Properly designed, energy efficient windows represent a cost-effective way to use solar energy for heating. Windows are an important element in passive solar home designs, which can reduce heating, cooling, and lighting needs in a house.
Passive solar design strategies vary by building location and regional climate. The basic techniques involving windows remain the same select, orient, and size glass to control solar heat gain along with different glazings usually selected for different sides of the house (exposures or orientations). For most U.S. climates, you want to maximize solar heat gain in winter and minimize it in summer.
Heating-Dominated Climates
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 (e.g., awnings) prevent excessive heat gain.
To be effective, south-facing windows usually must 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.
Windows on east-, west-, and north-facing walls are reduced in heating climates, while still allowing for adequate daylight. East- and west-facing windows are limited because it is difficult to effectively control the heat and penetrating rays of the sun when it is low in the sky. These windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used just to provide useful lighting. Low-emissivity window glazing can help control solar heat gain and loss in heating climates.
Cooling-Dominated 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.
The following types of glazing help reduce solar heat gain, lowering a window's SHGC
Low-E Tinted
Reflective
Spectrally Selective.
Most of these glazing types, except for spectrally selective, also help lower a window's VT.
