On the crawl space moisture control in buildings

Abstract

Received 5 December 2002, in revised form 24 January 2003 Abstract. This study considers transient effects of the moisture capacity and other properties of ground covers and base floor on relative humidity in cold-climate outdoor-air-ventilated crawl spaces. The objectives of the study were to find out how relative humidity can be reduced by optimal selection of ground covers and air change rates, and to evaluate the acceptability of achieved moisture conditions by means of mould growth analyses. Two buildings, one with a relatively warm and another with a relatively cold crawl space, were studied with the resistance- capacity network model including the heat and moisture transfer in crawl spaces. Thermal and moisture buffering effects of various ground covers and air change rates were simulated. In a relatively warm crawl space the moisture problems were easy to avoid - all ground covers gave clearly acceptable conditions at an air change rate of 0.5-2.0 ach. In the cold crawl space, the moisture conditions were much more critical. The acceptability of conditions was evaluated by calculating the mould growth index. To achieve acceptable moisture conditions, 15-30 cm light- weight expanded clay aggregate or 5-10 cm expanded polystyrene ground cover must be used. An air change rate of 0.5-1.0 ach provided the lowest relative humidity conditions during the heating season, and in the summer it was necessary to use an air change rate of 2.0-5.0 ach to warm up the crawl space. In critical conditions, thermal insulation and moisture capacity proved to be important properties of the ground cover allowing to achieve acceptable conditions.