Mold and Mildew have grown on to the floor joists of this crawl space. When the relative humidity within the crawl space reaches above 60%, alongside a temperature of about 70 degrees or higher, mold will begin to grow on any organic materials. Since approximately 50% of the air you breathe in a home comes from your crawl space, homeowners will smell the musty odors coming from mold growth, and mold spores will leak up into the home and irritate anyone with mold allergies, while prolonged exposure to mold can actually cause asthma.