It might be easy to overlook something you can look right through, but don't do it. Windows are a key element in a home and they deserve a second look.

The National Association of Home Builders recently issued a list of keys to saving energy in your home. Near the top of the list: look for "double-pane windows that have high-performance glass that helps reduce heat loss in the winter and heat gain in the summer."

There's good reason to take the NAHB's advice if you're buying a new home or considering replacement windows. The U.S. Department of Energy says windows can account for 10 to 25 percent of your heating bill. In fact, the Department notes that during the summer, "sunny windows make your air conditioner work two to three times harder."

Both the NAHB and the Energy Department recommend looking for double-pane windows which are low-e coated or solar control spectrally selective. What? Well, it's actually a lot easier than it sounds.

"Low-e" stands for "low-emissivity." Emissivity is a measurement of how much heat is emitted from an object by radiation. In other words, how much of the sunshine hitting your window gets spread throughout your room, or how much of your home's heat is lost to the outside. Low-e coatings on a window reduce that flow and give your air conditioner or furnace a break.

"Spectrally selective coatings" block the infrared portion of sunlight. Since the infrared portion accounts for most solar heating, blocking it allows the sun to shine in without heating up your house. The Energy Department says such coatings are great for people in hot climates, but you should avoid them if you live in the colder parts of the country.

In addition to energy savings, the right windows can improve your home life in a number of other ways. The Efficient Windows Collaborative web site, sponsored by government and industry, lists five other benefits derived from efficient windows: improved comfort, less condensation, increased light and view, reduced fading and lower HVAC costs. It also provides figures on possible dollar savings from high-performance windows.

So, how do judge something that's transparent? Look to the National Fenestration Rating Council. The NFRC describes itself as a non-profit, public/private organization that provides consistent ratings on window, door and skylight products. It's comprised of manufacturers, suppliers, builders, architects and designers, specifiers, code officials, utilities and government agencies. Windows certified by the NFRC carry a label that provides the information you need to make a wise window choice.

The label provides four ratings.

  1. The "U-factor", or how well a window keeps heat inside a building. A U-value of 0.35 or below is recommended for colder climates. The lower the U-value, the better the insulation.
  2. Solar heat gain, or a window's ability to block warming caused by sunlight. The rating is expressed as a number between 0 and 1. The lower the number, the better the window is at blocking the sun's heat. It's recommended that windows in hot climates have a low rating, while south-facing windows in cold climates should have a high rating.
  3. Visible light "transmittance," or how much light gets through a product. The higher the number, the more light is transmitted.
  4. Air leakage, or heat loss and gain through cracks in the window. The Energy Department recommends you select windows with a rating of 0.3 cubic feet per minute or less.

The Environmental Protection Agency also recommends looking for the Energy Star label on windows. Manufacturers and retailers who participate in the government program agree to promote energy-efficient products.

According to the EPA, "If all residential windows in the U.S. were replaced with Energy Star qualifying models, the nation would save $7 billion in energy costs over the next 15 years."

Carol Ochs is a Washington-based reporter who covers new home trends.

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