Thermal resistance and transmittance are calculated in accordance with EN ISO 6946 (Building components and building elements – Thermal resistance and thermal transmittance – Calculation method) and, by taking into account the possible additional correction factors ΔU in the national building codes.  Requirements are also given in the national building codes.

In the calculation of thermal resistance and thermal transmittance the design thermal values, λ_design of each component are used. These are determined in accordance with EN ISO 10456: Building materials and products – Procedures for determining declared and design thermal values and, by taking into account the possible additional correction factors Δλ in the national building codes. Tabulated values can also be used.