Whether replacing windows in an older house or choosing windows for a new house, your decisions on what type of windows to buy will be among the most important decisions you will make in terms of energy use. Because of the impact windows have on both heat loss and heat gain, proper selection of products can be confusing. To add to the complexity, window glazing technology has changed tremendously in recent years. The best window glazings today insulate almost four times as well as the best commonly available windows from twenty years ago.

Features to Look for in Energy-Saving Windows

Window technology has improved dramatically in recent years, with the net result of lowering your energy bills. Some of the most important energy features of windows are explained below.

• Multiple layers of glazing. Until the 1980s the primary way manufacturers improved the energy performance of windows was to add additional layers of glazing. Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer of glazing results in further improvement. Some of these windows use glass only; others use thin plastic films as the inner glazing layer(s).

• Thickness of air space. With double-glazed windows the air space between the panes of glass has a big effect on energy performance. A very thin air space does not insulate as well as a thicker air space because of the conductivity through that small space. Many window manufacturers have increased the thickness of the air space in their double-glazed windows from ¼" to ½" or more. If the air space is too wide, however, convection loops between the layers of glazing occur. Beyond about 1", you do not get any further gain in energy performance with thicker air spaces.

• Low-conductivity gas fill. By substituting a denser, lower conductivity gas such as argon for the air in a sealed insulated glass window, heat loss can be reduced significantly. Most major window manufacturers offer argon-gas fill as an option in their most popular windows. Other gases that have been or are being used in windows include carbon dioxide (CO₂), krypton (Kr), and argon-krypton mixtures.

• Tinted glass coatings. Tinted glass and tinted window films have long been used in commercial buildings to reduce heat gain through windows. Improved, lightly tinted windows are becoming more common for the residential market in southern (cooling-dominated) climates. These new glazings reduce the solar heat gain without reducing visibility as much as older tinted glass and films.

• Low-e coatings. More than any other single improvement, the invention and commercial development of low-emissivity (low-e) coatings in the 1980s revolutionized window technology. Thin, transparent coatings of silver or tin oxide permit visible light to pass through, but they effectively reflect infrared heat radiation back into the room. This reduces heat loss through the windows in the winter. A variety of low-e windows are now available for different climate zones and different applications in any particular location. Low-e windows with high solar heat gain coefficients are appropriate for northern climates where passive solar heating is advantageous, while “southern low-e” windows with low heat gain coefficients are appropriate in milder climates where summer cooling is more significant than winter heating.

• Edge spacers. The edge spacer is what holds the panes of glass apart and provides the airtight seal in an insulated glass window. Traditionally, these have been hollow aluminum channels, usually filled with desiccant beads. Aluminum has extremely high conductivity. That didn’t matter when the glazing did not insulate very well, but as better performing glazings were developed, proportionately more heat was lost through the edges. Since about 1990, a number of improved edge spacers have come onto the market. Some are made of thin-walled steel and have a thermal break. Others are made of silicone foam or butyl rubber. Generally, better edge seals are a low-cost option when ordering windows, and worth considering.

To learn more about energy-efficient windows, visit the Efficient Windows Collaborative.

Selecting New Windows for Your Home

Three key measures are used to report window energy performance. U-value (or "U-factor") is the measure of the amount of heat (in Btus) that moves through a square foot of window in an hour for every degree Fahrenheit difference in temperature across the window. The lower the U-value rating, the better the overall insulating value of the window. Solar heat gain coefficient (SHGC) is the measure of the amount of solar energy that passes through the window; typical values range from 0.4 to 0.9, and the higher the SHGC the greater the solar energy that passes through the window system. Windows with high SHGC (above 0.7) are designed for colder climates, while windows with low coefficients are designed for hotter climates. Air infiltration or air leakage is given in terms of cubic feet of air per minute per foot of window edge. The best windows have air leakage rating between 0.01 and 0.06 cfm/ft.

Two organizations offer guidance for consumers trying to make sense of the complicated windows market. The National Fenestration Rating Council (NFRC) provides objective data to help consumers make wise choices. The NFRC is a nonprofit collaboration of window manufacturers, government agencies, and building trade associations founded to establish a fair, accurate, and credible energy rating system for windows, doors, and skylights. Windows that have been rated by NFRC-approved testing laboratories and certified by independent certification and inspection agencies carry the NFRC label. The label includes window U-factor and SHGC and additional performance ratings (visible transmittance and air leakage).

ENERGY STAR offers guidance tailored to specific climate zones. Window, doors, and skylights qualifying for the ENERGY STAR label must meet requirements tailored for the country’s four broad climate regions: northern, north/central, south/central and southern. In addition, each ENERGY STAR window must carry the NFRC label, allowing comparisons of ENERGY STAR-qualified products on specific performance characteristics such as infiltration. To get efficiency gains beyond the ENERGY STAR levels, use the NFRC label or visit the NFRC website to find products with even tighter specifications.

Recommendations

• If you’re shopping for new windows, look for the NFRC label as your guide to their energy performance. Compare ratings on ENERGY STAR-labeled windows for your climate region.
• Remember that a window’s ability to insulate is given by its U-value, and the amount of solar energy it transmits is given by its solar heat gain coefficient—the lower the U-value and SHGC, the more efficient the window.
• Look for windows with these energy-saving features: double panes; low-e coatings; low-conductivity gas-fill between panes; and wood, vinyl, or fiberglass frames.
• Select windows with low air leakage ratings—between 0.01 and 0.06 cfm/ft.
• Consider different glazings for windows on different sides of your house to benefit from passive solar and maximize energy benefits. Install the lowest U-value windows you can afford on north-facing windows.Select windows with appropriate low-e coatings for your local climate on the east, west, and south sides of your house. If you do order different glazings for your different windows, be sure to keep track of which windows have which type of glazing because they will probably all look identical!
• To maximize energy performance, choose windows with larger unbroken glazing areas instead of multi-pane or true-divided-light windows. Applied grills that simulate true-divided-light windows are fine; they do not reduce energy efficiency.
• Choose windows with good warranties against the loss of the air seal. If the glazing seal is lost, not only will fogging occur, but also any low-conductivity gas between the layers of glass will immediately be lost.
• To ensure that your new windows perform as well as they should, hire skilled contractors to install them.

Page last updated September, 2005.