EREC Fact Sheet 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 optionenergy-efficient windowsavailable 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
availableall 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
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
The thermal resistance or conductance of the
frame and spacer materials
The "tightness" of the installation
(i.e., air leakssee 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
glazingor cutting and fitting window panes into frameshas 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
Plastic glazing materialsacrylic,
polycarbonate, polyester, polyvinyl fluoride, and polyethyleneare 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 frameusually
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.
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.
This document was produced for the U.S. Department of
Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national
laboratory. The document was produced by the Information Services Program, under the DOE
Office of Energy Efficiency and Renewable Energy. The Energy Efficiency and Renewable
Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL/DOE.
The statements contained herein are based on information known to EREC and NREL at the
time of printing. No recommendation or endorsement of any product or service is implied if
mentioned by EREC.