How Windows Are Rated for Energy Efficiency
Last updated in November 2016
Modern windows are constructed to reduce your home’s wintertime heat loss, enhance its wintertime heat gain from the sun, and suppress its summertime heat gain.
Within each window style and construction option, you’ll find windows available that provide different levels of energy efficiency—in general, the more efficient the window, the higher its price.
By choosing efficient windows, you conserve energy. That saves you money, saves world resources, and reduces your contribution to pollution. In addition, preventing heat from escaping your home will make your home more comfortable. You will eliminate cold areas around windows.
When choosing among windows, look for several factors that measure energy conservation performance:
Construction and Insulation
Modern “insulated” windows are constructed using two or three panes of glass, with air or an inert gas such as argon or krypton filling the space between panes. The air or gas is an insulator, more resistant to the passage of heat than solid glass. Usually one or more of the panes are glazed with a thin transparent film of metal that reduces the pane’s ability to radiate heat; this is called a low-E coating. In high-quality windows, the materials of the frame itself are poor heat conductors. The spacer material that separates the panes of glass around the edges is another issue. Aluminum, a common spacer material, is highly conductive, transferring heat from pane to pane; rigid foams and other less conductive materials replace aluminum in some windows.
This measures the ease with which heat passes through the window. In order to minimize heat loss, you want windows with a low U-factor. Old windows with a single pane of glass in an aluminum frame might have a U-factor of about 1.3; some new high-tech windows have U-factors as low as 0.1. In this area, the U.S. Department of Energy currently requires that new residential windows have U-factors of .27 or lower. Sometimes U-factors are calculated for the glass area only. That is not very useful; you should get information on the U-factor for the entire window assembly.
Solar Heat Gain Coefficient (SHGC)
This expresses the fraction of the sun’s radiation that is admitted through a window. In cold weather, a high SHGC number is desirable for windows that receive a lot of direct sun exposure. The heat from solar radiation a home takes in through a south-facing window may be great enough to more than offset heat loss through the window, making the window better than a well-insulated wall from an energy conservation standpoint. On the other hand, in warm weather a high SHGC number in windows that receive a lot of direct sun exposure is undesirable, as the heat from the solar radiation raises indoor temperatures, forcing air-conditioning systems to work harder. The ideal solution is to have a high SHGC on south-facing windows, which get direct exposure to the sun when the sun is low in the sky in the winter, and then shade these windows in the summer with a roof overhang or trees.
In general, as U-factor goes down (when more effective low-E coatings are added to the glass), so does the SHGC measurement. For a south-facing window you may decide to sacrifice some ability to prevent heat loss in favor of a window with high solar heat gain potential. An old window with a single clear pane of glass with no special low-E coating might have an SHGC rating of about 0.70; a double-glazed window with low-E coating on both panes might have an SHGC of about 0.35, meaning it transmits about half as much solar heat as the old model.
This is reported in cubic feet per minute per square foot of window (cfm/sq ft), as a measure of the movement of air (convection) between the inside and outside of a building through cracks in and around the window frame. Windows should have an air-leakage rating of 0.3 or less.
How They Affect Sunlight
The whole purpose of having windows, of course, is to create views and admit sunlight. But on the admitting-light front, not all windows are equal. Light passing through a window is affected by the glazing material (glass or plastic), number of layers of panes, and any coatings applied to the panes. Window-makers use “visible transmittance” (VT) to indicate how much light windows admit. VT ranges from more than 90 percent for clear glass to under 10 percent for windows with highly reflective coatings on top of tinted glass. You may have to sacrifice some visible light that would come through a window to achieve an acceptable level of energy efficiency. This means that what you see outdoors will not look as bright as it otherwise would. The diminished brightness may not bother you—might even please you—on sunny days, but could be undesirable at night.