Vapour control design of wooden structures including moisture sources due to air exfiltration

Sealed wooden structures such as unvented flat roofs have acquired a bad reputation due to frequently reported moisture damage caused by vapour convection. While better detailing and workmanship may considerably improve the air-tightness of a roof assembly, field observations indicate that it is impossible to achieve a perfect air barrier in building practice. Therefore, the new German standard on wood protection specifies a convective moisture source for vapour control design analysis of roofs with exterior seal. This convective source is added as a safety margin to the amount of condensate caused by vapour diffusion when Glaser type calculations or hygrothermal simulations are performed. The results show that applying such a safety margin to a sealed roof structure with vapour barrier leads to failure due to moisture accumulation. Replacing the vapour barrier by a moderate vapour retarder can provide enough drying potential towards the interior spaces to compensate the effect of the convective source under favourable conditions. The paper demonstrates how the exterior climate conditions, the roof cover layer and the height of the building (stack effect) influence the moisture performance of flat roofs.