Understanding Solar Classifications - COLORBOND® steel
As shown in this example for Surfmist®, the thermal efficiency of COLORBOND® steel cool roofing colours are indicated with two key measures: Solar Absorptance (SA) value and Solar Reflectance Index (SRI)1.
The Solar Absorptance (SA) expressed as a value between 0 and 1, indicates the proportion of incoming solar radiation that is absorbed by the roofing material. In this case, a lower SA indicates that the roofing colour choice can help keep the roof space and building cooler on hot days. It is used to classify materials under the National Construction Code (NCC). In Volume One of the NCC, which primarily regulates commercial, industrial and public assembly buildings, the SA of upper roof surfaces is required to be less than or equal to 0.45 to use the ‘deemed-to-satisfy’ pathway to compliance. All COLORBOND® steel cool roofing colours meet this specification.
The Solar Reflectance Index (SRI) provides a metric to compare the roof surfaces. It’s calculated using both the solar reflectance and thermal emittance values described above. The higher the SRI, the cooler the surface temperature of the roof will be in the sun.
- Solar absorptance has been measured in accordance with ASTM E903-96, Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres. The COLORBOND® steel Solar Absorptance values are nominal values based on new product and measured in accordance with ASTM E903-96. Solar Reflectance Index has been determined in accordance with ASTM E1980-11, Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces’.