The higher the R-Value the better thermal resistance the product will provide. Units: m2K/W.
R-Value = Thickness (m) / Thermal conductivity (W/mK)
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.
To achieve a specific thermal resistance, increased product thickness is more economically efficient than using a product with a lower thermal conductivity.