The presence of moisture is what leads to mold, which can be bad for your health and costly to remove. According to the Environmental Protection Agency, the way to control mold is to control moisture. The main source of crawl space moisture is ground evaporation, and ventilation allows this evaporation to dissipate. Proper ventilation not only exhausts moisture from the crawlspace but replaces it with fresh, drier air providing effective moisture control. Ventilation also helps to remove strong odors and dangerous radon gas from the crawlspace.