 How to select new replacement windows
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 weather stripping, followed by
replacing window frames.
Caulking and Weather stripping 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. Weather stripping 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 weather
stripping, 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 radiation. 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
semi-rigid 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.
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.
Window Frame and Spacer Materials
Window frames are available in a variety of
materials including vinyl, wood, fiberglass and aluminum. 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. Vinyl window frames, which are
made primarily from polyvinyl chloride (PVC), and 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. Click
here for more vinyl replacement window information. 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.
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. Aluminum frames
are ideal
for strength and customized window design. However they conduct heat at
a high rate 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. 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, weather stripping, 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. National Wood Window and Door Association
1400 East Touhy Avenue
Des Plaines, IL 60018-3305
(708) 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.
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