Probably the best windows for passive solar gain come from Germany, where building codes require higher-performance techniques than in the United States, and where the Passivhaus movement—which focuses on eliminating mechanical heating and cooling systems—is strong. Some reputable manufacturers are Internorm, Optiwin, Pazen EnerSign, and Unilux. These manufacturers offer triple-pane windows with U-factors in the 0.10 to 0.15 range combined with SHGC above 0.50. These units are expensive, however, at upwards of $100 per square foot of window space (not including shipping). What is available depends on what dealers are nearby. Remember that you only need low U-value, high SHGC windows for the south side of your home if you are building a passive solar home, so it may make sense to use a combination of windows from different manufacturers to keep costs reasonable. German manufacturers benefit from having ready access to low-iron glass (almost all iron oxide has been removed for clarity), developed primarily for that country’s booming PV market.
North American windows can be half the price of European ones, and shipping and lead times are substantially reduced, although design, operation, and energy efficiency are generally reported to be a notch below their German counterparts, according to NFRC certifications. There are dozens of window manufacturers, so a comprehensive review isn’t within the scope of this article. Accurate Dorwin, Duxton, Fibertech, Inline Fiberglass, and SeriousWindows are reputable North American manufacturers. U-values typically range from 0.15 to 0.30 for higher-quality windows.
So, where to begin? If you’re interested in highly energy-efficient windows, you probably won’t find them at your big-box hardware store, although acquainting yourself with the prices of average windows will give you a starting point. The best place to start is the Window Selection Tool on the Energy Efficient Windows Collaborative website. This is a location-specific tool that will generate a window availability list based on criteria such as triple-pane or a high SHGC. Your window purchase can rival buying a car in cost and complexity, and the more legwork you do for your specific project and location, the better off you’ll be.
Stephen Hren is a builder and writer. He is the author of Tales from the Sustainable Underground and coauthor of A Solar Buyer’s Guide for the Home and Office (see www.earthonaut.net).
Energy Efficient Windows Collaborative • efficientwindows.org • Window industry info
Passivhaus windows • bit.ly/HPwindows • Sourcing low U-value, high SHGC windows
National Fenestration Rating Council • nfrc.org • List of window manufacturers