Ground moisture evaporation in crawl spaces

Air change rate, humidity and temperature were monitored in a naturally and a mechanically ventilated crawl space of the test building, in order to determine the ground moisture evaporation rate and the mass transfer coefficient. The average value of the moisture evaporation with uncovered ground was 3.6–5.7 g/h m2, corresponding to the mass transfer coefficient 0.0012–0.0018 m/s. The mass transfer coefficient determined from evaporation showed sufficient agreement with the coefficient calculated from temperature differences. Normally it would have been sufficient to consider natural convection but with high air change, forced convection also had to be taken into account. The higher air change rates increased moisture evaporation, but still brought about lower relative humidity. The lowest relative humidity, 74.5% in summer, achieved with covered ground and air change of 3 ach, indicates that the control of moisture evaporation and thermal behaviour in summer are key elements in the moisture balance.