Solar heat gain coefficient

Understanding the solar transmittance through translucent and transparent materials such as glass is important for determining the solar heat gain into the space they enclose during sunny conditions. Solar heat gain can be beneficial in the winter, as it reduces the need for heating, but in the summer can cause overheating.

The total solar heat transmittance through transparent and translucent materials is equal to the solar heat that is transmitted through the material directly, plus the solar heat that is absorbed by the material and then re-emitted into the enclosed space. Until recently this was expressed in terms of a shading coefficient which described the amount of solar heat transmitted through a material compared to the amount of solar heat transmitted through a standard sheet of clear float glass 3mm thick.

However, manufacturers are now moving away from shading coefficients. In the USA, they are moving towards the use of solar heat gain coefficients (SHGC) and in Europe, window solar factors or g-values. In essence, these both represent the fraction of incident solar radiation transmitted by a window, expressed as a number between 1 and 0, where 1 indicates the maximum possible solar heat gain, and zero, no solar heat gain. The difference between US and European systems is that they use a different value for air mass.

Actual solar heat gain is dependent on the angle of incidence of solar radiation on the glazing (and so the proportion of diffuse and direct beam solar radiation) as well as the spectral make up of the solar radiation. However, as a simplified method, manufacturers will often only provide a spectrally-averaged solar heat gain coefficient for normally-incident solar radiation. Values for other angles of incidence and for diffuse solar radiation can then be estimated using standard equations or tables for similar windows.

Solar heat gain coefficients, as with g-values can refer to the centre-of-glass SHGC or can relate to the entire window, including frame.

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Comments

The definition of SHGC on this page is incomplete as it does not indicate whether the input gain is global (direct + indirect) or just direct, and it does not give the sun angle.

CORRECTION - Actual solar heat gain is dependent on the angle of incidence of solar radiation on the glazing (and so the proportion of diffuse and direct beam solar radiation) as well as the spectral make up of the solar radiation. However, as a simplified method, manufacturers will often only provide a spectrally-averaged solar heat gain coefficient for normally-incident solar radiation. Values for other angles of incidence and for diffuse solar radiation can then be estimated using standard equations or tables for similar windows.

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