Sunlight streaming through a window can lift your spirits and keep you in touch with nature and the changing seasons. But you can get too much of a good thing. That's why window glass matters when building a green, energy efficient house. While windows bring in generous amounts of natural daylight, that can be a drawback if the glass has no insulative or reflective properties to manage the ultraviolet (UV) light responsible for fading furnishings and the infrared light responsible for heat gain.When looking for new windows, Energy Star's® Climate Zone Map is a great place to start. Most manufacturers design windows based on these zones. Two numbers to look for when choosing energy efficient windows are the U-factor and solar heat gain coefficient (SHGC).
U-factor indicates the rate at which heat travels through glass. SHGC refers to the amount of heat that travels through glass. The lower the U-factor, the slower the rate of heat flow; the lower the SHGC, the less heat comes through. So, windows for northerly climates should have a low U-factor and relatively high SHGC while windows for southerly climates should have just the opposite. For example:
U-Factor SHGC U-Factor SHGC
North.....0.30 or less 0.40 or more North-Central......0.32 or less 0.40 or less
South.....0.60 or less 0.27 or less South-Central......0.35 or less 0.30 or less
Low-E and other coatings, thickness of glass, amount of space between panes, use of Argon gas and frame material all affect U-factor and SHGC. It may sound complicated, but choosing the best windows for your home can be easy. Just follow Energy Star® recommendations for your climate zone. And let the sun shine in (selectively, of course)!