Moisture and temperature conditions in cold lofts and risk of mould growth

The paper presents some results from a theoretical study of moisture and temperature conditions in cold lofts and the risk of mould growth on the roof underlay in various climates in Norway. The computational model developed for this study calculates the heat, air, and moisture balances within the loft considering parameters describing the building, building materials, outdoor climate as well as parameters influenced by the occupiers. Both stack effect and air leakage distribution are included in the model as air leakages play a key roll both for the moisture content of the indoor air and for the moisture transport in and out of the loft. In order to perform quantified parameter studies a mould growth potential is introduced. By using simplified relations between the relative rate of mould growth and temperature and relative humidity, respectively, a total mould growth potential is summarized for successive calculation periods. Traditionally cold lofts in Norway have been vented, by openings at the eaves, gables and the ridge, in order to prevent snow melting and to give the roof drying out capability. Closed lofts, without openings to the exterior, however, have several advantages like improved protection against spread of fire reduced risk of snow drifting into the loft and reduced heat loss due to reduced air flow through the roof insulation. The study shows that closed cold lofts may be constructed with low risk of mould growth. The roof however, has to be correctly designed using roof underlay with sufficiently low vapour resistance and