Understanding Solar Heat Gain Coefficient and Shading Coefficient
The solar heat gain constant and shading coefficient are interchangeably used as the property of building façade. However, both properties are different.
Let us explain the difference between the solar heat gain coefficient (SHGC) and the Shading Coefficient:
Solar Heat Gain Coefficient (SHGC)
SHGC of a glass indicates that how much percent of solar radiation incident upon a glazing incident upon a glazing assembly that ends up inside the building thermal energy.
Or we can say that it Is the ability of a material to resist the heat gain from solar radiation.
Being a ratio SHGC is Dimensionless and can range from 0 to 1 theoretically. But in actual practice products have SGHC between 0.2 to 0.9. The high value of SHGC is not desirable as it enables the high percentage of solar radiation reaching inside the building envelope.
SHGC considers both the heat from solar radiation transmitting through the glass as well as the thermal energy absorbed and transferred inward from the glass and the frame.
The SHGC also has a concern as if the SHGC value is very low the glass will become transparent leads to various drawbacks.
Shading Coefficient (SC)
SHADING COEFFICIENT (SC) is the ratio of the solar heat gain through a fenestration product to the solar heat gain through an unshaded 1/8-inch-thick clear double strength glass under the same set of conditions. In essence SC is an index of the effective solar rejection performance of a given glass type.
Still, the entire fenestration (i.e., combination of the exterior shading component, glass, and interior solar controls such as drapes or blinds) is taken into significance.
In case of glass assembly, SC is based only upon the glass portion of the glazing assembly – it does not consider the frame.
Like SHGC, SC is also a dimensionless ratio having a value rests between 0 to 1 for different materials. SC is also useful for expressing the effects of external or internal solar controls.
Vertical Fenestration U-factor (W/m2·K) and SHGC Requirements as per ECBC
|Climate||Maximum U-factor||Maximum SHGC||Maximum SHGC|
|Hot and Dry||3.30||0.25||0.20|
|Warm and Humid||3.30||0.25||0.20|
Although SHGC and SC are similar but differ in significant ways. Curiously, procedures for merging SC and SHGC values have not been entirely ratified. Design & construction professionals should always be aware of which specific metric is being referenced in any analysis.