Because crawl spaces are small and unventilated they are prone to excess moisture. Because excess moisture can lead to a variety of problems, you'll want to monitor your crawl space's moisture levels.
Problems Associated with Excess Moisture
Excess moisture in your crawl space can cause a host of problems. Perhaps the most worrisome is mold.
Not only can mold cause severe structural damage, particularly in homes where the crawl space is located underneath the house, but excess moisture may also cause skin and eye irritation, asthma and other respiratory problems.
Ideally, moisture levels should be kept below 50 percent relative humidity (RH). According to the Environmental Protection Agency, this will help prevent problems with mold, dust mites and other biological pollutants.
Moisture Control Methods
A dehumidifier can be used to reduce the amount of moisture in the air. Many dehumidifiers also include a device which reads the relative humidity level.
As an alternative, consider installing an exhaust fan. When shopping for an exhaust fan, look for one that produces a minimum of 10 air changes per hour.
For crawl spaces located under your home, consider laying a vapor barrier, available at most home supply stores, on the ground. A vapor barrier in a crawl space will help prevent ground moisture from moving up and condensing or entering the home.