When moisture buildup occurs, high moisture in crawl spaces can result. The most common source of moisture inside a crawl space is related to a combination of moisture vapor rising out of the uncovered ground and warm, high-humidity, air entering the crawl space through the perimeter foundation vents during the summer months. Whenever warm, humid air enters the crawl space, the warm air temperature drops about 20 degrees or more since the temperature in the crawl space is much cooler than outside air.
When the temperature drops, the warm, humid air must give up its moisture and this moisture condenses on cool objects such as air conditioning ducts, the wood floor (exposed to interior air conditioning) and the ground surface. As this moisture collects, the humidity in the crawl space increases. Wood is a hygroscopic material, meaning that it tries to reach a moisture equilibrium with the surrounding atmosphere–either absorbing moisture in high humidity or off-gassing moisture in low humidity.