Mold needs 3 things to grow; 1. Organic Material (wood joists or the paper backing of insulation) 2. warm temperatures, and 3. High Relative humidity (over 60%). Crawl Spaces in Greater Richmond often meet these conditions especially during the hot humid summers Virginia is accustomed to and many homes see mold growth on their joists. If the joists reach 19% moisture content, wood rot can begin.