To control mold it's important to keep a crawlspace humidity level below 70 percent. Readings taken in wood at 16% - 20% range indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture. The excess moisture source should be corrected if found.
Readings in the 20% - 28% - range taken in wood indicate that conditions are border-line for decay. Surface molds may develop.
A scientific study conducted by Advanced Energy (www.crawlspaces.org) concludes that Southeastern US homes with closed crawl spaces perform much better in terms of air quality, structural health and energy-efficiency than their vented crawl counterparts.