Solar Reflectance Index

This is the best measure for the effectivity of Heat Reflective Paints. It is a value that incorporates both solar reflectance and emittance in a single value to represent a material's temperature in the sun.

SRI quantifies how hot a surface would get relative to standard black and standard white surfaces. It is calculated using equations based on previously measured values of solar reflectance and emittance as laid out in the American Society for Testing and Materials Standard E 1980. It is expressed as a fraction (0.0 to 1.0) or percentage (0% to 100%).

From: U.S. Environmental Protection Agency

So How Is SRI (Solar Reflectance Index) Calculated?

The index is based on a mathematical formula that combines solar absorbance and solar flux (reflectivity), thermal emissivity (emittance of heat), the Stefan Boltzman constant, as well as other coefficients.

The SR Index is based on:

• Standard black (reflectivity 5%, emissivity 90%) having an index of 0, and
• Standard white (reflectivity 80%, emittance 90%) having an index of 100.

But very hot surfaces can actually have negative values below 0 on the SRI and very cool surfaces can be rated with an index of more than 100, like the better SRI paints we've reviewed.