What’s New Selecting Windows
in Building for Energy Efficiency
New window technologies have increased energy
Efficiency benefits and comfort, and have provided more practical
options for consumers. This selection guide will help
homeowners, architects, and builders take advantage of
the expanding window market. The guide contains three
sections: an explanation of energy-related window
characteristics, a discussion of window energy
U.S. Department of Energy
performance ratings, and a convenient checklist for
electing the right window for a unintentional (infiltration). (See the
S specific home invariably
requires tradeoffs between dif-
ferent energy performance features,
Window Energy Glossary for expla-
nations of these terms.)
and with other non-energy issues. An Insulating Value
INSIDE understanding of some basic energy The non-solar heat flow through a
concepts is therefore essential to window is a result of the temperature
Solar Control 6 choosing appropriate windows and difference between the indoors and
skylights. As illustrated on the fol- outdoors. Windows lose heat to the
Window Energy lowing page, three major types of outside during the heating season and
Rating and Labeling 10 energy flow occur through windows: gain heat from the outside during the
(1) non-solar heat losses and gains in cooling season, adding to the energy
Window Checklist 13 the form of conduction, convection, needs in a home. The effects of non-
and radiation; (2) solar heat gains in solar heat flow are generally greater
Window Energy the form of radiation; and (3) airflow, on heating needs than on cooling
Glossary 15 both intentional (ventilation) and needs because indoor-outdoor tem-
perature differences are greater dur-
ing the heating season than during
the cooling season in most regions of
the United States. For any window
product, the greater the temperature
difference from inside to out, the
greater the rate of heat flow.
A U-factor is a measure of the rate
of non-solar heat flow through a win-
dow or skylight. (An R-value is a
measure of the resistance of a win-
dow or skylight to heat flow and is
the reciprocal of a U-factor.) Lower
U-factors (or higher R values), thus
indicate reduced heat flow. U-factors
allow consumers to compare the
insulating properties of different win-
dows and skylights.
The insulating value of a single-
pane window is due mainly to the
The three major types of
energy flow that occur
through windows: (1) non-
solar heat losses and gains in
the form of conduction, con -
vection, and radiation; (2)
solar heat gains in the form of
radiation; and (3) airflow, both
intentional (ventilation) and
thin films of still air on the interior and nologies aimed at decreasing U-factors.
moving air on the exterior glazing sur- These technologies include low-emittance
faces. The glazing itself doesn’t offer (low-E) coatings and gas fills.
much resistance to heat flow. Additional A low-E coating is a microscopically
panes markedly reduce the U-factor by thin, virtually invisible, metal or metallic
creating still air spaces, which increase oxide coating deposited on a glazing sur-
insulating value. face. The coating may be applied to one
In addition to conventional double-pane or more of the glazing surfaces facing an
windows, many manufacturers offer win- air space in a multiple-pane window, or to
dows that incorporate relatively new tech- a thin plastic film inserted between panes.
The coating limits radiative heat flow
between panes by reflecting heat back
into the home during cold weather and
back to the outdoors during warm weath-
er. This effect increases the insulating
value of the window. Most window man-
ufacturers now offer windows and sky-
lights with low-E coatings.
The spaces between windowpanes can
be filled with gases that insulate better
than air. Argon, krypton, sulfur hexafluo-
ride, and carbon dioxide are among the
High-performance windows make energy-
efficient homes possible with greater
freedom of design than in the past.
gases used for this purpose. Gas fills add nounced influence on the U-factors of
only a few dollars to the prices of most windows and skylights. As a result, frame
windows and skylights. They are most and spacer options have also multiplied as
effective when used in conjunction with manufacturers offer improved designs.
low-E coatings. For these reasons, some Window frames can be made of alu-
manufacturers have made gas fills stan- minum, steel, wood, vinyl, fiberglass, or
dard in their low-E windows and sky- composites of these materials. Wood,
lights. fiberglass, and vinyl frames are better
The insulating value of an entire win- insulators than metal. Some aluminum
dow can be very different from that of the frames are designed with internal thermal
glazing alone. The whole-window U-fac- breaks, non-metal components that reduce
tor includes the effects of the glazing, the heat flow through the frame. These ther-
frame, and, if present, the insulating glass mally broken aluminum frames can resist
spacer. (The spacer is the component in a heat flow considerably better than alu-
window that separates glazing panes. It minum frames without thermal breaks.
often reduces the insulating value at the Composite frames may use two or more
glazing edges.) materials (e.g. aluminum-clad wood,
Since a single-pane window with a vinyl-clad wood) to optimize their design
metal frame has about the same overall U- and performance, and typically have insu-
factor as a single glass pane alone, frame lating values intermediate between those
and glazing edge effects were not of great of the materials comprising them. Frame
concern before multiple-pane, low-E, and geometry, as well as material type, also
gas-filled windows and skylights were strongly influences thermal performance
widely used. With the recent expansion of properties.
thermally improved glazing options Spacers can be made of aluminum,
offered by manufacturers, frame and spac- steel, fiberglass, foam, or combinations of
er properties now can have a more pro- these materials. Spacer thermal perfor-
Aluminum Frame without thermal break (with conventional spacer)
Alum. Frame with thermal break (with conventional spacer)
Wood or Vinyl Frame (with insulated spacer)
GLAZING TYPE U-FACTOR (Btu/hr-ft2-˚F)
Single glass ----- 1.07 1.30
1/2-inch air space 0.48 0.62 0.81
Double glass, e = 0.20*,
1/2-inch air space 0.39 0.52 0.70
Double glass, e = 0.10*, Representative Window
1/2-inch air space 0.37 0.49 0.67 U-Factors
*e is the emittance of the
Double glass, e = 0.10*, low-E coated surface.
1/2-inch argon space 0.34 0.46 0.64 Values are for 3-foot-by-5-
foot windows. U-factors vary
Triple glass, e = 0.10 on two somewhat with window size.
panes*, 1/2-inch argon spaces 0.23 0.36 0.53
Source: ASHRAE Handbook—
Quadruple glass, Fundamentals, American Society of
e = 0.10 on two panes*, Heating, Refrigerating, and Air-
Conditioning Engineers, Inc., Atlanta,
1/4-inch krypton spaces 0.22 ----- ----- GA, 1993.
Outdoor air temperature and indoor air rela-
tive humidity combinations at which conden-
sation will occur on the center of the glass for
single, double, and triple glazings, some with
low-E coatings and gas fills.
On or above each curve, the conditions are right
for condensation. Below each curve, condensa -
tion will not occur on that glazing type as long
as the glazing is exposed to room air circula -
tion. Values are based on winter conditions:
70˚F indoor air temperature, 15 mph outdoor
air velocity, and no incident solar radiation.
Source: WINDOW 4.1 (a computer program for calculating
the thermal and optical properties of windows), Lawrence
Berkeley National Laboratory, Berkeley, CA, 1994.
mance is as much a function of geometry Preventing Condensation
as of composition. For example, some Air can hold varying amounts of water
well-designed metal spacers insulate vapor or moisture. The warmer the air is,
almost as well as foam. the more moisture it can hold. The
The table on page 3 shows representa- amount of moisture in the air, expressed
tive U-factors for window glazing, frame, as a percentage of the maximum amount
and spacer combinations under winter the air could hold at a given temperature,
design conditions. Due to their orientation is called its relative humidity. For health
and their greater projected surface areas, and comfort, indoor air should contain
domed and other shaped tilted and hori- some moisture. The relative humidity
zontal skylights have significantly higher should generally be between 30% and
U-factors than do vertical windows of 40% at normal room temperature.
similar materials and opening sizes. The relative humidity of air can be
increased by adding more moisture or by
reducing the temperature. When the rela-
tive humidity reaches 100%, the air can
hold no more moisture, and water begins
to condense from it. The temperature at
which this condensation occurs is called
the dew point temperature of the air.
When moist air comes in contact with a
cold surface in a home, it may be cooled
to its dew point temperature, resulting in
condensation on the surface.
Single-glazed windows characteristical -
ly suffer from water condensation prob -
lems and the formation of frost on the
inside surface of the glass in winter.
Windows don’t cause condensation, but Recommendations for Selecting
historically they have been the first and Window U-Factors
most obvious place it occurs. This is When shopping for windows and sky-
because windows generally have lower lights, pay close attention to whether the
thermal resistances than insulated walls, U-factor listed by the manufacturer
ceilings, and floors. As a result, their applies to the glazing only or to the entire
inside temperatures are usually lower than unit. If it is for the glazing only, the over-
those of other surfaces in a home during all U-factor may be considerably higher
cold weather. If the air in a home is humid because of the frame and spacer effects.
enough, water will condense from it when These effects increase with decreasing
it is cooled at a window surface. total window area. Compare different
Condensation is most often thought of as window types or makes by their total U-
a cold climate winter problem. However, factors, which are best obtained from
in hot, humid weather, moisture can con- NFRC labels. (See Window Energy
dense on the outside surface of a poorly Rating and Labeling, page 10.) New win-
insulated window in an air-conditioned dow energy ratings and the RESFEN
building. computer program can be used to estimate
Left unchecked, condensation can dam- the relative energy usage associated with a
age window frames, sills, and interior particular window type and U-factor.
shades. Water can deteriorate the sur- Avoid aluminum-frame windows with-
rounding paint, wallpaper, plasterboard, out thermal breaks if possible. Even in
and furnishings. In severe cases, it can milder climates, these windows tend to
seep into adjoining walls, causing damage have low inside surface temperatures dur-
to the insulation and framing. ing the heating season, giving rise to con-
The indoor air coming in contact with densation problems. Aluminum-frame
energy-efficient windows is less likely to windows with properly designed thermal
be cooled to its dew point temperature breaks can be used in moderate climates.
because the inside surface temperatures Wood, vinyl, and fiberglass are the best
remain higher during cold weather than do frame materials for maximum insulating
those of windows with single glazing, tra- value.
ditional metal spacers, and metal frames. Single-pane windows are impractical in
The figure on page 4 illustrates condi- heating-dominated climates. In these
tions under which condensation will form regions, multiple-pane, low-E, and gas-
on the center of the glass of five glazing filled window configurations are advis-
types with widely varied U-factors. The able. In most climates, glazings with low-
graph shows clearly that the risk of con- E coatings and gas fills will be a choice
densation at the center of the glass is that provides significant energy savings in
reduced as the insulating value of the a cost-effective product. Low-E and gas
glass increases. Even at an outdoor air fills have now become a common option
temperature of -30˚F, the indoor air rela- for many manufacturers, which reduces
tive humidity must be nearly 70% before their added cost. The resultant total win-
condensation will form on the triple glaz- dow U-factor should be 0.5 or lower and
ing with two low-E coatings and a gas fill. preferably below 0.4 for maximum energy
On the other hand, at an outdoor tempera- savings. Consumers should select win-
ture of 10˚F, condensation will form on dows with long warranty periods, which
the single glazing at an indoor relative indicate sound window design and con-
humidity of only 18%. struction, and a reduced probability of
Condensation is even more likely to insulating glass seal failure or gas leak-
occur at window spacers and frames, age, which would reduce performance.
which are usually less insulating than the Remember that lower window and sky-
corresponding glazings. With so many light U-factors mean less energy con-
insulating glazing types available, efforts sumption, lower utility bills, and greater
to prevent condensation have shifted comfort in the living space.
toward the development of better insulat-
ing spacers and frames.
Daily total solar heat gain
through 1/8-inch clear sin -
gle glass for various win -
dow orientations on very
clear days at 40˚N lati -
tude (for example,
Columbus, Ohio, and
Source: ASHRAE Handbook—
Fundamentals, American Society
of Heating, Refrigerating, and Air-
Conditioning Engineers, Inc.,
Atlanta, GA, 1993.
Solar Control 40˚N latitude. South-facing windows
Solar transmission through windows and allow the greatest and potentially most
skylights can provide free heating during beneficial solar heat gain during the heat-
the heating season, but it can cause a ing season, while admitting relatively lit-
home to overheat during the cooling sea- tle of the solar heat that contributes to
son. Depending upon orientation, shading cooling requirements during the cooling
and climate, solar-induced cooling costs season. The reverse is true for skylights
can be greater than heating benefits in and east- and west-facing windows. North
many regions of the United States. In fact, exposures transmit only minimal solar
solar transmission through windows and heat at any time. The ultimate importance
skylights may account for 30% or more of of these climatic and orientation effects
the cooling requirements in a residence in will depend on the type of glazing under
some climates. consideration.
Because the sun’s position in the sky The Solar Heat Gain Coefficient
changes throughout the day and from one (SHGC) is a measure of the rate of solar
season to another, window orientation has heat flowing through a window or sky-
a strong bearing on solar heat gain. The light. (AShading Coefficient (SC) is the
figure above shows the solar heat gain previous standard indicator of a window’s
through 1/8-inch clear single glass for var- shading ability and for simple glazings is
ious window orientations on very clear approximately equal to the solar heat gain
days in the heating and cooling seasons at coefficient multiplied by 1.15.) Solar heat
gain coefficients allow consumers to com-
pare the solar heat gain properties of dif-
ferent windows and skylights. The solar
heat gain coefficient accounts for both the
transmissive glazing element, as well as
the opaque frame and sash.
Additional glazing layers provide more
barriers to solar radiation, thus reducing
the solar heat gain coefficient of a win-
dow. Tinted glazings, such as bronze and
green, provide lower solar heat gain coef-
The NFRC label provides useful informa -
tion—such as the window’s U-Factor,
Solar Heat Gain Coefficient, Visible
Light Transmittance, and Air Leakage
rating—to help you choose wisely.
ficients than does clear glass. Low-E coat- or space. Mirror-like reflective glazings
ings can be engineered to reduce window are commonly used in office buildings,
solar heat gain coefficients by rejecting but only occasionally chosen for resi-
more of the incident solar radiation. dences. While they may have very low
Spectrally selective glazings, including solar heat gain coefficients, they block so
some low-E coated glazings with low much of the light and view that they are
solar heat gain coefficients and new light not normally desirable in homes.
blue and light blue-green tinted glazings, The table below shows representative
block out much of the sun’s heat while solar heat gain coefficients and visible
maintaining higher visible transmittances transmittances for glazings with typical
and more neutral colors than more heavily wood or vinyl frames and aluminum
tinted bronze and grey glazings. High- spacers. Aluminum-frame windows of
transmittance, low-E coatings, used in comparable size and glazing type general-
conjunction with a tinted outer glass layer, ly have slightly higher solar heat gain
also reduce solar heat gain by preventing coefficients because of their thinner
the absorbed heat from reaching the interi- frames and greater glazing areas.
Solar Heat Gain Coefficient
glass total glass total
GLAZING TYPE only window only window
Single glass, clear 0.90 0.66 0.86 0.66
Single glass, bronze tint 0.68 0.50 0.73 0.56
Single glass, green tint 0.82 0.60 0.71 0.55
Single glass, clear, solar control 0.25 0.18 0.40 0.33
Double glass, clear, 1/2-inch air space 0.81 0.59 0.76 0.59
Double glass,bronze tint 0.62 0.45 0.62 0.49 Representative Window
outer pane, 1/2-inch air space Solar Heat Gain
Coefficients and Visible
Double glass, green tint 0.75 0.54 0.60 0.47 Transmittances
outer pane, 1/2-inch air space Values are for a 3-foot-by-
5-foot horizontal slider
Double glass, clear, 0.76 0.55 0.65 0.51 window with wood or vinyl
e = 0.20*, 1/2-inch air space frames and aluminum
spacers. Solar heat gain
Double glass, 0.45 0.33 0.35 0.29 coefficients vary somewhat
“Southern” low-E, e=0.08*, with window size.
on tint, 1/2 inch air space
*e is the emittance of the low-
E coated surface; emittance
Double glass, spectrally selective, 0.72 0.52 0.40 0.33 will vary slightly with specific
e = 0.04*, 1/2-inch argon space products.
Triple glass, clear, 0.68 0.50 0.49 0.39 Source: WINDOW 4.1 (a computer
e = 0.08 on two panes*, program for calculating the ther -
mal and optical properties of win -
3/8 to 1/2-inch air or argon spaces dows), Lawrence Berkeley National
Laboratory, Berkeley, CA, 1994.
Ultraviolet Protection tion levels at that time on south-facing
Ultraviolet radiation is the main compo- windows, especially those with adequate
nent of sunlight that can fade and damage roof overhangs.
drapes, carpets, furniture, and paintings Skylights and east- and west-oriented
when transmitted through windows and windows may warrant lower solar heat
skylights. Efforts to produce window gain coefficients since they transmit the
glazings that transmit less ultraviolet most solar heat during cooling periods. In
energy have met with some success. In most climates, there is not much point in
general, windows and skylights with plas- spending more money to obtain lower
tic glazing layers or low-E coatings solar heat gain coefficients for north-fac-
reduce ultraviolet transmission. Even ing windows.
without any ultraviolet radiation, sunlight In hot, sunny climates, select windows
can still cause fading of fabrics and other with spectrally selective glazings to pro-
furnishings. vide low solar heat gain coefficients with-
out loss of light. Darker tinted glazings
Recommendations for also provide lower solar heat gain coeffi-
Solar Control cients, but they will yield somewhat
decreased outdoor visibility, particularly
Consumers should consider two aspects at night. Where glare is a concern, this
of window selection to control solar effect may be desired, but under other
gain—the selection of the window itself conditions it may not. In climates where
and the choice of interior or exterior cooling loads are large, look for windows
shading devices. Traditional windows with a SHGC of 0.4 or less. To maintain
with clear glass required the use of shad- good light transmittance and visibility,
ing devices to obtain adequate perfor- select windows whose glazings have visi-
mance, especially when the orientation ble transmittance of 0.6 or higher.
admitted substantial sunlight in summer. In some hot climates, where winters
However, modern high-performance win- are mild, it might seem reasonable to
dows can do such a good job of control- select a single-glazed window with a low
ling sunlight that the importance of these Solar Heat Gain Coefficient rather than a
shading systems is reduced. more typical double glazing. However,
Window Solar Heat Gain Coefficients single glazings have a more limited range
should ideally be selected according to of solar control (even if laminated glass
orientation, but it may not always be and glue-on plastic films are considered),
practical to do so. If south exposures are so a double-glazed window with appro-
to admit beneficial solar heat during the priate glazing choice as noted above, may
heating season, their Solar Heat Gain be the best overall solution, even in hot
Coefficients should be high. These high climates.
coefficients will not usually result in Exterior or interior shading devices—
overheating problems during the cooling such as awnings, louvered screens, sun-
season because of the lower solar radia- screens, venetian blinds, roller shades,
and drapes—are essential for shading
clear glass, and can complement and
enhance the performance of windows
with low Solar Heat Gain Coefficients.
One advantage of many shading devices
is that they can be adjusted to vary solar
heat transmission with the time of day
and season. But windows with “built-in”
lower Solar Heat Gain Coefficients pro-
Commercially available windows must
meet numerous standards and build -
ing regulations addressing condensa -
tion resistance, sound control, main -
tenance requirements, and overall
durability of the unit.
vide better visibility and require less man- and skylight frames, sash, and glazings.
agement and maintenance in today’s busy This leakage can account for up to 10%
households. of the energy usage in a home. The air-
Exterior shading devices are more tightness of a window depends on both
effective than interior devices in reducing the characteristics of the window—such
solar heat gain because they block radia- as sash type and overall quality of win-
tion before it passes through a window. dow construction—and the quality of the
Light-colored shades are preferable to installation. Operable windows with com-
dark ones because they reflect more, and pressing seals are generally more airtight
absorb less, radiation. Horizontally orient- than purely sliding seals, because of the
ed adjustable shading devices are appro- way the sash element seals against the
priate for south-facing windows, while framing.
vertically oriented adjustable devices are An air leakage rating is a standardized
more effective for shading windows on measure of the rate of infiltration through
east and west orientations. a window or skylight under specific envi-
ronmental conditions. Air leakage ratings
allow consumers to compare the airtight-
ness of different windows and skylights
Sash Type Effective as manufactured products; they do not
Open Area* account for leakage between the installed
product and the wall or roof. Lower air
Casement 90% leakage ratings indicate greater airtight-
Awning 75% ness.
Horizontal sliding 45% Airflow Recommendations
Single-hung 45% In milder climates, or in spring and fall in
Double-hung 45% more severe climates, operable windows
can provide ventilation, improve comfort,
Representative Window Ventilation Areas and reduce the need for air conditioning.
*Effects of window screens are not Operable windows are often specified to
included. meet building code requirements for
emergency egress. Although operable
Source: R.K. Vieira and K.G. Sheinkopf, Energy- windows are sometimes useful in house-
Efficient Florida Home Building, FSEC-GP-33- hold areas with high moisture production,
88, Florida Solar Energy Center, Cape such as bathrooms, kitchens, and laundry
Canaveral, FL, 1988.
rooms, exhaust fans provide more reliable
control throughout the year.
Select windows with air leakage rat-
Ventilation and Airtightness ings that meet or exceed standard industry
Airflow through and around windows requirements of 0.37 cfm/ft 2 to minimize
occurs by design as ventilation and inad- discomfort from uncontrolled infiltration.
vertently as infiltration. The use of win- Select windows with even lower values
dows for natural ventilation is as old as for particularly windy sites or harsh cli-
architecture itself. Opening windows, par- mates. Check the seals between window
ticularly on opposite sides of a living components for airtightness. To minimize
space, can cool a home for free. The sash infiltration around installed windows, fol-
type of a window influences the ventila- low the manufacturer’s installation proce-
tion airflow rate through the window rela- dures carefully and seal and caulk joints
tive to its size. Some common sash types and cracks.
and their effective open areas for ventila-
tion purposes are shown in the table
above. Casement windows are especially
effective for ventilation because they tend
to direct the greatest airflow into the liv-
ing space when fully open.
Infiltration is the uncontrolled leakage
of air into a building from the exterior
through joints and cracks around window
Window Energy Rating and Labeling
any windows, skylights, and
glazed doors now bear energy rat-
ings or labels, similar to those
Q How will designers and homeown -
ers use these energy labels?
being placed on household appliances, to
assist consumers in selecting energy-effi-
cient products. The labels have been
A Energy labels will show a variety of
product performance attributes,
enabling designers to compare and
developed by a non-profit group, the select products directly, based on
National Fenestration Rating Council each project’s specific energy per-
(NFRC). The following interview with formance needs. Until now, design-
NFRC staff, provides homeowners, archi- ers have had to spend too much time
tects, and builders with some important trying to understand a mixed bag of
information on these new window energy rating techniques, test methods, and
ratings. performance claims. A nationwide
system for rating whole-product
Q Why are energy ratings or labels
important for windows and sky -
energy performance will not only
give designers the energy informa-
tion they seek, but will also permit
direct product comparisons.
A Fenestration—windows, skylights,
and glazed doors—can account for
over 25% of the heating and cooling
Homeowners have faced a simi-
lar dilemma. When selecting fenes-
tration products for a remodeling
energy bills in a typical home. project or new construction, home-
Designers, builders, and homeown- owners have had no way to compare
ers have never had a tool for deter- the energy performances of two
mining or comparing the energy per- products directly. This difficulty has
formances of fenestration products to been compounded by the different
assist them in their purchase deci- energy rating techniques employed
sions. Many manufacturers offer a by the various industry segments.
variety of energy-efficient products, Window energy labels will enable
but have not been able to demon- consumers to compare products
strate their superiority through com- directly, regardless of glazing and
parable performance ratings. frame type.
Q How will the energy ratings be
A The energy ratings are determined
using advanced computer tools
developed in the United States and
Canada, combined with standard-
ized product performance testing.
The WINDOW 4.1 program, devel-
oped at Lawrence Berkeley National
In evaluating the cost of new or
replacement window units, the aesthet -
ic, functional, and energy performance
characteristics come together. To
make a better decision, it is useful to
think of the life-cycle cost of the unit.
National Fenestration Rating Council
1300 Spring Street, Suite 120
Silver Spring, MD 20910
The Telephone: (301) 589-6372
Fax: (301) 589-0854
When shopping for windows, look for the Web: http://www.nfrc.org
National Fenestration Rating Council
(NFRC) label as your guide to buying energy-efficient windows. The NFRC is a non-profit public/pri-
vate collaboration that provides contractors and homeowners with standardized, unbiased meth-
ods of comparing various brands and types of windows.
Below is an example of an NFRC label. All the parts of the label are described, but the U-factor and
Solar Heat Gain Coefficient, which rate the efficiency of the entire window (glass and frame), are
the most important in helping choose the best window for your purposes. All labels have U-factors;
Solar Heat Gain Coefficients and Visible Light Transmittance are now being added. Air leakage rat-
ings and annual heating and cooling factors will be added in the near future.
The NFRC insignia is This box contains the name of the
your assurance that Independent Certification and
this window has been Inspection Agency (IA) selected
independently rated. by the window manufacturer.
The U-factor is a
measure of heat
transmission due to
a temperature dif-
ference. The smaller Name of the window
the U-factor, the manufacturer.
less heat is trans-
Description of the
The Solar Heat Gain
to which this label is
Coefficient is a
measure of the rate
of solar heat flow
through the window. Since ratings can
vary depending on
The Visible Light the typical size of
Transmittance value windows in different
is a measure of the building types, values
fraction of visible are given for both
light that passes residential and com-
through the window. mercial applications.
The Air Leakage rating is a measure of the rate of infiltra-
tion through this particular window.
Laboratory, is one of the fundamen- a result of this effort, consumers
tal building blocks of the rating sys- across the country now have energy
tem. This program and the FRAME rating labels on windows, skylights,
program are used to calculate the U- and glazed doors analogous to those
factors and Solar Heat Gain on automobiles, appliances, and
Coefficients of windows. Air leakage insulation.
and other energy performance attrib-
utes are also being rated. Soon,
homeowners will see two new rat-
ings, the Fenestration Heating Rating
Q Are these computer tools available
to the public?
(FHR) and Fenestration Cooling
Rating (FCR), which provide a com-
parative index of heating and cooling
A Yes. These computer tools are avail-
able through the National
Fenestration Rating Council for use
season energy use. The RESFEN by building energy professionals,
program, developed at Berkeley Lab, engineers, architects, and others.
can also be used to estimate the NFRC also provides detailed train-
annual energy consumption and ing for manufacturers and design
costs associated with a particular professionals in the proper use of
window type and orientation in a these window-related computer
specific geographic location, based tools. For more information on these
on local utility costs. computer programs, contact:
Q Who is responsible for implementing
the window energy performance rat -
ing and labeling program?
National Fenestration Rating Council
1300 Spring Street, Suite 120
Silver Spring, MD 20910
Telephone: (301) 589-6372
Fax: (301) 589-0854
A The National Fenestration Rating
Council has developed and is imple-
menting this rating and labeling sys-
tem. NFRC is a non-profit coalition
of manufacturers, builders, state and
federal energy officials, private and
Q Where might I see NFRC labels ref -
erenced or used?
government laboratories, utilities,
consumers, and others working
together to develop a nationwide
A Several state building codes and
other organizations with an interest
in promoting energy efficiency, such
energy performance rating system
as utilities, are already referencing
that is fair, accurate, and credible. As
NFRC ratings. The ratings are a pre-
requisite for some special programs,
such as low-interest financing to pur-
chase energy-efficient windows.
Look for labels on products dis-
played in your local building materi-
als supply store or window store.
NFRC ratings are listed in the prod-
uct literature, available from many
window manufacturers, architects, or
Choosing a better-performing window
to save on fuel costs will also improve
comfort and performance in other
areas. When shopping for windows, look
for the NFRC Label as your guide.
For Design, Specification, and Installation
his checklist guides homeowners, architects, and builders in selecting residential
T windows and skylights. Selecting the right window can be difficult because of the
many factors involved and the great variations in climate, utility costs, and occu-
pant needs. Check boxes are provided for marking entries during the selection or design
process. Note that each entry below does not apply to all circumstances and that some
general guidance may appear to be contradictory because all of the detailed conditions
cannot be specified. Users should mark the items that apply to their particular needs.
Other local sources of information for window selection are utilities, state and local
building code officials, design professionals, and building materials suppliers.
Insulating Value and Condensation If shading devices are to be used to sup -
Resistance plement the use of high-performance win -
t Look for NFRC U-factor ratings and dows, consider the following points:
labels to guide window selection. t Select light-colored shading devices to
t Select double-pane windows in all cli- minimize solar heat gains.
mates where heating is needed. Select t Select exterior shading devices to mini-
double- or triple-pane windows with mize the inward flow of absorbed solar
low-E coatings and gas fills in cold cli- heat.
mates to reduce heat losses and con- t Select interior shading devices to
densation. reduce solar heat gains while providing
t To reduce frame and edge heat losses for privacy and aesthetics, or when
and condensation in all climates where exterior shading devices cannot be
heating is needed, select windows with used.
wood, vinyl, fiberglass, or properly t Select horizontally oriented shading
designed, thermally broken aluminum devices for south-facing windows and
frames. vertically oriented shading devices for
t Use heavy drapes, thermal shades, or east- and west-facing windows.
thermal shutters to provide additional t Specify overhangs, exterior awnings, or
window insulation in cold climates. the planting of deciduous trees and
shrubs to shade south-facing windows
Solar Control and Ultraviolet during the summer while allowing ben-
Protection eficial solar heat gains during the win-
t Look for NFRC Solar Heat Gain ter.
Coefficient ratings and labels to guide
window selection. Daylight and View
t Select windows with spectrally selec- t Look for NFRC Visible Light
tive glazings (special tints or modified Transmittance ratings and labels to
low-E coatings) to reduce solar heat guide window selection.
gains (SHGC less than 0.4) while t Select window size, location, and glass
maintaining high visible transmittance type to provide adequate daylight lev-
(glass transmittance greater than 0.6). els in each space.
t Select tinted windows to reduce solar t Select windows with high visible trans-
heat gains and control glare by lower- mittances (greater than 50%) to maxi-
ing visible transmittance. mize outward visibility.
t Select special glazings (with plastic t Specify window sizes and positions in
layers or low-E coatings) to reduce walls to take advantage of desirable
ultraviolet transmission in rooms with views.
materials subject to fading. (If this is a
critical concern, consult expert assis- t Position windows away from bright
tance.) external surfaces that create glare.
Ventilation and Airtightness t Select laminated glass or tempered
t Select operable windows for rooms requir- glass with screens for skylights and for
ing substantial ventilation during mild windows near doors or close to the
weather and to meet building code egress floor.
requirements. t Select windows with locks or latches
t Select casement or awning windows to that can be easily opened from the inte-
maximize effective ventilation area. rior but cannot be opened from the
t Select awning windows to better exclude
precipitation while ventilating.
Maintenance, Durability, and
t Position operable windows in opposite Lifetime
walls of living spaces to maximize cross-
ventilation. t Check warranties for indication of
durability and lifetime before selecting
t Select fixed windows or windows with windows and skylights.
compression seals to minimize infiltration.
t Check the quality of window construc-
t Select windows and skylights with contin- tion.
uous edge seals to minimize infiltration.
t Use protective paints, stains, or
t Seal and caulk around window and sky- sealants on wood window and skylight
light frames and sash to reduce infiltration. frames or select clad wood products.
Follow the manufacturer’s installation
instructions. t Follow the manufacturer’s instructions
to maintain glazing, sash, frame, and
hardware in good repair.
t Position windows away from external Installation
sources of extreme noise.
t Check all applicable building codes
t Select double- or triple-pane windows before installing windows and sky-
with panes of unequal thickness, laminated lights.
glass, or gas fills to minimize noise from
the exterior. t Follow the manufacturer’s installation
Privacy, Safety, and Security
t Select interior shading devices that
obscure direct view for additional privacy. t Consider the relative effects on utility
bills when selecting windows and sky-
t Check building codes on fire, wind-load- lights. Contact the NFRC (see Window
ing, and seismic safety before selecting Energy Rating and Labeling, page 10)
and positioning windows and skylights. or consult energy specialists or utility
representatives for estimates of the
energy and cost savings provided by
energy-efficient windows and sky-
t Consider the effects on the resale value
of a home when selecting windows and
t Check local, state, and federal energy
efficiency programs and utility energy
conservation programs for economic
incentives for installing energy-effi-
cient windows and skylights.
New materials and improvements to all
parts of the window assembly have
contributed to their high performance.
These double-glazed windows with vinyl
frames have a high insulating value.
Window Energy Glossary
Air Leakage Rating: A measure of the rate person’s body can lose heat to a cold win-
of infiltration around a window or skylight dow or skylight surface in a similar way.
in the presence of a strong wind. It is
expressed in units of cubic feet per minute R-Value: A measure of the resistance of a
per square foot (cfm/ft 2) of window area material or assembly to heat flow. It is the
or cubic feet per minute per foot (cfm/ft) inverse of the U-factor (R = 1/U) and is
of window perimeter length. The lower a expressed in units of hr-ft 2-˚F/Btu. A high
window’s air leakage rating, the better its window R-value, has a greater resistance
airtightness. to heat flow and a higher insulating value.
Conduction: The flow of heat through a Shading Coefficient (SC): A measure of
solid material, such as glass or wood, and the ability of a window or skylight to
from one material to another in an assem- transmit solar heat, relative to that ability
bly, such as a window, through direct con- for 1/8-inch clear, double-strength, single
tact. glass. It is equal to the Solar Heat Gain
Coefficient multiplied by 1.15 and is
Convection: The flow of heat through a expressed as a number without units
circulating gas or liquid, such as the air in between 0 and 1. A window with a lower
a room or the air or gas between window- Shading Coefficient transmits less solar
panes. heat, and provides better shading.
Fenestration: A window or skylight and Solar Heat Gain Coefficient (SHGC):
its associated interior or exterior elements, The fraction of solar radiation admitted
such as shades or blinds. The placement of through a window or skylight, both
window openings in a building wall is one directly transmitted, and absorbed and
of the important elements in determining subsequently released inward. The Solar
the exterior appearance of a building. Heat Gain Coefficient has replaced the
Shading Coefficient as the standard indi-
Gas Fill: A gas other than air placed cator of a window’s shading ability. It is
between window or skylight glazing panes expressed as a number without units
to reduce the U-factor by suppressing con- between 0 and 1. A window with a lower
duction and convection. Solar Heat Gain Coefficient transmits less
solar heat, and provides better shading.
Glazing: The glass or plastic panes in a
window or skylight. Spectrally Selective Glazing: A specially
engineered low-E coated or tinted glazing
Infiltration: The inadvertent flow of air that blocks out much of the sun’s heat
into a building through breaks in the exte- while transmitting substantial daylight.
rior surfaces of the building. It can occur
through joints and cracks around window U-Factor (U-Value): A measure of the
and skylight frames, sash, and glazings. rate of heat flow through a material or
assembly. It is expressed in units of
Low-Emittance (Low-E) Coating: Btu/hr-ft2-˚F or W/m2-˚C. Window manu-
Microscopically thin, virtually invisible, facturers and engineers commonly use the
metal or metallic oxide layers deposited on U-factor to describe the rate of non-solar
a window or skylight glazing surface pri- heat loss or gain through a window or
marily to reduce the U-factor by suppress- skylight. Lower window U-factors have
ing radiative heat flow through the win- greater resistance to heat flow and better
dow or skylight. insulating value.
Radiation: The transfer of heat in the form Visible Transmittance: The percentage or
of electromagnetic waves from one sepa- fraction of visible light transmitted by a
rate surface to another. Energy from the window or skylight.
sun reaches the earth by radiation, and a
Selecting Windows for
Energy Efficiency What’s New
New window technologies have increased in Building
energy benefits and comfort, and have provided
more practical options for consumers. This Energy
selection guide will help homeowners,
architects, and builders take advantage of the
expanding window market. The guide contains
three sections: an explanation of energy-related
window characteristics, a discussion of window
energy performance ratings, and a convenient We would like to acknowledge the many win-
dow and glazing industry reviewers of this doc-
checklist for window selection. ument. We appreciate the time they took to
assure the usefulness of this document. Special
thanks to V&WPatio Door & Window Co., Inc.
of Berkeley, California, for making one of their
Resources window retrofit installations available for photo
illustration of this brochure. This work was
supported by the U.S. Department of Energy,
In seeking information concerning windows and energy efficiency in general, Assistant Secretary for Energy Efficiency and
Renewable Energy, Office of Building
there are several local resources worth investigating: Technology, State and Community Programs,
Office of Building Systems.
• Local utilities
• State or municipal energy agencies This document was prepared as an account of work
sponsored by the United States Government. While this
• Regional universities with architecture, construction, or extension programs document is believed to contain correct information,
neither the United States Government nor any agency
• Bookstores thereof, nor The Regents of the University of
• Product literature at home improvement centers California, nor any of their employees, makes any war-
ranty, express or implied, or assumes any legal respon -
• Local chapters of the American Institute of Architects sibility for the accuracy, completeness, or usefulness of
any information, apparatus, product, or process dis -
• Local builder’s associations closed, or represents that its use would not infringe pri-
vately owned rights. Reference herein to any specific
commercial product, process, or service by its trade
Recommended Web Sites name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recom -
Search for specific window and glazing manufacturers and trade organiza - mendation, or favoring by the United States
Government or any agency thereof, or The Regents of
tions by name on the World Wide Web. the University of California. The views and opinions of
authors expressed herein do not necessarily state or
American Architectural Manufacturers Association: http://www.AAMAnet.org reflect those of the United States Government or any
agency thereof, or The Regents of the University of
American Institute of Architects: http://www.aia.org California.
American Solar Energy Society: http://www.ases.org/solar Produced for DOE’s Office of Energy Efficiency and
Renewable Energy by the Lawrence Berkeley National
American Society of Heating, Refrigerating & Air Conditioning Engineers: Laboratory, a DOE national laboratory.
Home Energy Magazine: http://www.homeenergy.org
National Association of Home Builders: http://www.nahb.com PUB-788 January 1997 - 5000
National Fenestration Rating Council: http://www.nfrc.org
National Wood Window and Door Association: http://www.nwwda.org For More Information
National Research Council of Canada:
http://www.cisti.nrc.ca:80/irc/irccontents.html Energy Efficiency and
Natural Resources Canada: http://www.NRCan.gc.ca Renewable Energy Clearinghouse
P.O. Box 3048
Passive Solar Industries Council: http://www.psic.org Merrifield, VA 22116
U.S. Department of Energy: http://www.eren.doe.gov (800) DOE-EREC
Recommended Reading (703) 893-0400 fax
Residential Windows: A Guide to New Technologies and Energy Performance,
by John Carmody, Stephen Selkowitz, and Lisa Heschong, W.W. Norton &
Company, 1996; http://www.wwnorton.com. Printed on recycled paper
using soy-based inks.