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Solar gain (also known as solar heat gain or passive solar gain) refers to the increase in temperature in a space, object or structure that results from solar radiation. The amount of solar gain increases with the strength of the sunlight, and with the ability of any intervening material to transmit or resist the radiation.
Objects struck by sunlight absorb the short-wave radiation from the light and reradiate the heat at longer infrared wavelengths. Where there is a material or substance (such as glass) between the sun and the objects struck that is more transparent to the shorter wavelengths than the longer, then when the sun is shining the net result is an increase in temperature — solar gain. This effect, the greenhouse effect, so called due to the solar gain that is experienced behind the glass of a greenhouse, has since become well known in the context of global warming.
When discussing the properties of windows, doors and shading devices, shading coefficients are commonly mentioned properties. Shading coefficients measure the solar energy transmittance through windows.
- g-value is the coefficient commonly used in Europe to measure the solar energy transmittance of glass - called a Solar Factor on some window literature (%) i.e. 53% = 0.53;
- Solar Heat Gain Coefficient (SHGC) is used in the United States and most commonly refers to the solar energy transmittance of a window or door as a whole, factoring in the glass, frame material (wood, aluminum, etc.), sash (if present), divided lite bars (if present) and screens (if present). SHGC may also refer to the solar energy transmittance of the glass alone (sometimes more specifically termed center-of-glass SHGC), in which case it is analogous to g-value.
- Shading Coefficient was succeeded by SHGC in the United States, but older windows and doors may still refer to their SC value. The relationship between SHGC and SC may be approximated as: SHGC = SC × 0.87;
g-values and SHGC values ranges from 0 to 1, a lower value representing less solar gain. Shading coefficient values are calculated using the sum of the primary solar transmittance (T-value) and the secondary transmittance. Primary transmittance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insolation, the amount of radiation that the window receives. The secondary transmittance is the fraction of inwardly flowing solar energy absorbed in the window (or shading device) again compared to the total solar insolation.
Solar gain and building design
In the context of passive solar building design the aim of the designer is normally to maximise solar gain within the building in the winter (to reduce space heating demand), and to control it in summer (to minimise cooling requirements). Thermal mass may be used to even out the fluctuations during the day, and to some extent between days.
In direct solar gain systems, the composition and coating of the building glazing can also be manipulated to optimise the greenhouse effect, while its size, position and shading can be used to optimise solar gain. Solar gain can also be transferred to the building by indirect or isolated solar gain systems.