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High Performance Windows
High performance windows are designed to prevent air leakage, and in warmer climates, minimize solar heating. Air leakage and undesired solar healing are some of the most important factors in having an energy efficient home, as they account for wasted heat in cold climates, and cool air in warmer climates.
According to the Environmental Protection Agency, windows can be responsible for as much as one fourth of all the energy used to heat a home, and one half of the energy used to keep a home air conditioned. Considering one home causes, on average, 22,000 pounds of greenhouse gas emissions every year, taking measures to minimize energy use should be a top environmental priority for individuals, as well as for society as a whole.
Buying energy efficient replacement windows is a simple way to drastically reduce both greenhouse gas emissions and energy bills. In order to decipher how environmentally-friendly a model is, you have to know, what is a good Energy Star rating for windows. The Energy Star sticker found on all qualified products will provide ratings data. This information can be used to compare different models, and to find out the perfect design for each consumer's needs.
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Rating Requirements and Factors
A window is an Energy Star qualified model if it meets specific requirements set by the Energy Star division of the US Department of Energy and the Environmental Protection Agency. These highly efficient windows must be manufactured with durable frame materials, such as fiberglass or vinyl. They must have at least two panes of glass, although some models have three or four. High performance windows are coated to reflect infrared and ultraviolet light, sealing warm air inside in the winter, and outside in the summer, and protecting furniture from sun damage. Not all, but some Energy Star models are filled with insulating gases, such as argon or krypton.
Once the base requirements are met, each model is given a rating. There are two primary figures in the Energy Star rating system for windows — the U-Factor and the solar heat gain coefficient (SHGC). The U-Factor is the rate at which a window conducts non-solar heat. A lower figure signifies greater energy efficiency. The SHGC represents how much solar energy passes through the glazing material. Basically, a low number means a window is more capable of shading solar light from entering the home. In warm climates, a low number is desired. In cooler climates, this number is not as important.
Other secondary figures include air leakage and visible transmittance. Air leakage is how much air leaks through the window. A tighter, more efficient model will have a lower figure. Visible transmittance (VT) is the amount of visible sunlight entering the home. Like the SHGC, a low VT number is particularly desirable in warmer climates.
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Energy Star Climate Zones
The Energy Star rating system divides the United States into four different zones — Northern, North-Central, South-Central, and Southern. To determine your specific zone, see the official Energy Star government website. Once the zone is known, use the criteria listed on the website to find out what is a good Energy Star rating for windows in your area. For example, in Northern climates, a good U-Factor is less than .3, while the SHGC can vary. In the Southern zone, the U-Factor must be less than .6, and the SHGC, less than .27.
In cooler areas, the U-Factor and air leakage are the most important; in warmer areas, the SHGC and VT are most important. Although specific requirements vary with region, as a general rule, the lower the numbers, the better the window. All of the figures represent how much light, heat, energy, or air is able to move through or around the window. The models with the smallest figures are the most tightly constructed, well-coated, and thoroughly insulated, high performance windows.
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"High Performance Windows." (US Environmental Protection Agency, December, 2000).
"Energy Performance Ratings for Windows, Doors, and Skylights." (US Department of Energy.) <energysavers.gov/your_home/windows_doors_skylights/index.cfm/mytopic=13320>.
"Energy Star Qualification Criteria." (Energy Star, April 7, 2009).
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photo by: Olga Dietrich ( CC/flickr) <flickr.com/photos/_dietrich/2402860858/>.