Crawl spaces are prone to moisture for a variety of reasons including improper ventilation, leaky duct work or piping and local weather conditions. Crawl spaces are also prone to moisture coming up through the soil.
Crawl spaces are unexposed to sunlight and not well ventilated. Their floors are often bare soil. This is a natural recipe for moisture. Additionally, some of the most common solutions for keeping moisture levels in crawl spaces down actually exacerbate the problem.
A frequent line of thinking in home construction or remodeling is that the more vents a crawl space has the better its circulation and the drier it will be. Numerous vents are added along the base of a home to facilitate air circulation through the crawl space with the only result being greater amounts of moisture going in. This is due to air not moving laterally through crawl spaces, but rather moving vertically through the home above. This is known as the "stack effect."
To combat the stack effect, some homeowners attach plastic sheeting to the floor joists in an attempt to keep moisture from entering the home above the crawl space. This causes moisture to condense on the plastic sheeting and become trapped up against the wood, causing rot and fungus to develop. In rainy areas clogged rain gutters prevent water from being effectively moved away from the side of a home, and this water may enter the crawl space.
A ventilation fan with a humidity detector is an effective way to keep crawl space moisture levels down.