Moisture conditions in highly insulated outdoor ventilated crawl spaces in cold climates

Abstract

This study simulated the effects of thermal insulations on the ground, in the foundations and in the base floor in order to achieve as dry conditions as possible in outdoor air ventilated crawl spaces in a cold climate. The objectives of the study were to find out how the thermal capacity and resistance and the placement of insulation layers affect relative humidity (RH) in the crawl space. The possibilities to reduce the air change rate to very low levels corresponding to natural ventilation, and to control RH by minimising the thermal capacity present in the crawl space were simulated. Mold growth analyses were used as performance criteria to predict the acceptability of the hygrothermal conditions in the studied crawl spaces. Most of the simulations were carried out with a low U-value for the base floor which normally results in a low temperature and high RH in the crawl space during the summer. The results show that there are two alternative ways to use ground covers in combination with air change in order to achieve acceptable conditions in crawl spaces. First, a traditional thin layer of lightweight expanded clay aggregate (LWA) or expanded polystyrene (EPS) may be used, but in this case, the air change rate has to be increased in the summer at least to 1 ach. An alternative solution is to use thicker ground covers, such as at least 30 cm lightweight aggregate or 10 cm polystyrene, and a low air change rate of approximately 0.5 ach all the year round. This solution provides even lower RH values and mold growth index values because the thick ground covers effectively insulate the massive ground whereby the crawl space warms up quickly when the warm season starts.