Controlling Home Moisture for Indoor Air Quality
Page 19
UNHEATED crawls
space
HEATED crawl
space 
Crawl spaces need moisture control similar to that used in a basement to keep the soil in the crawl space dry. There should be a vapor retarder covering the floor of the crawl space that is bonded to the footing wall. A vent should be available to remove moisture if it accumulates in the crawl space. The vent is normally closed to keep moisture from the outdoor air entering the crawl space.