How do you calculate thermal insulation? It is measured in m2. K/W. The R-Value is calculated as R = l/λ where l = the thickness of the material in metres and λ (lambda) is the thermal conductivity of the material in W/m.K. Most Common Thermal Insulation Materials Insulation Material Price / Sq. Ft. R-Value / Inch Fiberglass $ R-3.1 […]