How Heat Reflective Paint Works
It’s not magic—it’s physics. And good marketing sometimes forgets the science.
The Two Things That Matter
Emissivity & Reflectance
A proper heat reflective paint does two jobs: it reflects sunlight before it turns into heat, and it emits any absorbed heat back into the atmosphere instead of trapping it. That’s reflectance and emissivity.
The best way to know if a paint is doing both? Look at its Solar Reflectance Index (SRI). That number tells you how well the paint reflects and releases heat, rolled into one useful score.
If the product doesn’t list its SRI, that’s a red flag. You’re buying guesswork instead of performance.
Solar Reflectance Index
SRI – 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%).
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:
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.
Not Heat Reflective Paint
If a coating retains heat, it will pass it to the substrate and into your house! Emissivity is the quality of the coating to emit energy by radiation.
Where a coating cannot reflect 100% of the solar radiation, what it does with the heat energy not reflected (absorbed) will affect the temperature inside your roof. That means that the greater the emissivity of the coating, the less heat is passed into your home.
Emissivity is one-half of the Solar Reflectance Index (SRI) calculation.
Insulating Paint
Solar Reflection measures the amount of solar radiation (heat energy) that is reflected by the paint. Obviously the greater the reflection, the less chance of heat entering your home.
Solar reflection is one half of calculating the Solar Reflectance Index (SRI).