High-performance Windows
(Page 2 of 5)
December/January 2005
By Greg Pahl
Multiple panes of glass are the best way to increase R-values because they increase the number of barriers between the outdoors and the interior of your home. More than two panes are rarely necessary except in extreme climates. Microscopically thin layers of clear metal oxides applied to the glass during production, called “low-emissivity” or “low-e” coatings, further help to reduce radiant heat transfer between panes of glass by reflecting some or all of the infrared radiation and heat flow exchanged between glazing layers. These coatings stop the heat, but let most of the sunshine through. Conversely, low-e coatings also keep your home’s warmth inside during the winter.
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“Most low-e coatings are designed to make a minimal sacrifice of visible transmission [clarity] while providing dramatic improvements in U-value and solar heat gain,” says Howdy Goudey, an engineer with the Lawrence Berkeley National Laboratory (LBNL). “Sometimes a very small reduction in visible transmission of a low-e window is noticeable if it’s viewed side by side with a clear glazing, but it would be nearly impossible to distinguish an entirely low-e glazed house from one without low-e.”
The second major factor is solar heat gain, also referred to as the solar heat gain coefficient (SHGC). Put simply, the SHGC measures how much of the sun’s energy (both visible and near-infrared light) striking the window transmits as heat into the room. The SHGC is based on a scale of zero (for no heat gain) to 1 (for 100-percent heat gain). The SHGC also measures how much solar heat passes through the frame, rather than just the glass.
In cold Northern climates, you will want high solar heat gain to capture as much of that passive solar energy as you can; an SHGC of 0.55 and higher is recommended for this region. In hotter Southern climates, however, a lower SHGC of 0.4 or less is suggested to keep the solar energy out. In climates where heating and cooling are equally important, an SHGC between 0.4 and 0.55 is best.
The third factor that affects a window’s energy efficiency is air leakage. This is the air that flows through cracks between the sash and the frame, or at the point where the window connects to the wall. The lower the leakage value, the better; look for windows with air-leakage rates of less than 0.3, a measurement of the cubic feet of air that passes through a square foot of window area per minute.
Through the Glass Darkly
To achieve an even lower SHGC than is possible with low-e coatings, it is necessary to start rejecting significant portions of visible light energy using tinted glass. Glass tints are generally the result of colorants added to the glass during production, although some tints also are produced by adhering colored films to the glass following production (but this will negate most window warranties). Tinted glass has drawbacks — it absorbs heat, which can actually increase the SHGC unless the windows have multiple panes. “In controlling solar gain, it is much more effective to reflect solar radiation using low-e coatings rather than to absorb it, because this eliminates the problem of dissipating the absorbed energy,” Goudey says.
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