Glass Facts

The glass in a window is the first line of defense against uncomfortable extremes of hot and cold, as well as glare from the sun. "Comfort," where windows are concerned, is not a scientific measurement, but the National Fenestration Rating Council (NFRC)—a nonprofit organization that rates the performance of windows, doors, and skylights—is working on ways to quantify it. "Comfort is the big issue now," says NFRC executive director Jim Benney. "It's not only about efficiency anymore."

Window glass has come a long way in the past couple of decades. Very few windows today are sold with single panes, which are mostly used for historical accuracy and need good storm panels to work well. Most windows are dual-pane and sandwich an inert gas that doesn't conduct heat well, like argon or krypton, between the two pieces of glass to create an insulated panel. More expensive triple-pane windows up the efficiency with a third layer of glass in the sandwich—good for very cold climates or places where noise is an issue.

Extra panes and insulating gases are only half the energy-saving armor. Most new window glass also has a low-E (for "low-emissivity") coating on one or both of the sheets of glass. Low-E coatings are factory-applied on the inside of the panes (soft coat) or incorporated into the glass itself (hard coat). While soft coat performs better, hard coat lasts longer. Low-E can also be retrofitted by applying a film to the outside of the window or installing a storm panel with hard-coat glass.

Originally developed to slow down the escape of household heat during cold weather—increasing a window's insulating ability, or U-factor—today's coatings also cut down on the heat from the sun that enters the house, known as solar heat gain. (See "Deciphering the Sticker," left, for how these features are rated.) As a bonus, the coatings also block most of the sun's ultraviolet rays, which can fade wood and fabrics.

Cold climates, such as that at the TOH TV project house site, present a conundrum: How to cut solar heat gain in summer but still take advantage of it in winter, to offset heating bills? The short answer is, you can't have it both ways. But TOH plumbing and heating expert Richard Trethewey says it's more important to control solar heat gain in summer. "It's not worth fighting for the extra heat in the winter, because it's too spotty—it won't warm up the whole house," he says. "You'll save more if you don't have to crank the air-conditioning in the summer." While low-E coatings add 10 to 15 percent to the cost of windows, they reduce energy loss by 30 to 50 percent, which means, depending on the type of heating fuel being used, they could pay for themselves in 5 to 10 years. In fact, Michael Koenig, manager of the technical group at Andersen Corporation, recommends a simple strategy: If you consistently use air-conditioning to beat the heat wherever you live, buy windows with both a U-factor and a solar heat gain rating around 0.35.

That advice falls roughly in line with the U.S. Department of Energy and Environmental Protection Agency's guidelines for Energy Star?certified windows suitable in all 50 states. Energy Star windows generally perform in the top 10 percent of all windows, says Koenig, and those rated for the entire country will provide the best comfort in a wide range of weather conditions. The ones chosen for the Cambridge house—dual-paned, argon-filled, and low-E—fall into that category.

The newest generation of low-E is also what's known in industry lingo as "spectrally selective," meaning it keeps out heat without cutting down on light—an issue with early versions of low-E, which darkened the window glass. While windows aren't labeled as having a spectrally selective coating, it's safe to assume that any new window that doesn't have a visible tint and has good ratings on U-factor and solar heat gain coefficient likely has it.
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