Q:What are R-values of building materials? Can R-values be added to get the total R-value? The R-value is an insulating property of a material, also referred to as the thermal resistance. Based on particular materials that manufacturers use, the R-values vary considerably. For R-values individual to your project always check manufacturer literature. R-Values can be added. R-final (or R-total) is calculated as the sum of the single components: R1 + R2 +… + RN = R-final for example R-5 + {Read More}