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Essentially, it is the ratio of solar gain (due to direct sunlight) passing through a glass unit to the solar energy which passes through 3mm Clear Float Glass. It is referred to as an indicator to how the glass is thermally insulating (shading) the interior when there is direct sunlight on the panel or window.
The shading coefficient (SC) depends on the color of glass and degree of reflectivity. It also depends on the type of reflective metal oxides for the case of reflective glass. Sputter-coated reflective and/or sputter-coated low-emissivity glasses tend to have lower SC compared to the same pyrolitically-coated reflective and/or low-emissivity glass.
It is usually a value ranging from 1.00 to 0.00, but experiments[which?] show that the value of the SC is between 0.98~0.10. The lower the rating, the less solar heat is transmitted through the glass, and the greater its shading ability.
It is known to designers and architects that the SC value plays a significant role in the selection of glass, specially at high-temperature areas. Usually at those areas, low SC is needed to lower the solar heat gain through the glass. It works with the direct sunlight, and with the absence of sunlight SC loses its significance in design.
Window standards have moved away from Shading Coefficient to Solar Heat Gain Coefficient (SHGC), which is defined as that fraction of incident solar radiation that actually enters a building through the entire window assembly as heat gain. To perform an approximate conversion from SC to SHGC, multiply the SC value by 0.87.