For labeling energy-efficient windows, ENERGY STAR® has established minimum energy performance rating criteria by climate. However, these criteria don't account for a home's design, such as window orientation.
In heating-dominated climates, major glazing areas should generally face south to collect solar heat during the winter when the sun is low in the sky. In the summer, when the sun is high overhead, overhangs or other shading devices prevent excessive heat gain.
To be effective, south-facing windows should have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer. See Energy Performance Ratings to learn more about these ratings.
Windows on east-, west-, and north-facing walls should be minimized while still allowing for adequate daylight. It is difficult to control heat and light through east- and west-facing windows when the sun is low in the sky, and these windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used only for lighting. Low-emissivity (low-e) window glazing can help control solar heat gain and loss in heating climates.
In cooling climates, particularly effective strategies include preferential use of north-facing windows and generously shaded south-facing windows. Windows with low SHGCs are more effective at reducing cooling loads.
Some types of glazing help reduce solar heat gain, lowering a window's SHGC. Low-e coatings—microscopically thin, virtually invisible metal or metallic oxide layers deposited directly on the surface of glass—control heat transfer through windows with insulated glazing. Tinted glass absorbs a large fraction of incoming solar radiation through a window, reflective coatings reduce the transmission of solar radiation, and spectrally selective coatings filter out 40% to 70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted. Except for spectrally selective, these types of glazing also lower a window's VT. See Window Types to learn more about glazing, coatings, tints, and other options when selecting efficient windows.
If you're constructing a new home or doing some major remodeling, you should also take advantage of the opportunity to incorporate your window design and selection as an integral part of your whole-house design—an approach for building an energy-efficient home.