Buying Energy-Efficient Windows Isn't an Open-and-Shut Case
Thursday, October 12, 2006
Q I don't want to go through another winter with the drafty old windows in my house. Besides appearance, what should I look for when buyin g new ones?
A Shopping for windows gets a bit complicated because there is no one-size-fits-all "best" window. People living in the North might want their houses to soak up as much free heat from the sun as possible, while those struggling to control air-conditioning costs in the South need windows that let as little heat through as possible. In the D.C. area, heating and cooling use about equal amounts of energy, so windows usually need to perform somewhere between the two extremes.
Here, new windows should have an insulating ability (known as U-factor) no higher than 0.40 and a solar heat gain coefficient (a measure of how much heat from the sun passes through) no higher than 0.55, numbers that fall in the middle of the scale. All windows rated by the federal Energy Star program meet these criteria, but many lesser-quality windows are still being sold, so don't automatically assume that everything you see passes muster. Always look for the label indicating how the windows performed on standard industry tests set through the National Fenestration Rating Council.
Although it might theoretically be possible for double-pane windows with clear glass to meet the Energy Star standards, manufacturers usually do it in part by adding ultra-thin, mirrorlike coatings to the glass. These "low-E" (for "low emissivity") coatings reflect certain wavelengths of light. Most of the visible light still gets through, so interior rooms appear bright, but the coatings can be fine-tuned to reflect various amounts of heat. They also reflect most ultraviolet light, which fades furnishings.
Besides the coatings, other components of a window, from the frame materials to the number of panes and the type of gas between them, affect performance. When you're walking around in a window showroom, it's hard to keep track of all the pluses and minuses of each option. It helps to do some online research beforehand. For a great overview, visit the Web site of the American Council for an Energy-Efficient Economy ( http:/
Then home in on specifics about how different window features are likely to affect your heating and cooling bills. At http:/
The site even points you to specific brands that meet your criteria. For example, you can see that a window with three panes of glass, a moderate-gainlow-E coating, a combination of argon and krypton gas between panes, and a frame built of wood with a low-maintenance exterior cladding will probably keep your annual heating and air-conditioning costs slightly under $1,000 a year. But if you opt instead for an identical-looking window that has a coating with low solar gain,those annual costs will rise above $1,000. That low-E coating wouldn't allow you to take much advantage of free heat from the sun during the winter.
Averages don't tell the entire story, however. You might want to specify different low-E coatings for different faces of your house, for example, to block some of the heat that blazes in during the summer through west-facing windows in your kitchen and dining room.
Or you might be inclined to follow the example of Bill Prindle, deputy director of the American Council for an Energy-Efficient Economy. When he shopped for windows for his own home in Washington, he chose a relatively high solar gain because deciduous trees keep his windows nicely shaded in the summer. By the time the leaves fall, he appreciates the heat that gets through the glass.