Since 1977, when Sweden introduced its stringent energy code, almost all new homes in Sweden have been equipped with triple-glazed windows. Here in the U.S., where energy codes are more lax, triple-glazed windows are still rare.
For a minority of U.S. builders, however — especially cold-climate builders of superinsulated homes — triple-glazed windows are considered essential. Since few U.S. manufacturers offer high-solar-gain triple-glazed windows, most Americans get these windows from Canadian manufacturers.
Look for a low U-factor and a high SHGC
In any climate, a window with a low U-factor performs better than one with a high U-factor. The lower the U-factor, the better. (For more information on low-U-factor windows, see “Passivhaus Windows.”)
Most cold-climate builders want windows with a high solar heat gain coefficient (SHGC) and a high visible light transmittance (VT). After all, solar gain helps heat a house during the winter. During the summer, when solar heat gain is less desirable, a properly sized roof overhang will shade south-facing windows during the hottest hours of the day. (For more information on the desirability of windows with a high SHGC, see “High-Solar-Gain Glazing” and “Windows That Perform Better Than Walls.”)
Most low-U-factor windows have a SHGC that is unacceptably low — at least for cold climates. Designers of cold-climate houses have to balance conflicting needs — the need for windows with a very low U-factor and as high a SHGC as possible. Most builders end up choosing triple-glazed windows with a U-factor ranging from 0.19 to 0.26 and a SHGC ranging from 0.39 to 0.47.
The higher the visible light transmittance (VT), the better. Windows with a low VT look gray and depressing. To get an idea of what range you’re looking for, consider the advice of Robert Clarke, a technical specialist at Serious Energy and the former president of Alpen Windows. According to…