Seasonal Variation in Airtightness of Two Detached Houses Available to Purchase

Fan pressurization tests on two unoccupied houses were conducted once every two weeks for a period of a year (May 1982 to July 1983) to determine the seasonal variation in airtightness. Both houses are of insulated wood frame construction. House No. 1 was built with more insulation than is required by the local building code, and a polyethylene vapor barrier was applied with special care to improve its airtightness. House No. 2, a less airtight house, was built with various wall construction features and a polyethylene vapor barrier in only two walls.

Indoor relative humidity, indoor and outdoor air temperatures, and moisture content of the stud and top plates of the wood framing were measured at the time of airtightness testing to determine whether a correlation exists between these factors and house airtightness. The results indicate that air leakage varies throughout the year, with the minimum value in late summer and fall and the maximum value in winter and early spring. The difference is more pronounced in the leakier house. There is also indication of a rough correlation between envelope airtightness and indoor humidity ratio.