The article presents a comparative analysis of international and Russian regulatory procedures for determining the design thermal conductivity of building materials and products. It was shown that the methods for determination of design thermal values, accepted by international standards, are more appropriate. Design according to Russian standards leads to an overestimation (degradation) of the design characteristics of thermal insulation materials, resulting in an unjustified increase of 10–15% of the thickness of the insulating layer in the building envelope. In addition, Russian regulations of determining the design values of the thermal conductivity of building materials assume unfounded and unsupported by real operating conditions values of the weight ratio of moisture in the material. The authors believe that in order to improve the situation, in determining the design values of the thermal conductivity of building materials, Russian standards should be harmonized with international standards.