The transmittance of a wall is calculated according to the UNI EN ISO 6946:2008 standard. The transmittance is given by the inverse of the total areal thermal resistance RT:

U = 1/RT

The total areal thermal resistance, RT, of a flat building component, consisting of thermally homogeneous layers perpendicular to the thermal flow, is calculated through the following equation:

RT = Rsi + R1 + R2 +…Rn + Rse

  • Rsi is the internal surface resistance (UNI EN ISO 6946:2008)
  • Rse is the external surface resistance (UNI EN ISO 6946:2008)
  • R1, R2 … Rn are the n thermal resistances of each layer, calculated as the ratio between the thickness of the respective material layer and its thermal conductivity.

Surface resistances must comply with UNI EN ISO 6946:2008 based on the direction of thermal flow. However, the Rsi value corresponding to horizontal thermal flow can be used for all surfaces when:

  • the direction of thermal flow is uncertain or subject to variations.
  • the entire building is modeled with a single calculation.

In the case of calculating the resistance of internal components or components located between the internal environment and an unheated environment, Rsi applies on both sides.


Values of individual products useful for thermal calculation:

Thermal conductivity 0.2 W/(mK)
Thermal resistance of adhesive-coating: thickness = 0.002 m _ Rsanakoll 0.010 (m2k) / W
Transmittance of adhesive-coating: thickness = 0.002 m _ Usanakoll 100 W / (m2k)

Thermal conductivity 0.031 W/(mK)
Thermal resistance: thickness = 0.005 m _ Rsanatex 161 (m2k) / W
Transmittance: thickness = 0.005 m _ Usanatex 6.2 W / (m2k)
Emissivity 0.59