Fiberglass insulation is pervasive but is a bad choice for a crawl space, or any other area with natural ambient moisture (humidity) issues. First, the paper backing is exactly the type of organic material that lets mold thrive. Second, the insulation itself absorbs the water in the air until it gets too heavy to stay up, and then simply falls to the ground. Waterproof and inorganic insulation is needed in a crawl space.