Shading coefficients basically refer to the fraction of solar heat gain that passes through a transparent solar aperture compared to the amount of solar radiation incident upon it and are expressed as a decimal value without units between 0 and 1.

There are a couple of different types of coefficient that each use a different reference point for comparison.

Solar Heat Gain Coefficient (SHGC)

The solar heat gain coefficient (SHGC) refers to the fraction of solar radiation admitted through an entire window, door, or skylight assembly — either transmitted directly or absorbed and subsequently released as heat into the space behind. The reference point in this case is the total solar radiation incident on the external surface of the aperture assembly.

The lower the SHGC, the less solar heat the object transmits and the greater its shading ability. An object with a high SHGC is more effective at transmitting solar heat gains inside during the winter. An object with a low SHGC is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Therefore, what SHGC you need for a window, door, or skylight should be determined factors such as the climate, surface orientation, and external obstructions.

Shading Coefficients (SC)

The shading coefficient (SC) refers to ratio of solar heat gain through a specific glazing system compared to the total solar heat gain through a single layer of clear, double-strength glass. The reference point here is the transmission of a standard glazing system as opposed to the incident radiation.

This value is being phased out in favour of the SHGC, and is approximately equal to the SHGC multiplied by 1.15 (or divided by 0.87, being the SHGC of single, clear, double-strength glass).