What is the Solar Reflectance Index (SRI)?
The Solar Reflectance Index (SRI) shows how well a material reflects the sun's heat. Therefore, the temperature of this material increases slightly when exposed to sunlight.
Thus, while a value of 0 is assigned for a black reference material, a value of 100 is assigned for a white reference material. In other words, for the first case, the black reference material is characterized by being very poorly reflective and capable of widely dissipating the absorbed heat. In the case of the white reference material, on the other hand, it is characterized by high reflectivity and minimal heat emission.
More specifically, according to the standard ASTM: E1980-11, the Solar Reflectance Index (SRI) is the relative ST (Steady State Surface Temperature) of a surface with respect to a standard white (SRI=100) and a standard black (SRI=0) in standard Solar and solar conditions.
Thus, the higher the Solar Reflectance Index (SRI) value obtained in laboratory testing of a surface, the lower the heat energy transmitted by said surface. Therefore, this material will help mitigate the urban heat island effect.
Solar albedo and thermal emissivity
While solar reflectance allows the ability of a material to reflect solar radiation to be evaluated, the emissivity of a material is the ability of said material to emit infrared radiation.
More specifically, solar albedo is the fraction of solar flux reflected by a surface, according to the aforementioned rules. On its own, thermal emissivity is the ratio of the radiative flux emitted by a surface at a given temperature to the flux emitted by a blackbody at the same temperature.
Therefore, a surface with low solar reflectance has the characteristic of absorbing a large amount of solar energy. A portion of this absorbed energy is conducted to the ground and buildings. On the other hand, another part is convected into the air - a fact that causes its temperature to rise. Another part of this absorbed energy is radiated into the sky.
Likewise, the standard states that low-emissivity surfaces do not radiate effectively into the sky and therefore heat up.
That's why, as the standard explains, determining solar reflectance and thermal emissivity, and subsequently calculating the solar reflectance index (SRI), can help designers and consumers choose the right materials for their energy-efficient buildings and communities .
