An Explanation of Window Glazing
Homeowners and home builders choose a window glazing option depending on their geographical location. Energy Star, a U.S. Environmental Protection Agency program, uses an energy performance rating to gauge the energy efficiency of windows according to climate. Check the labeling before you purchase windows to make sure they're suitable for your area. There are four window-glazing options: single-glazed, double-glazed, low-E-glazed and triple-glazed.
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R-Value and U-Value
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The R-value of windows generally ranges from R-1 to R-3.2, with the higher value representing a better insulating property with more resistance to heat flow. Windows are also measured by their U-value, a measurement of the rate of heat loss. The lower the U-value, the more energy-efficient the window.
Single-Glazed Windows
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Usually seen only in older homes, a single-glazed window consists of one pane of glass. As a result, nearly all solar energy---light and heat---passes through this glass, and energy loss is considerable. Typically paired with storm windows in winter and screens in summer, it provides little energy efficiency, with only a R-1 insulating value.
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Double-Glazed Windows
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Built directly into the window frame are two panes of glass separated by an air space in double-glazed windows. These R-2 windows work well in most parts of the country, and energy heating bills can be slashed by as much as 25 percent compared with single pane.
Low-E-Glazed Windows
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Low-E, low-emissive glass, is a microscopically thin metallic film set between the two panes of glass in a double-glazed window. With a R-3.2 value, it nearly doubles the efficiency of double-glazed windows by blocking major heat loss. Because nearly all ultraviolet light is also blocked, it dramatically reduces the fading of your home furnishings.
Triple-glazed Windows
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Triple-glazed windows are a good bet if you live in an exceptionally cold climate. As the name implies, these R-2.9 windows are composed of three layers of glass, and although your initial cost might be hefty, it will eventually pay for itself with the savings in heating costs.
Background
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High energy costs precipitated the popularity of insulated windows---windows containing more than one pane of glass. Eighty percent of residential windows sold in the U.S. were double-glazed, and 15 percent were triple-glazed, by the late 1980s, according to Mark Miller et al., authors of "Audel Complete Building Construction," 2004. They further add that because of concern about global warming and the strong interest in energy conservation, extensive window research and tougher energy codes are ongoing.
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- Photo Credit Image by Flickr.com, courtesy of Jeremy Levine
