Though windows are only one target among several products and practices toward a top-notch thermal envelope, there are still rules of thumb to follow to optimize their impact.
Southern Exposure: An all-day exposure, per the sun's path. In heating (i.e., cold) climates, leverage it with a better U-factor (ideally 0.20 or less) but less-efficient SHGC (perhaps 0.50 or higher) to boost heat gain in the winter and offset heating energy; use overhangs or other shading devices to cut down gain in the summer, when the sun is higher in the sky. In cooling (i.e., hot) climates, spec windows with U-factors and SHGC ratings of 0.30 or better and use shading tactics. "If I have to choose between blocking the summer sun and some solar gain in the winter, I'll elect to block it," says Texas custom builder Don Ferrier.
Western Exposure: Solar gain mostly in the late afternoon. Bob Saxler, architectural marketing manager at Andersen Windows, advises builders to focus on this elevation first, as it is the most difficult to control. If possible, orient the house and floor plan away from this exposure, such as situating utility areas, bathrooms, and, ideally, the garage on that side, and specify small and fewer operable (ideally casement) windows with efficient U-factors and SHGC ratings to mitigate solar gain and provide some measure of passive ventilation. If you have a view to the west, he says, boost the SHGC even more and look for multiple shading opportunities inside and out.
Northern Exposure: In this hemisphere, the least opportunity for solar gain. A dual-pane window with a standard low-E coating on the inner face of the outside pane (cold climate) or the outer face of the inside pane (hot climate), is sufficient. "We always recommend a low-E window for north-facing windows for its insulating value alone," says Val Brushaber, director of product management, certification, and architectural development for Hurd Windows & Doors. The number and size of windows can be dictated by views, exterior aesthetics, and floor plan as much as thermal efficiency, though fewer windows is always better in that regard. North is also notorious for prevailing winds, so think about air infiltration and passive ventilation through casement windows (instead of hung units) or fixed windows to lessen leakage.
Eastern Exposure: Rich in daylight, but far cooler than its opposite exposure. You can dial up the SHGC rating to 0.40 or more, especially in heating or mixed climates, while a U-factor of 0.30 is plenty to retard thermal transfer through the window.
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