The effect of thermal transmittance of building envelope and material selection of wind barrier on moisture safety of timber frame exterior wall

• Moisture safety of low thermal transmittance timber frame walls can be achieved.
• Capillary active insulation decreases the risk of mould growth.
• Lower thermal transmittance increases the risk of mould growth.
• The drying out period is longer in walls with lower thermal transmittance.
• The nZEB needs a more careful hygrothermal design than the current practice.

The nearly zero energy buildings (nZEB) ideology of the future obliges, first and foremost, that heat losses should be reduced remarkably compared to the present levels. The current study examines the potential hygrothermal risks and their effect on highly insulated timber frame exterior walls under cold climate conditions. The focus is on the timber frame exterior walls with thermal transmittances between 0.17 and 0.08 W/(m2 K), with different material combinations and boundary conditions, where the risk of mould growth as a performance criterion was used. A careful selection of materials allows to design moisture safe timber frame exterior walls and provide low thermal transmittance. It was found that the relative humidity and the risk of mould growth are higher and the drying out period is longer in walls with lower thermal transmittance when insulation thickness is the only changed parameter. Wind barriers with higher thermal resistance and water vapour permeability indicated a lower increase of mould growth risk in structure in the course of reduction of the thermal transmittance. Therefore, the building envelope of the future nZEB needs a careful hygrothermal design.