High-performance Windows
(Page 3 of 5)
December/January 2005
By Greg Pahl
Tinted glass usually is selected for aesthetic purposes. Reducing visible transmission means reducing natural interior lighting, which ultimately means you’ll have to use additional interior lighting and deal with its associated energy load and heat input, Goudey says. In general, choose windows with high visible transmission that freely admit daylight, minimizing the need for artificial lighting. “Most people want as much visible light as possible,” says Christopher Mathis, president of MC Squared, an energy-efficiency consulting firm in Asheville, N.C. “In general, I don’t think that tinted glass is a good choice for residential settings.” At the very least, get a sample of the glass before you order your windows. Take the sample outside and look through it carefully from a variety of angles before you make a final decision. For special situations when tinted glass is desirable, the visible transmission range can run from as much as 90 percent for almost clear glass down to a rather dim 15 percent for some heavy-tinted varieties.
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The appropriate type of low-e coatings and tints actually can be different for different parts of your home. In a cold climate, select windows with high solar heat gain properties for the south side of your home, and windows with lower heat gain for the north, east and west sides. If you have questions, seek the advice of a dealer or professional consultant.
Between the Panes
Determining the right combination of coatings for your windows is important, but what’s between them also is a matter of concern. All double-pane glazing is held apart by a “spacer.” Less expensive thermal-pane windows typically use aluminum spacers. Unfortunately, aluminum is highly conductive, which causes the edges of these windows to be colder and encourages condensation, or on cold days, frost formation on the inside edges of the glass. This moisture can induce the growth of mold and rot. To diminish this problem, higher quality windows use less conductive materials such as stainless steel, foam, plastic or rubber — all of which can increase the edge temperature by about 5 degrees and improve the U-value of the entire window unit by about 10 percent. Insulated “superwindows” with three or more layers of glazing and low-e coatings can eliminate condensation.
Another popular strategy to improve the insulating properties of the window is to replace the air inside the thermal pane with a gas such as argon (the least expensive and most commonly used) or krypton; both are harmless to humans. Some concerns have been expressed about leakage of these gases over time, which would reduce energy efficiency. But this is not a major problem with most high-quality windows. “There are some very, very low leakage rates of a fraction of a percent per year,” says Stephen Selkowitz, department head of the Environmental Energy Technologies Division at the LBNL. “In 20 years this might amount to a 10-percent change in the gas.” That would be an unmeasurable impact on the total performance of the window.
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