Energy-Efficient
Windows
Windows bring light, warmth, and beauty
into buildings and give a feeling of openness and space to living areas. They
can also be major sources of heat loss in the winter and heat gain in the
summer. In 1990 alone, the energy used to offset unwanted heat losses and gains
through windows in residential and commercial buildings cost the United States
$20 billion (one-fourth of all the energy used for space heating and cooling).
However, when properly selected and installed, windows can help minimize a
home's heating, cooling, and lighting costs. This publication describes one
option—energy-efficient windows—available for reducing a home's heating and
cooling energy requirements.
Controlling Air Leaks
When air leaks around windows, energy is
wasted. Energy is also transferred through the centers, edges, and frames of
windows. Eliminating or reducing these paths of heat flow can greatly improve
the energy efficiency of windows and, ultimately, of homes. Several options
are available to reduce air leaks around windows; the least expensive options
are caulking and weatherstripping, followed by replacing window frames.
Caulking and Weatherstripping
Caulks are airtight compounds (usually
latex or silicone) that fill cracks and holes. Before applying new caulk, old
caulk or paint residue remaining around a window should be removed using a
putty knife, stiff brush, or special solvent. After old caulk is removed, new
caulk can then be applied to all joints in the window frame and the joint
between the frame and the wall. The best time to apply caulk is during dry
weather when the outdoor temperature is above 45° Fahrenheit (7.2° Celsius).
Low humidity is important during application to prevent cracks from swelling
with moisture. Warm temperatures are also necessary so the caulk will set
properly and adhere to the surface.
Weatherstripping is a narrow piece of
metal, vinyl, rubber, felt, or foam that seals the contact area between the
fixed and movable sections of a window joint. It should be applied between the
sash and the frame, but should not interfere with the operation of the window.
For more information on caulking and weatherstripping, contact the Energy
Efficiency and Renewable Energy Clearinghouse (EREC).
Replacing Window Frames
The type and quality of the window frame
usually affect a window's air infiltration and heat loss characteristics. Many
window frames are available—all with varying degrees of energy efficiency.
Some of the more common window frames are fixed-pane, casement, double- and
single-hung, horizontal sliding, hopper, and awning.
When properly installed, fixed-pane
windows are airtight and inexpensive and can be custom designed for a wide
variety of applications. But, because they cannot be opened, fixed-pane
windows are unsuitable in places where ventilation is required.
Casement, awning, and hopper windows
with compression seals are moderately airtight and provide good ventilation
when opened. Casement windows open sideways with hand cranks. Awning windows
are similar to casement windows except that their hinges are located at the
tops of the windows instead of at the sides. Hopper windows are inverted
versions of awning windows with their hinges located at the bottom. Windows
with compression seals allow about half as much air leakage as double-hung and
horizontal sliding windows with sliding seals.
Double-hung windows have top and bottom
sashes (the sliding sections of the window) and can be opened by pulling up
the lower sashes or pulling down the upper sash. Although they are among the
most popular type of window, double-hung windows can be inefficient because
they are often leaky. Single-hung windows are somewhat better because only one
sash moves. Horizontal sliding windows are like double-hung windows except
that the sashes are located on the left and right edges rather than on the
tops and bottoms. Horizontal sliding windows open on the side and are
especially suitable for spaces that require a long, narrow view. These
windows, however, usually provide minimal ventilation and, like double-hung
windows, can be quite leaky.
Reducing Heat Loss and
Condensation
Manufacturers usually represent the
energy efficiency of windows in terms of their U-values (conductance of heat)
or their R-values (resistance to heat flow). If a window's R-value is high, it
will lose less heat than one with a lower R-value. Conversely, if a window's
U-value is low, it will lose less heat than one with a higher U-value. In
other words, U-values are the reciprocals of R-values (U-value = 1/R-value).
Most window manufacturers use R-values in rating their windows.
Usually, window R-values range from 0.9
to 3.0 (U-values range from 1.1 to 0.3), but some highly energy-efficient
exceptions also exist. When comparing different windows, you should ensure
that all U-or R-values listed by manufacturers: (1) are based on current
standards set by the American Society of Heating, Refrigeration, and
Air-Conditioning Engineers (ASHRAE), (2) are calculated for the entire window,
including the frame, and not just for the center of the glass, and (3)
represent the same size and style of window.
The following five factors affect the
R-value of a window.
- The type of glazing material (e.g.,
glass, plastic, treated glass)
- The number of layers of glass
- The size of the air space between the
layers of glass
- The thermal resistance or conductance
of the frame and spacer materials
- The "tightness" of the
installation (i.e., air leaks—:see previous discussion).
Types of Glazing Materials
Traditionally, clear glass has been the
primary material available for window panes in homes. However, in recent
years, the market for glazing—or cutting and fitting window panes into
frames—has changed significantly. Now several types of special glazings are
available that can help control heat loss and condensation.
Low-emissivity (low-e) glass
has a special surface coating to reduce heat transfer back through the window.
These coatings reflect from 40% to 70% of the heat that is normally
transmitted through clear glass, while allowing the full amount of light to
pass through.
Heat-absorbing glass
contains special tints that allow it to absorb as much as 45% of the incoming
solar energy, reducing heat gain. Some of the absorbed heat, however, passes
through the window by conduction and reradiation.
Reflective glass has
been coated with a reflective film and is useful in controlling solar heat
gain during the summer. It also reduces the passage of light all year long,
and, like heat-absorbing glass, it reduces solar transmittance.
Plastic glazing materials—acrylic,
polycarbonate, polyester, polyvinyl fluoride, and polyethylene—are also
widely available. Plastics can be stronger, lighter, cheaper, and easier to
cut than glass. Some plastics also have higher solar transmittance than glass.
However, plastics tend to be less durable and more susceptible to the effects
of weather than is glass.
Storm windows can
increase the efficiency of single-pane windows, the least energy-efficient
type of glazing. The simplest type of storm window is a plastic film taped to
the inside of the window frame. These films are usually available in
prepackaged kits. Although plastic films are easily installed and removed,
they are easily damaged and may reduce visibility. Rigid or semirigid plastic
sheets such as plexiglass, acrylic, polycarbonate, or fiber-reinforced
polyester can be fastened directly to the window frame or mounted in channels
around the frame—usually on the outside of the building. These more durable
materials are also available in kits.
For more information about advanced
types of glazing materials, contact EREC.
Layers of Glass and Air
Spaces
Standard single-pane glass has very
little insulating value (approximately R-1). It provides only a thin barrier
to the outside and can account for considerable heat loss and gain.
Traditionally, the approach to improve a window's energy efficiency has been
to increase the number of glass panes in the unit, because multiple layers of
glass increase the window's ability to resist heat flow.
Double- or triple-pane windows have
insulating air- or gas-filled spaces between each pane. Each layer of glass
and the air spaces resist heat flow. The width of the air spaces between the
panes is important, because air spaces that are too wide (more than 5/8 inch
or 1.6 centimeters) or too narrow (less than 1/2 inch or 1.3 centimeters) have
lower R-values (i.e., they allow too much heat transfer). Advanced, multi-pane
windows are now manufactured with inert gases (argon or krypton) in the spaces
between the panes because these gases transfer less heat than does air.
Multi-pane windows are considerably more
expensive than single-pane windows and limit framing options because of their
increased weight.
Frame and Spacer Materials
Window frames are available in a variety
of materials including aluminum, wood, vinyl, and fiberglass. Frames may be
primarily composed of one material, or they may be a combination of different
materials such as wood clad with vinyl or aluminum-clad wood. Each frame
material has its advantages and disadvantages.
Though ideal for strength and customized
window design, aluminum frames conduct heat and therefore
lose heat faster and are prone to condensation. Through anodizing or coating,
the corrosion and electro-galvanic deterioration of aluminum frames can be
avoided. Additionally, the thermal resistance of aluminum frames can be
significantly improved by placing continuous insulating plastic strips between
the interior and exterior of the frame.
Wood frames have higher
R-values, are not affected by temperature extremes, and do not generally
promote condensation. Wood frames do require considerable maintenance in the
form of periodic painting or staining. If not properly protected, wood frames
can swell, which leads to rot, warping, and sticking.
Vinyl window frames,
which are made primarily from polyvinyl chloride (PVC), offer many advantages.
Available in a wide range of styles and shapes, vinyl frames have moderate to
high R-values, are easily customized, are competitively priced, and require
very low maintenance. While vinyl frames do not possess the inherent strength
of metal or wood, larger-sized windows are often strengthened with aluminum or
steel reinforcing bars.
Fiberglass frames are
relatively new and are not yet widely available. With some of the highest
R-values, fiberglass frames are excellent for insulating and will not warp,
shrink, swell, rot, or corrode. Unprotected fiberglass does not hold up to the
weather and therefore is always painted. Some fiberglass frames are hollow;
while others are filled with fiberglass insulation.
Spacers are used to
separate multiple panes of glass within the windows. Although metal (usually
aluminum) spacers are commonly installed to separate glass in multi-pane
windows, they conduct heat. During cold weather, the thermal resistance around
the edge of a window is lower than that in the center; thus, heat can escape,
and condensation can occur along the edges. To alleviate these problems, one
manufacturer has developed a multi-pane window using a 1/8-inch-wide (0.32
centimeters-wide) PVC foam separator placed along the edges of the frame. Like
other multi-pane windows, these use metal spacers for support, but because the
foam separator is secured on top of the spacer between the panes, heat loss
and condensation are reduced. Several window manufacturers now sandwich foam
separators, nylon spacers, and insulation materials such as poly-styrene and
rockwool between the glass inside their windows.
Additional Options for
Reducing Heat Loss and Gain through Windows
Movable insulation, such as insulating
shades, shutters, and drapes, can be applied on the inside of windows to
reduce heat loss in the winter and heat gain in the summer. Shading devices,
such as awnings, exterior shutters, or screens, can be used to reduce unwanted
heat gain in the summer.
In most cases, these window treatments
are more cost-effective than energy-efficient window replacements and should
be considered first. Additional information on window treatments is available
from EREC.
Conclusion
Reducing heat loss or gain in homes
often includes either improving existing windows or replacing them. Low-cost
options available for improvement are caulking, weatherstripping, retrofit
window films, and window treatments. Replacing windows will involve the
purchase of new materials, which should adhere to certain energy efficiency
standards.
Different combinations of frame style,
frame material, and glazing can yield very different results when weighing
energy efficiency and cost. For example, a fixed-pane window is the most
air-tight and the least expensive; a window with a wood frame is likely to
have less conductive heat loss than one with an aluminum frame; double-pane,
low-e window units are just as efficient as triple-pane untreated windows, but
cost and weigh less.
No one window is suitable for every
application. Many types of windows and window films are available that serve
different purposes. Moreover, you may discover that you need two types of
windows for your home because of the directions that your windows face and
your local climate. To make wise purchases, first examine your heating and
cooling needs and prioritize desired features such as daylighting, solar
heating, shading, ventilation, and aesthetic value.
Source List
The following resources provide more
information on energy-efficient windows.
American Architectural
Manufacturers Association (AAMA)
2700 River Road, Suite 118
Des Plaines, IL 60018
(708) 202-1350
Developed a testing procedure [AAMA 1503]
for measuring the thermal transmission properties of aluminum-, vinyl-, and
wood-framed windows.
American Society of Heating,
Refrigerating, and Air-Conditioning Engineers (ASHRAE)
1791 Tullie Circle, NE
Atlanta, GA 30329
(404) 636-8400
ASHRAE's Handbook of Fundamentals
contains tables citing heat transfer, light transmittance, and shading
properties for various window types and materials.
For information about many kinds of
energy efficiency and renewable energy topics, contact:
The Energy Efficiency and
Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
Email: doe.erec@nciinc.com
EREC provides free general and technical
information to the public on the many topics and technologies pertaining to
energy efficiency and renewable energy.
National Fenestration Rating
Council (NFRC)
962 Wayne Avenue, Suite 750
Silver Spring, MD 20910
(301) 589-6372
Developed the Procedure for Determining
Fenestration Product Thermal Properties (NFRC 100-91). These procedures are
now being used in NFRC's window certification and efficiency labeling programs,
which have already been adopted by three states.
Window and Door
Manufacturers Association
1400 East Touhy Avenue
Des Plaines, IL 60018-3305
847-299-5200
Issues seals of approval for manufacturers of wood-framed windows.
U.S. Department of Energy (DOE)
Building Systems and Materials Division
EE-421
1000 Independence Avenue, SW
Washington, DC 20585
(202) 586-9214
Developed the WINDOW computer program,
which aids window manufacturers and building designers in optimizing the thermal
and daylighting performance of window systems. For their certification and
labeling programs, the NFRC uses the WINDOW computer program and DOE-supported
research and testing to determine the thermal and optical properties of windows.
Vinyl Window and Door Institute
355 Lexington Avenue
New York, NY 10017
(212) 351-5400
Developed performance standards and
certification program for manufacturers of vinyl-framed windows.
Reading List
"Low-E Glass—Why the Coating Is
Where It Is," Energy Design Update, pp. 5–7,March 1990.
"No Pane, No Gain (Window Technology:
Part One)," Popular Science, pp. 92–98,June 1993.
"Through the Glass Darkly," Popular
Science, pp. 80&3150;87, July 1993.
- U.S. Department of Energy (DOE)
- National Renewable Energy Laboratory (NREL)
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