Citizens Action Coalition Education Fund

Almost half of all homes in the U.S. have inefficient, single-pane windows. Inefficient windows can account for up to 25% of your heating bill in the winter, and can make your air conditioner work up to three times harder in the summer. Although high performance windows may cost more initially, they save you money in the long run. In addition, they increase the value of your home.

If you are shopping for new windows, look for the National Fenestration Rating Council (NFRC) label. The NFRC provides information for windows and skylights similar to fuel economy ratings on new cars, allowing you to compare the energy ratings of various windows. An NFRC label means the window’s performance is certified.

Window rating systems help you evaluate three main factors that affect the energy efficiency of windows. "The first is heat flow through the glass and frame, which is generated by the temperature difference between the inside of your home and the outside environment. . . . Heat transferred through a window is expressed with U-values; the lower the U-value, the better. The resistance to heat flow is expressed as an R-value. . . In general, the higher the R-value, the better the insulation."

Higher R-values are achieved with multiple panes of glass. Double-pane windows insulate almost twice as well as single-pane windows. In a double-pane window, the air space between the panes of glass provides insulation, reducing the amount of hot or cold air being conducted from one side of the window to the other.

The second major factor affecting a window’s energy efficiency is solar gain, also called the solar heat gain coefficient (SHGC). "Put simply, the SHGC measures how much of the sun’s energy (both visible and near-infrared light) striking the window transmits as heat into the room. . . . The SHGC also measures how much solar heat passes through the frame, not just the glass."

"The third factor that affects a window’s energy efficiency is air leakage. This is the air that flows through cracks between the sash and the frame, or at the point where the window connects to the wall. The lower the leakage value, the better."

Low-emissivity, or low-e, coatings have revolutionized window technology over the last three decades. "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." Low-e glass keep heat inside in winter and outside in summer. They also reflect damaging ultraviolet light, helping to protect interior furnishings from fading.

Some energy-efficient windows are filled with low-conductivity gas, such as argon, krypton, and yes, even CO2. These lower conductivity gases insulate better than regular air.

Tinted glass and tinted window films can further help to reduce solar gain by reflecting light back outside. Tinted glass has its limitations, however, as it tends to absorb some heat and reduces overall visibility, but may be an effective choice for certain applications or in certain areas.

Another factor to consider when purchasing windows is edge spacers. Edge spacers keep the window’s glass panes the correct distance apart. Traditionally, edge spacers were made of aluminum, a highly conductive material. In more recent years edge spacers made of foam, fiberglass and vinyl have been developed. These materials further reduce heat flow and prevent condensation.

If you are not ready to replace your existing windows, there are a number of inexpensive measures you can take to improve their performance:

• Weatherize windows in winter. Use a heavy-duty, clear plastic sheet, either on a frame, or taped to the inside of your window frames during cold weather months.
• Install tight-fitting insulating shades on drafty windows.
• Passive solar heating and cooling. Use shades and curtains to block heat loss through windows in cold weather and to block passive solar heat in the summer.
• Install exterior storm windows – they can reduce winter heat loss by up to 50%. Low-e storm windows can save even more energy.
• Install awnings on south- and west-facing windows to block heat from the sun in warmer months.

• U.S. Department of Energy, Energy Efficiency and Renewable Energy: Energy Saver$: Tips on Saving Energy & Money at Home, p. 18 (June, 2005).
• Pahl, Greg, High Performance Windows, The Mother Earth News Guide to Homes, p. 69 (Summer, 2007)
• American Council for an Energy Efficient Economy, Consumer Guide to Savings: New Windows, found at: http://www.aceee.org/consumerguide/windows.htm