How Energy Efficient are Double Glazed Windows?
Double-glazed windows have two panes of glass, separated by a cavity containing air or an inert gas such as argon. The primary purpose of double glazing is to save energy.
A calculation system called the U Value is used to measure energy efficiency. That is, it shows how effectively a window (or another part of a structure) retains heat; in other words, the amount of heat energy it gains or loses. The lower the U value, the better: Obviously, the more energy efficient it is, the more energy it saves.
For example, a typical U value for single glazing might be 5.6. With double glazing, the U value may vary, from about 1.5 to 2.8, depending upon the characteristics of the glass used, and upon whether the cavity between the panes of glass is filled with argon gas or with air.
Energy efficiency of windows, whether double or single glazed, is considerably affected by the type and thickness of glass used. Low-e or low emissivity glass has a coating that reduces the amount of ultraviolet or infrared light that passes through it, without blocking the passage of visible light.
Reducing the emissivity improves the insulating ability of the glass. Thus a double-glazed window made with low-e glass will have a lower U value than one made of uncoated glass and will provide an additional reduction in the cost of heating or cooling the home, and also a greater reduction in the building’s carbon footprint.
Double glazing greatly reduces the amount of heat transmitted through a building’s windows, by 20-25 percent and perhaps as much as 50 percent. This means that less fuel will be needed to heat or cool the building, saving money for the owner as well as reducing the building’s carbon footprint, a very desirable effect.
Energy efficient glazing has additional benefits: It reduces condensation on the inside of windows, and it provides additional insulation from external noises, which can be an important factor.
A related property of windows is their Solar Heat Gain Coefficient (SHGC). Although not the same as energy efficiency, it is important to know that a lower SHGC indicates that a window transmits less solar heat. This is helpful in summer, when less solar heat is desired, but not so desirable in winter, when the additional warmth may be welcomed.
A consumer can recognize products that meet Department of Energy and Environmental Protection Agency criteria by the Energy Star designation, labeled according to the zone for which they are suited.
Additionally, the NFRC (National Fenestration Rating Council) label provides a means for consumers to compare among Energy Star products, as it gives product information regarding U-factor ratings, and also other properties (e.g., SHGC, Visible Light Transmittance or VT, and sometimes air leakage and condensation resistance).
Research and attention to product labeling will enable a consumer to determine the optimal value for money when choosing among various double-glazed windows; by being informed, engaged and attentive he or she can arrive at the best product choice for his or her particular circumstances.
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