Transient heat transfer and moisture load in cold attic constructions - A CFD analysis

Mold growth in cold attic constructions has become an increasing problem in Sweden and other countries with cold climates due to the demands on energy efficient building envelopes. Highly insulated building envelopes with cold ventilated attics lead to colder climates in the attic space, which increases the risk for mold growth. In this paper, the transient heat transfer process including natural and forced convection is investigated for a 24-hour cycle in a virtual 3D model of a ventilated attic construction by the use of CFD technology. The momentum and energy equations along with the realizable k-epsilon turbulence model are solved with commercial CFD software. Transient simulations are performed for daily temperature variations for various ventilation rates and a hygrothermal analysis is conducted to estimate the risk for mold growth at the inner roof boundary. A virtual model of a ventilated attic with an underlying layer of porous medium is used for the simulations and the CFD approach is based on a validated model from previous research. The results are the temperature field and the risk for mold growth for different ventilation rates presented as a function of time during the cycle