R value for High reflectivity Paint

Hi Majeed, thanks for your question. R-value (aka Resistance Value) is one of four measures of heat transfer. The others are convection, radiation and infiltration. The reflectivity of a surface will impact heat transfer through radiation.

R-Value measures insulation effectiveness or heat transfer effectiveness through a material or assembly. The higher the R-Value, the more resistant the material is to heat transfer.

A roof or wall that is well insulated (has a high R-Value) may improve its ability to reject radiant heat transfer on extremely hot, sunny days by having a reflective paint added to it.

The reflectivity of a paint can be measured by the Solar Reflectance Index - SRI, which is a scale of 0-100 with 100 being (e.g. a white roof/wall) the most reflective and 0 being the least reflective (e.g. a tar roof/black wall).

So, R-Value measures the ability of a material to reject heat transfer, and SRI measures a material's ability to reject solar heat gain, which can cause radiant heat transfer. Here's more information on R-Value.