Dehumidifiers are used to control the humidity in damp environments that are regularly exposed to moisture, such as basements or crawl spaces. Studies show that up to 50% of the moisture in most homes is found in the crawl space and is usually caused by cracks, leaks and faulty plumbing. Crawl spaces and basements are typically cooler than the rest of the house, which means when temperatures drop, vapors condense on the walls and surfaces of these areas.