Prediction of ventilation, heat transfer and moisture transport in attics.

The attic space of most residential buildings is well insulated from the house below, and therefore experiences extreme temperature conditions on seasonal and diurnal cycles. This can cause moisture entering the attic through the ceiling from the house, or through other leaks from outside, to accumulate in or on interior wood surfaces. This moisture accumulation leads to structural degradation and the growth of micro-organisms.

An attic simulation model has been developed for this study as a practical method of examining attic moisture problems. The model was validated using measured data from the Alberta Home Heating Research Facility. After validation the attic simulation model was used in parametric simulations over a wide range of ambient weather conditions to examine strategies for moisture control. The new methods of attic simulation and the other important contnbutions of this study are:

• the development of a two zone attic ventilation model that calculates the house, attic and interzonal (ceiling) flow. The ventilation model calculates wind shelter using a new wind shadow method and calculates the shelter and wind pressure coefficients as continuous functions of wind direction.
• the coupling of heat transfer and ventilation models.
• the use of a complete transient mass balance for the attic moisture that includes the wood surfaces.
• the use of combined ventilation heat transfer and moisture transport models.
• calculation of ventilation rates, temperatures and moisture levels using envelope leakage, indoor temperature and relative humidity, and ambient weather conditions
• the development of a comprehensive data base of measurements for evaluating attic models and identifying important parameters for attic moisture accumulation.

The results of the simulations have shown that increased attic ventilation rates are not always a useful strategy for controlling moisture problems and that appropriate strategies are dependent on climate and attic construction. At low ventilation rates the capacity for moisture removal is small and at high ventilation rates the additional cooling of the attic causes more moisture deposition. The optimum ventilation rate for a maritime climate has been found using the model developed for this study.