Attic ventilation and moisture.

Ventilation with outdoor air has been the accepted method for controlling moisture and overheating in residential attics. This is recognized in the Building Code which requires that the total vent area must be l/300th of the attic floor area. However, how much ventilation is provided and how this ventilation affects moisture accumulation in an attic are not well understood. The present study attempts to provide some answers to these questions. The study consists of two parts. First, detailed measurements were carried out in two separate attics located at a field test site. The full-sized attics were instrumented to measure ventilation rates, indoor-attic exchange rates, temperatures, and moisture contents with these measurements extending over a two year period. Second, a model was developed to predict the moisture performance of an attic for a given leakage configuration and weather conditions. The model consisted of three interactive sub-models: a ventilation model that predicts attic ventilation rates (and interior house ventilation rates) for the above inputs, a thermal model that predicts the temperature distribution, and a moisture model that predicts moisture distribution within an attic. The model was verified with the field data and showed reasonably good agreement with measurements. The model was used in a number of simulations. First, several parametric simulations were carried out where both attic configuration and climatic para.meters were varied systematically; these included attic leakage, use of mechanical ventilation, outdoor temperature and relative humidity, wind speed, and cloud cover. These results helped to identify which parameters where important in influencing moisture levels in attics. A second set of season-long simulations were carried out using actual weather data for a number of Canadian cities in different climatic zones. This report highlights some of the measurements and model calculations that were performed. An appendix to this report provides a more extensive compilation of these results and is available from the Canada Mortgage and Housing Corporation. ·