Greg Kallio gives advice on choosing windows
My teenage daughter has a good sense of humor. After listening to my ramblings on sustainable building, she claims my ideal home is a well-insulated cave. Yes, an abode with no windows to let heat escape or enter and where the ambient temperature is a constant 55 degrees. Body heat alone could nearly heat the place, and no air-conditioning would be needed. Except for a few appliances and well-placed lights, this home would certainly qualify as a zero-energy building. How did she get this idea? Maybe it’s because I once calculated the air-conditioning load of a well-insulated home with and without windows in our climate zone. She probably remembers me muttering that the windows were responsible for a two-fold increase in cooling load and were surely evil in the eyes of the energy gods.
Despite all my energy-wise rants, I’ll admit our home is not the most energy-efficient structure in town. Back in 2000 when we built the place, I made sure we had high R-value insulation in the attic and walls, radiant barrier roof sheathing, a high-efficiency air-conditioner, low U-factor (low heat loss) windows, and a whole-house fan. However, I neglected several important design features. My worst energy sin was not paying attention to window placement and solar shading.
After 12 years among the beautiful but shady pines and cedars of Forest Ranch, we craved light. So we designed our 2,391-square-foot home with ample windows—343 square feet to be exact. This yields a window-to-floor-area ratio of about 14 percent, most of which is south-facing. The 2008 California Building Energy Efficiency Standards (Title 24) limits total glass area to a generous 20 percent.
Having windows on southern exposures is actually good for wintertime passive solar gain, but they should be properly shaded from the summertime sun. This is where we went wrong. The best way to shade windows is with extended roof overhangs. If you are handy with geometry, it’s not too difficult to determine an overhang length that shades a window from the high summer sun but allows the lower winter sun to shine in. If windows or glass doors are located well below the roof overhang, shading can be accomplished with awnings, removable reflective covers or landscaping.
When selecting windows for our climate zone make sure they have a U-factor of 0.35 or less. This typically corresponds to a double-glazed, argon-gas, low-e type. Window areas on north, east and (especially) west exposures should be minimized while still allowing for adequate daylight. In particular, east and west exposures should have a solar heat gain coefficient (SHGC) of 0.35 or less due to low sun angles. Southern exposures should have the most glass area with shading to block the direct rays of summer sun. These windows should have a relatively high SHGC (0.5 or greater) to allow good transmission of wintertime rays. Furthermore, deciduous trees can help shade east, west and southern exposures from the summer sun. For more information, check out www.efficientwindows.org/selection.cfm.