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 Window technology has improved dramatically in recent years, with the net result of lowering your energy bills. Some of the most important energy features of windows are explained below.
Multiple layers of glazing
Until the 1980s the primary way manufacturers improved the energy performance of windows was to add additional layers of glazing. Double glazing insulates almost twice as well as single glazing. Adding a third or fourth layer of glazing results in further improvement. In the 1970s, with rising concern over energy, triple-glazed and even quadruple-glazed windows entered the market. Some of these windows use glass only; others use thin plastic films as the inner glazing layer(s).
Thickness of air space
With double-glazed windows the air space between the panes of glass has a big effect on energy performance. A very thin air space does not insulate as well as a thicker air space because of the conductivity through that small space. During the 1970s a lot of window manufacturers increased the thickness of the air space in double-glazed windows from 1/4" to 1/2" or more. If the air space is too wide, however, convection loops between the layers of glazing occur. Beyond about 1", you do not get any further gain in energy performance with thicker air spaces.
Low-conductivity gas fill
By substituting the air in a sealed insulated glass window for a denser, lower conductivity gas such as argon, heat loss can be reduced significantly. The largest window manufacturer in the country today, Andersen Windows, uses argon gas-fill in all of its insulated glass windows, and most major manufacturers offer argon gas-fill as an option. Other gases that have been or are being used in windows include carbon dioxide (CO2), sulfur hexafluoride (SF6), krypton (Kr), and argon-krypton mixtures.
Tinted glass coatings
|Tinted glass and tinted window films have long been used in commercial buildings to reduce heat gain through windows. Improved, lightly tinted windows are becoming more common for the residential market in southern (cooling-dominated) climates. These new glazings reduce the solar heat gain without reducing visibility as much as older tinted glass and films.
Low-e Coatings
More than any other single improvement, the invention and commercial development of low-emissivity (low-e) coatings in the 1980s revolutionized window technology. Thin, transparent coatings of silver or tin oxide permit visible light to pass through, but they effectively reflect infrared heat radiation back into the room. This reduces heat loss through the windows in the winter.
A variety of low-e windows are now available for different climate zones and different applications in any particular location. Low-e windows with high solar gain coefficients are appropriate for northern climates where passive solar heating is advantageous, while southern low-e windows with low heat gain coefficients are appropriate in milder climates where summer cooling is more significant than winter heating.
The different properties of these low-e glazings sometimes make it advisable to choose different types of glazing for different sides of your house. We realize it may be difficult to find a builder or contractor interested in customizing window glazings for the four sides of your house. However, we encourage you to begin thinking about new windows from an informed energy perspective. For example, if you want to benefit from passive solar heating, for the south side choose windows containing a top-performing low-e glass with a high solar heat gain coefficient. On the north, install the lowest U-value windows you can afford. Or to keep things simpler, you can order the same glazings for the east-, west-, and north-facing windows.
Some window manufacturers now produce both "northern" and "southern" climate low-e products. But other window manufacturers may still offer just one type of low-e glazing as their standard, and charge extra for substitutions—if they provide options at all. So a decision to choose different glazings for windows of different orientations may require some extra shopping around. If you do order different glazings for your different windows, be sure to keep track of which windows have which type of glazing, because they will probably all look identical!
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