As the insulation acts as a sponge, it holds moisture against the wood joists raising the moisture content of the joists. Mold requires organic material, warm temperatures, and relative humidity above 60% to grow. Often we see mold growing on the paper backing of the insulation and on wood joists in the crawl space. As the moisture content in the joists increases, the probability for wood rot becomes more likely. Once the joists reach 19% moisture wood rot begins.