Although much has been written about passive solar design, and some mention is made of selecting glazings appropriate to the building aspect, sourcing windows with glazing both optimized for passive solar buildings, and reasonably priced can be less than straight forward. Window manufacturers in the U.S. tout the insulative properties of their windows (their u-value) to reduce heating loads, and how well they exclude solar heat (low Solar Heat Gain Coefficient, or SHGC) to reduce cooling loads. Both of these qualities are achieved with a combination of double glazing, and low-e coatings. However, by using glazings optimized for low u-value and high SHGC, south facing windows can contribute significantly to the winter heating of a house. Most factory wood and clad-wood window offerings in the U.S. only include low-e glazings optimized to exclude solar heat. This is presumably because the vast majority of homes are designed without regard for the sun, a single window brand may be distributed across very diverse climate zones, and the prescriptive energy codes dictate low u and SHGC values, but give no credit for passive heating. It is easier to offer a handful of glazings which will work reasonably well at meeting code, at the lowest cost, in both heating and cooling dominated locations.
On one of our first passive solar designs to be built we clearly told the client (and builder) that they needed to chose glazing appropriate for the window’s aspect. She chose one of the premier residential window brands for her new guest house and art studio, located in the high desert of the southern Utah. During the first winter the building was occupied she contacted me wondering why it required more supplemental heating than I had predicted. Upon investigation, I realized the window’s high performance glazing was as described above, and excelled at heat exclusion. Since then we have designed a number of homes in that neighborhood, all of which successfully use tuned glazing, and which perform as designed.
Fortunately, there are a few windows available well suited to passive solar homes, and hopefully more in the future. All these companies offer glazings suitable for non-south windows as well. Loewen offers an array of double and triple glazings, including triple clear which meets code minimum for u-value in most climate zones, and offers a high solar heat gain coefficient (SHGC). Eagle windows are available (although this isn’t mentioned in their literature) with Alpen’s heat mirror glazings, which have suspended films tuned for different purposes. Experience with ordering this seems to vary between dealers. Cardinal recently introduced a low-e film optimized for solar heat gain, Low-E 179. Semco offers it as an option; ask and encourage other brands who use Cardinal glazing to do so. Serious Materials is a relative newcomer whose offerings look very promising, addressing glazing tuning as well as overall window efficiency. A number of high performance Canadian fiberglass windows are available as well, although at a significant premium.
A high SHGC glazing can let in over twice as much heating energy as a glazing optimized for cooling, with only a small reduction in insulative performance. After designing a passive solar home with a large collector area of south facing windows, appropriately shaded in the summer, take the time to spec appropriate windows, and educate the client and contractor about the importance of the glazing choice.
Kalen Jones is a founder and principal at With Gaia Design. With Gaia provides sustainable architecture and landscape architecture design, consulting, and education services. Their focus is passive solar homes, civic and commercial site design.