Heat, air and moisture transport modelling in ventilated cavity walls

Abstract

Cavity walls are a widely used external wall type in north-western Europe with a good moisture tolerance in cool humid climates. In this work, a cavity wall configuration with a brick veneer outside leaf and a wood fibre board inside leaf is analysed with a newly developed coupled computational fluid dynamics–heat, air and moisture model. Drying of the outside or inside cavity leaf, both for summer and winter conditions was analysed. The new model was compared with a widely used simulation tool for building envelope analysis (WUFI®) that uses a simplified modelling approach for the convection in the cavity. The study showed that the simplified model overestimated the drying and moistening rates of the cavity wall compared to the detailed model. For both models the drying of the outer leaf was mainly determined by the outside conditions, and the outside leaf dried out mainly to the outside and not to the cavity. For the inside leaf, however the cavity ventilation was of major importance in drying. The study revealed that the simplified model could not be used to evaluate the drying potential of a ventilated cavity because it overestimated the ventilation effect systematically. The simplified model would in such case indicate lower moisture contents than in reality and consequently lower risk for mould growth, wood rot or other structural damage. Only detailed modelling of the convection in the cavity, as in the new model, leads to a correct evaluation of ventilated cavity walls.