What windows are best for a passive solar home?
Even in a place as cold and dark as Fairbanks, you can get net solar gain through south-facing windows every month of the year except December. So “passive solar” homes are definitely possible in Alaska, but only if you can minimize the amount of heat you lose through the windows during the long dark days of winter. So how do you do this?
One solution is insulated exterior shutters. With the right shutters on your home, you can benefit from net solar gain for 11 months.
Unfortunately a heavily insulated shutter system has yet to be invented, but in typical Alaska style people are building their own versions–everything from a piece of two-inch blue foam to a raise-able shutter than can be engaged from the inside with a hand crank. Keep in mind that, as with all sources of energy, you will make maximum use of solar gain by having an extremely well-insulated building shell. An under-insulated, leaky house will not get the same results.
In a cold climate, you want a window with a low U-value, which represents the rate of heat transfer through the glass (and is usually listed on a sticker on the window or is available from the dealer). Currently, highly efficient homes in Fairbanks (rated 6 Star) are required to have windows that are U-2.5 or lower, which typically translates to a triple-pane window.
When it comes to glass and coatings, nearly all windows geared toward a cold climate will have some kind of low emissivity (low-e) coating that reflects radiant heat back inside the home. The coating allows some short-wave infrared energy from the sun to enter the house while minimizing how much long-wave infrared radiation escapes from the heated space through the glass, a balancing act that depends on the types of coatings used and which side of the pane they are on. When it comes to windows, it’s worth the time to do the research before making a purchase.
Another question is how much of the wall’s surface area should be glass. To comply with 6 Star home requirements, the total window surface area should not exceed 15% of the above-grade wall area. If you exceed 15%, you will have to make up for those energy losses somewhere else, which usually means adding more insulation to another part of the building. The location of the glass also factors in; south-facing works best while north-facing should be minimized.
For more information on passive solar design, check out “A Solar Design Manual for Alaska” produced by the University of Alaska Fairbanks Cooperative Extension Service.