The R-value (= thermal resistance) of an insulation system reflects the extent to which the insulation succeeds in preventing heat transport. The higher the R-value, the better the insulation system. To calculate the R value of a structure which is composed of several layers, we will add the R values. Formula: R value = insulation thickness / λ value. Indicate here the thickness and the lambda value (in W/mK) of the insulation to calculate the R value.