In ever crawl space, we take a humidity reading. This tells us the relative humidity (RH) of the space, or how much moisture is in the air. This crawl space has a RH of 72%. At about 60% RH, mold and mildew growth can begin to grow as long as it has warm temperatures and an organic food source. In the crawl space, this food source is typically the wood floor joists or paper backing of the insulation. Moisture is also being absorbed by the fiberglass insulation and wood sub structure of your home.