insulation to bottle up heat within the storage mass until
needed.

■

Coordinate with Daylighting

Try to exploit daylighting if you use passive heating,

since the sunlight is coming into the space anyway. By
the same token, you need to consider lighting conditions
in a passive solar installation to avoid intolerable
brightness and glare.

The methods you use to control sunlight for

illumination are quite different from the methods you
use for passive heating. Daylighting is desirable any
time that sunlight is available, but passive heating is
desirable only when there is a heating load in the
building. Illumination requires much less glazing area
than passive heating, but more careful distribution of
sunlight. Daylighting requires still more auxiliary
devices, such as light diffusers.

■

Coordinate with Electric Lighting

Refer to

Subsection 9.5

for methods of controlling

electric lighting to exploit daylighting.

■

Coordinate with Heating and Cooling Equipment

To avoid wasting heating and cooling energy, be

sure to design the thermostatic controls of the

1424

11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

conditioning equipment so that they do not fight the
passive heating system. Passive heating results in swings
of temperature. If the passive system is designed
properly, the temperature swings remain small enough
to avoid discomfort. Design the thermostatic controls
to keep heating and cooling equipment turned off as
long as temperatures remain within acceptable limits.
This is called “deadband.” See

Measure 4.3.4.2

for

details.

■

Longevity of Materials and Installation Methods

Your choice of the materials and installation

methods has a major effect on longevity and
maintenance requirements. There is a strong temptation,
when buying large expanses of glazing, to use short-
lived materials to reduce cost. Your successors will curse
you for this if you succumb. If you cannot afford the
right materials, forget about passive solar. See

Measure

8.3.2

about selecting materials for longevity.

■

Water Leakage

Large expanses of glazing tend to be vulnerable to

water leakage, especially if the glazing is non-vertical.
See

Measure 8.3.2

■

Wind

Shading devices and movable insulation for passive

solar systems have a large amount of surface area, which
makes it important to design them strongly. In many
applications, wind is the greatest impediment to using
external devices.

■

Snow

Fortunately, skylights tend to shed snow, provided

that they have even a modest slope. Heat loss through
the glazing, combined with the insulating property of
snow itself, causes the bottom layer of snow to melt and
slide off the smooth surface of the glazing. Also, sunlight
penetrates snow and warms the surface underneath.

Nonetheless, snow can be very heavy. Skylights

should be designed to resist the weight of an overnight
wet snowfall. If insulating covers are used, they can be
designed to carry the snow load. This requires reliable
controls that respond automatically to snowfall. Snow
melts and turns to ice, so external mechanisms must be
designed to avoid jamming by ice.

■

Maintenance

In theory, passive solar systems should require

minimal maintenance. Good design places priority on
achieving this ideal. If mechanisms are necessary, such
as movable shading devices and thermal shutters, design
these for ruggedness and easy maintenance.

■

Esthetics

Daylighting and passive solar heating have a major

effect on both the exterior appearance and internal
layout. Blending these elements into the design of a
building requires imagination and a fine esthetic sense.
These qualities are not prevalent in contemporary
architecture, and many buildings heated by passive solar
are lovely only to their designers. Sadly, many attempts
at passive solar design have been so ugly that they
degrade the value of the building. The only solution is
for owners to be aware of this potential problem, and to
cast a critical eye on the esthetic aspects of the design.

■

Cost Efficiency

The clever designer will attempt to satisfy all the

functions symbolized in Figure 1, while minimizing the
hardware and complexity required. For example, it may
be possible to dispense with specific thermal storage
devices if the mass of the building is exploited effectively
for thermal storage. Considerable thermal storage may
be added cheaply by increasing the quantities of
inexpensive masonry materials in floor slabs and walls.

In new construction, it may be possible to minimize

materials costs by substituting glazing for other roof
and wall surfaces.

Document Outline

Front Matter
Keys to the Measures, Ratings, and Selection Scorecard
How to Use the Energy Efficiency Manual
- A Personal Note: The Right Way to Do Energy Conservation
- Expression of Gratitude
Table of Contents
Section 1: Boiler Plant
Section 2: Chiller Plant
Section 3: Service Water Systems
Section 4: Air Handling Systems
Section 5: Room Conditioning Units & Self-Contained HVAC Equipment
Section 6: Building Air Leakage
Section 7: Building Insulation
Section 8: Control and Use of Sunlight
Section 9: Artificial Lighting
Section 10: Independent Energy-Using Components
Section 11: Reference Notes
Group 1. Energy Management Tools
- 10. Clock Controls and Programmable Thermostats
- 11. Personnel Sensors
- 12. Placards
- 13. Energy Management Control Systems
- 14. Control Signal Polling
- 15. Infrared Thermal Scanning
- 16. Measurement of Liquid, Gas, and Heat Flow
- 17. Energy Analysis Computer Programs
Group 2. Energy Sources
- 20. Fossil Fuels
- 21. Electricity Pricing
- 22. Low-Temperature Heat Sources & Heat Sinks for Heat Pumps and Cooling Equipment
- 23. Non-Fossil Energy Sources
- 24. Characteristics of Sunlight
Group 3. Mechanical Equipment
- 30. Boiler Types and Ratings
- 31. How Cooling Efficiency is Expressed
- 32. Compression Cooling
- 33. Absorption Cooling
- 34. Refrigerants
- 35. Centrifugal Pumps
- 36. Variable-Speed Motors and Drives
- 37. Control Characteristics
Group 4. Building Envelope
- 40. Building Air Leakage
- 41. How Insulation Works
- 42. Vapor Barriers
- 43. Insulation Selection
- 44. Insulation Integrity
- 45. Insulation Economics
- 46. Daylighting Design
- 47. Passive Solar Heating Design
Group 5. Lighting
- 50. Measuring Light Intensity
- 51. Factors in Lighting Quality
- 52. Comparative Light Source Characteristics
- 53. Lighting Efficiency Standards
- 54. Incandescent Lighting
- 55. Fluorescent Lighting
- 56. H. I. D. and L. P. S. Lighting
- 57. Light Distribution Patterns of Fixtures
Index

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