A finite element based hygrothermal model consisting of several interconnected components with varying number of spatial dimensions was applied to analyze the time-dependent temperature and humidity conditions of a mechanically depressurized and ventilated crawl space. Purpose of the depressurization is to prevent the intrusion of radon or other insanitary particles into indoor air. However, in typical foundation structures the depressurization will cause airflow from soil into the crawl space air and it may convey excessive moisture making the hygrothermal conditions potential for mould growth or other moisture-induced biological damage, which is not considered to be acceptable even with the depressurization. Although in general the forced convection of humidity from soil presumably increases relative humidity in crawl space, significant heat capacity of the ground may warm the air flowing into the crawl space and thus decrease the relative humidity. Overall effect of the depressurization on the conditions in crawl space is therefore not trivial. Because a ful-scale three-dimensional finite element analysis of heat, mass and momentum transfer in crawl space and its surroundings would require excessive computational resources, several simplifications were necessary to apply in the model. According to the numerical results, the airflow through drainage layer into crawl space does not seem to have severe effect on the crawl space conditions. Conversely, in cold periods the relative humidity in crawl space is very low because of the air temperature is increased while flowing through the drainage layer.