To evaluate the radiative thermal performance of a roof and therefore its ability to have a cooling effect (or not), two important properties are measured:

• Solar reflectance (or albedo), corresponding to the percentage of reflected solar rays.
• Thermal emissivity, allowing the coating to reject the little heat absorbed towards the sky, in far infrared radiation.

These two properties must be maximized to obtain the most effective cool roof effect. They are between 0 and 1 (or 0% and 100%).

To characterize a reflective coating, an index is used: the Solar Reflectance Index (SRI) – not to be confused with solar reflectance – which is a calculation that takes into account both the solar reflectance property and the thermal emissivity property of a material. The higher the index, the greater the cool roofing effect. We can consider that a reflective coating is very efficient from a SRI higher than 100 (value corresponding to standard white, snow).

As an example, a bituminous membrane roofing has an SRI close to zero and a raw metal roofing has an SRI of about 50.