The effects of air change and ground covers on crawl space moisture balance in a cold climate are discussed in this paper. The objectives were to assess the suitability of outdoor air-ventilation in the crawl spaces of apartment buildings, to determine the optimum air change rate with and without ground covers, and the effect of the ground covers' thermal insulation on moisture behaviour. Measured data from the test building was used to develop the crawl space model in a modular simulation environment, where the parametric simulations were carried out. The air change rate was varied between 0-10 ach to study moisture behaviour with covered and uncovered ground surface. Moisture evaporation was included in the moisture balance, but moisture storage in the constructions and in the ground was not. The results show that evaporation and thermal behaviour are the key elements determining the resulting relative humidity in crawl spaces. If moisture evaporation is entirely prevented, the crawl space may be left unventilated. In other cases, ventilation is always required to remove the moisture. The higher air change rates increased moisture evaporation from uncovered ground, but still brought about lower relative humidity. The rise from 0.5 to 3 ach increased evaporation from 2.4 to 4.9 g/m2 h and decreased the highest monthly average of relative humidity from 81 to 74%. For the uncovered ground the lowest relative humidity was achieved at 2-3 ach air change in winter, but in the summer, the higher the air change the lower the, relative humidity. Ground covers made it possible to decrease relative humidity and to increase temperature significantly due to reduced moisture evaporation. Ground cover with thermal insulation was in principle more effective than without insulation as it provided a slightly higher temperature rise in summer. The studied 5 cm expanded polystyrene layer reduced moisture evaporation to 0.3-0.4 g/m2 h, increased crawl space temperature by 2-3°C and decreased relative humidity below the 60% level when air change was 0.5-1 ach. In general, outdoor air-ventilation demonstrated very high performance in the crawl space of the studied apartment building when a ground cover was applied.