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Window thermal issues and SIP structures

An article called "Building Envelope," originally written by engineer Keith Elder for the November 2000 issue of www.energyusernews.com, was recently summarized in Panel Building Update, a monthly newsletter written by Perma-R Products. The article focused on generic heat-loss fractions from various components of homes. According to Perma-R's Butch Johnson, this article generated a lot of interest and numerous calls, specifically about the thermal role of windows in a SIP structure.

Elder points out that in a well-insulated home, one built to ASHRAE Standard 90.1, the energy loads in the Pacific Northwest caused by windows alone (53%) is slightly larger than energy loads from all other building shell components combined (infiltration 27%; roof and walls 16%; floor 4%). Assuming this ratio is relatively on target, then more attention should be paid to window choices and design issues. Here are the key guidelines about windows:

1. Minimize window area. Since a good wall insulates roughly seven times more effectively than a good window, reduce the window area to just that required for egress, daylight and views. Less is better. This reduces initial costs, saves on operating costs, and improves year-round comfort.
2. Maximize window U-values. The colder the climate and the higher the cost of your heating fuel, the lower the U-value should be. For Minneapolis, shoot for windows with an NFRC* rating of U-0.30 or lower (R-3.3 or higher). In Boston, Chicago and Denver, select U-0.35 windows. In Dallas, Atlanta and many other moderate locations, you can settle on U-0.38 to U-0.42 windows. (Window U-values are lowered, and interior comfort is vastly improved, when low-e coatings are applied to the glazing. Note that most wood windows with metal claddings lose some thermal protection due to the connection of the metal cladding details.)
3. In your climate, "tune your glass" for solar gains. Today, the Solar Heat Gain Coefficient (SHGC) of windows is rated by the NFRC*. The higher the SHGC (0.60 for clear glass), the more the solar heat energy striking the outside of an entire window unit is transmitted through the glass; the lower the SHGC (0.25), the less solar heat energy is transmitted through the glass. Pick higher SHGC glass for cold climates, lower SHGC for hot climates.
4. On your site, either properly orient your windows or tune your glass. A recent housing performance study in Fort Collins (CO) shows that homeowners suffer significant comfort problems due to haphazard orientation of windows. (One couple declared "the sun is my enemy," due to uncontrollable solar gains in their family great room.) There are three key solutions here. First, improve orientation of windows; south facing is better (when combined with overhangs), east- and west-facing is usually worst, north is good in hot climates. Second, install windows with a SHGC which optimizes the comfort requirements of the home; e.g., reduce heat gains from east and west windows. Third, optimize the exterior shading devices above windows to allow less summertime solar gain and more wintertime solar gain.

Incorporating these window features in your design and product selection process should help you reduce the very large energy penalty associated with many window choices and applications. Furthermore, it will help you get the most out of your SIP investment.

* NFRC stands for National Fenestration Rating Council. Most windows for new homes have been rated by the NFRC. That rating includes the U-value and SHGC. Using the NFRC rating system is the only way to properly evaluate the overall energy performance of a window.