The effect of ventilation on moisture behaviour in the traditional outdoor-air ventilated crawl space of blocks of flats with uncovered and moist ground surface is discussed in this paper. The objectives were to compare in real conditions the mechanical supply and mechanical extract ventilation to natural ventilation, to determine the rate of ground moisture evaporation, and to test the reduction of humidity with plastic sheet cover. The study was made between April 1997 and October 1998 when the conditions in naturally and mechanically ventilated crawl spaces of the test building were monitored. The air change rate in the crawl space was monitored continuously, as were temperature and humidity. This made it possible to assess moisture evaporation rate from the ground soil. Additionally, evaporation from some types of soil, crushed stone, gravel and granulated clay was measured in laboratory tests. The reported results account for the behaviour of air change and moisture balance, and give certain validity to arguments for optimum ventilation and the reduction of ground moisture evaporation. It was demonstrated that air change is only one important parameter affecting humidity in crawl spaces. Ground moisture evaporation was related to air change rate and pressure conditions: a higher air change led to higher moisture evaporation. Pressure conditions in the crawl space affected humidity notably; these were varied by using supply and extract fans and were monitored continuously during the measurements. Supply ventilation led to the lowest relative humidity, and extract ventilation brought about even higher humidity than did natural ventilation. No high relative humidity values were measured in summertime, and only brief condensation peaks over a few days were detected. During the summer, the relative humidity level with natural ventilation and uncovered ground was less than 85%, and with ground cover or with balanced ventilation it did not exceed 80%. It seems that with supply or balanced ventilation at 1-3 ach and with ground cover applied it is possible to maintain the relative humidity level under 80% in the outdoor-air ventilated crawl spaces of blocks of flats. (C) 2000 Elsevier Science S.A. All rights reserved.