TechNote - Floors above Crawl Spaces: Reducing the Risk of Moisture Accumulation within Wood Floor Assemblies

Floors above vented and open crawl spaces in hot-humid climates are susceptible to moisture accumulation that could lead to mold, mildew, and decay within the floor assembly, even where crawl spaces are constructed in accordance with building codes.

Water vapor generally migrates from areas of higher temperature and relative humidity to areas of lower temperature and relative humidity (vapor drive). In a humid climate, during the cooling season, the vapor drive is from the crawl space to the relatively cool and dry indoors through the floor assembly. A vapor impermeable floor covering can trap moisture in the wood subfloor Conditions within the crawl space can lead to condensation and high wood moisture content.

This TechNote provides design recommendations to reduce the risk of moisture accumulation within wood floors above crawlspaces in hot-humid climates – IECC climate zones 1A, 2A, and lower portions of 3A below the warm-humid line. The factors that affect moisture accumulation include season, indoor temperature, types of floor insulation and floor covering, and type of crawl space. The focus of this TechNote is selecting floor insulation and coverings to control moisture migration and allow drying, should the floor get wet, for vented and open crawl spaces.