Modeling attic humidity as a function of weather, building construction, and ventilation rates

A dynamic model for predicting attic relative humidity (RH) and roof sheathing moisture content (MC) was developed for microcomputer application. The model accepts standard hourly weather data and building design parameters as input. Model predictions gave good agreement with measured data from a house located in Madison, Wisconsin. Solar radiation varies with roof orientation and plays an important role in determining moisture transfer to and from the roof sheathing. Opposing roof surfaces must be differentiated in attic humidity models to account for the effect of solar radiation. The model described in this paper is capable of such differentiation. Snow accumulation on a roof can significantly alter the temperature and moisture conditions in an attic, but further research is needed to understand the effect of a snow layer on attic temperatures. Various scenarios were simulated with this model to determine the effect of building practice and ventilation strategies on roof sheathing MC. Direct control of RH in the living space by ventilation is very effective in lowering attic moisture conditions. Where natural ventilation is not adequate, a timer-controlled attic fan shows great promise for ensuring efficient and economical attic ventilation.