The thermal resistance rating, or R-Value, is the measure used most commonly in the building and construction industry to determine a material’s ability to resist the transfer of heat.
Thermal Resistance is the most important material characteristic that should be defined when specifying insulation. From the formula for calculating, it is apparent that there are two factors affecting the thermal resistance: the thickness of the insulation and the thermal conductivity of the material. Simply specifying thickness of material is not enough.