Open windows or vents don't usually provide enough ventilation to warm a crawl space or basement adequately to prevent condensation. They can actually introduce warm moist air, which condenses on the cooler wall surfaces and ground. The best solution is to close windows and stop air from leaking into the crawl space. To be sure that moisture is not evaporating from the soil, cover the soil with heavy plastic. Overlap the plastic and weight it down or seal it at the joints and around the edges. Operate a dehumidifier to lower the moisture level in the basement or crawl space if condensation or dampness is still occurring.