Without windows, a house wouldn't be a home -- it'd be a very dark box.

Windows bring the daylight in, add architectural detail and give us a view of the world around us -- but how "green" are they? Can they be made more energy efficient?

Appearance is usually top priority when people choose windows for a new home or remodeling project. Awning, double-hung or casement? Tilt-in or slide? Wood or vinyl frames?

But it turns out there's more to a window than style or finish.

"When we look at the green aspect of a window, we need to look beyond the materials that make up that window and examine how well it performs in terms of energy efficiency," explains Anthony Grisolia, home-building quality control manager at IBACOS [www.ibacos.com], a leading building-science and research firm.

How well a window "performs" refers to how well it keeps the outside out and the inside in -- how much heat it loses when it's cold outside, and how much it gains when it's hot. Inefficient windows can account for 50 percent of your home's heating and cooling needs.

In the old days -- say, before 1950 -- windows in North America and other temperate climes were made with a single layer of glass, sometimes made of multiple smaller glass panels, framed by a sash made of wood or aluminum.

These older windows were pretty, but drafty and created a lot of condensation because they provided little insulation.

Today's windows are different -- most are double- or triple-glazed, often with the air between the layers sucked out and replaced by a less temperature-conductive gas such as argon or krypton.

"Windows are a big piece of the puzzle in making a house green," explains Grisolia. "A window has less resistance to heat transfer than the insulation in your walls."

According to Grisolia and the National Fenestration Rating Council [http://www.nfrc.org/getratings.aspx], there are two ratings that determine how energy efficient a window is:

— The U-factor measures how well a window prevents heat from escaping. U-factor ratings generally fall between 0.20 and 1.20 -- the lower the number, the greater the resistance to heat flow and the better its insulating value.

— The solar heat gain coefficient (SHGC) measures how well a window blocks heat from sunlight while letting in visible light. The SHGC is a number, or fraction, between 0 and 1 -- the lower a window's SHGC, the less solar heat gets through.

Both the U-factor and the SHGC can be clearly identified on the energy-performance labels, supplied by the NFRC, found on most new windows. Buyers can look up windows without labels at the NFRC Web site.

So what do these numbers mean to you? That depends on where you live.

"In Southern climates, where you want less heat from the sun to get into the house, a low SHGC is most important," says Grisolia. "In Northern climates, where you want to keep the cold of winter out of the house, a low U-value is most important."

The federal Department of Energy and Environmental Protection Agency have also gotten on board through their Energy Star program, which takes the U-factor and SHGC into account.

"When selecting a window that is right for you, you first need to determine which climate zone you live in," says Pat O'Brien, residential green expert with Pella Windows and Doors.

Go to the Energy Star Web site, punch in your Zip code or address and look at the resulting color-coded zone map. You'll find a U-Factor and SHGC that work best in your zone.

"Simply identify your zone and note the corresponding U-Factor and SHGC for that zone," says O'Brien, "When purchasing windows, be sure the numbers on the NFRC label meet or exceed the Energy Star program's recommendations."

"When it comes to windows, you get what you pay for," adds Grisolia. "Energy-efficient windows save you money each and every month on your utility bills and make you feel more comfortable and your living environment more enjoyable."

Then again, how your windows are put in is just as important as the energy ratings.

"Proper installation is critically important with windows and doors. Poorly installed windows and doors won't perform effectively, or last as long as they should, no matter how well they're built," explains O'Brien.

So before you choose a window, do your homework. Find out which zone you live in and the appropriate energy ratings. Hire a qualified contractor to install them properly, and your windows will definitely be "green."