The U-value, or thermal transmittance, is the rate of heat transfer through the structure. The rate will vary depending on the building structure, material, thickness of wood and insulation. The better the insulation of the building is, the lowest the thermal transmittance will be. The U-value thus indicates if a building is efficient when retaining the thermal energy and shows the quality of the insulation. For wooden log, the average U-value of a 180 mm log is estimated at 0,60 W/m²K, meaning that this value can easily be decreased and reached 0,17 W/m²K if the insulation is adapted.
A prefabricated external wall element with wooden facade presents a U-value of 0,16 W/m² whereas the U value of a prefab external wall elements with plastered facade reaches 0,13 W/m²K.