What Is It?
Solar gain is the positive contribution to the heating of a building’s interior made by the sun’s energy passing through a window.
Placement and Orientation
Where the windows are situated in the design of the building and the direction they face are significant factors affecting solar gain. South facing windows take advantage of solar gain in winter because the sun is low in the southern sky. North facing windows will have virtually no exposure to the direct winter sun and are usually net losers of energy. South and west facing windows can cause overheating in summer as a result of unwanted solar gains from the hot afternoon sun.
A simple yet often overlooked factor that affects solar gain is the amount of glazing in the window. A window with a wide frame and numerous small panels divided by mullions and muntins has a much smaller total glazing area to capture solar energy than a window of the same size, but with a narrower frame and single, undivided glazing area. The greater the proportion of glazing to frame area, the greater the amount of sun light that passes through, and the greater the potential for solar gain.
A single glazed window will allow the most light and energy from the sun to pass through. A double glazed window, compared to a single glazed window, reduces solar gain by about 10%. A triple glazed window reduces it a further 10% for a total of 20% less than a single glazed window. However it is important to understand that the gains in thermal performance offered by double and triple glazed windows over single glazed windows far outweigh the reduction in solar gain.
Window technology such as low-E coating also can reduce solar gain for double glazed windows by 10% to 40%, depending on the coating.
The shading of a window will affect solar gain as well. Roof overhangs and well placed deciduous trees planted in front of south and west facing windows will protect against overheating in summer but allow beneficial solar gain during the winter months.
Curtains and drapes can control the amount of sun light entering a building as well as the amount of heat loss that occurs when the sun is not shining. Keeping curtains open on sunny winter days allows maximum solar gain and then closing them at night, or on overcast days, reduces heat loss. Care must be taken to ensure proper installation that allows adequate air circulation between the curtain and the window to avoid condensation.