If your eyes glaze over when conversation turns to U-value and low-E, wake up to what's new in window glass.
Double-pane insulated glass with low-E coating is a major energy saver. Here, This Old House general contractor Tom Silva installs a tilt-in double-hung sash
Window glass, called glazing, has come a long way since the days when windows had only a single pane between you and the elements. Today's energy-efficient windows come with glazing "systems" that incorporate multiple panes of glass, gas fillings, and high-tech, heat-sensitive coatings. Sorting through the options takes a bit of effort, but it more than pays off in increased comfort and reduced energy costs.
Windows with just one layer of glass suck energy dollars from a house and offer little protection against heat or cold. That explains why few new single-panes are sold today. If you opt for single-pane windows because of their traditional look, choose ones in well-made wood frames, and combine them with snug-fitting storm panels to boost their energy efficiency.
Double-Pane Insulated Glass
Most new windows sold today are dual-pane glass (sometimes called insulated glass or by the formerly trademarked name Thermopane), in which a layer of inert gas — typically argon or krypton — is sealed between inner and outer panes. The gas is a poor thermal conductor, so it slows the passage of heat through the glass, eliminating the need for storm windows.
Triple-Pane Insulated Glass
More efficient still are triple-pane windows, which seal two layers of gas within the frame. Triple-panes are excellent performers in extreme northern climates (and good sound- reducers for houses near highways), but the extra pane of glass makes for a very thick, heavy, and expensive sash.
Low-E and Heat Mirror Glass
Adding panes and gas-filled spacers is one way to get a more efficient window. Another way is with low-emissivity (low-E) coating, an invisible layer of metallic oxide that reduces the amount of heat that passes through the glass. Virtually all new windows sold today offer this feature.
Depending on the climate, low-E coatings can be tailored to let the sun's energy in or to block it out. In cold regions, where heating costs dominate, homeowners can choose coatings that maximize the amount of heat transmitted from the outside, called solar heat gain; in hot regions, low-solar-gain coatings can keep cooling costs down.
While low-E coatings add 10 to 15 percent to the cost of a window, research has shown that they cut energy expenditures by about 25 percent over plain insulated glass. That means that depending on the type of fuel used in a home, low-E windows can pay for themselves in 5 to 10 years. It's possible to buy dual-pane glass without a low-E coating, but Tom Silva doesn't recommend it. "The extra cost is minimal, and it's really beneficial in the long run," he says, pointing out that because low-E coatings also filter ultraviolet radiation, they protect furniture, rugs, and artwork from fading.
Another high-tech glazing system, called Heat Mirror, can match or even exceed the energy efficiency of triple-pane windows — without the extra weight. It's made by suspending a sheet of low-E film between panes of insulated glass. Superglass, the most expensive glazing system on the market and one of the best insulators, suspends two layers of Heat Mirror between panes of glass with gas-filled spacers.
Evaluating Energy Efficiency
You can't judge a window's efficiency just by looking at it. Fortunately, most new windows sold in the United States carry a label from the National Fenestration Rating Council, a nonprofit organization that certifies energy performance. The label shows how the window performs in three key areas.
If puzzling through the label is daunting, the EPA's Energy Star program makes judging efficiency easier. An Energy Star sticker means a window has met the minimum standard for insulating ability specifically for the climate of the region where it's sold. Energy Star windows generally exceed building code requirements and cost about 15 percent more than windows without the rating.
You can also go to http://windows.lbl.gov and download a program called Resfen. The software, which is free (PC-compatible only), lets you calculate how windows with different glazing systems and frame materials will affect your energy bill.
Understanding Window Ratings
Window glazing is rated on three criteria: how well it insulates, how much light it lets through, and how effectively it blocks heat from the sun. By taking these values into account, you can choose the best windows for your climate, and even tailor windows for specific rooms in your house.
Where to Find It
Heat-Smart Plus Window Systems
Loewen Windows Stenbach
Palo Alto, CA
National Fenestration Rating Council
Silver Spring, MD
Energy Star Program:
U.S. Department of Energy and U.S. Environmental Protection Agency