A crawl space is a type of basement in which one cannot stand up. The height may be as little as one foot and the surface is often soil. They offer a convenient access to water pipes, sewer lines, heating ducts, substructures and a variety of other areas that may be difficult or expensive to access otherwise.
Water from the damp ground and moisture seeping through porous concrete can create a perfect environment for mold/mildew to form on any surface in the crawl space, especially cardboard boxes, wood floors and surfaces, drywall and some types of insulation.
It is extremely important to divert rain water away from a residence in order to limit water penetration into the crawl space. Any moisture present in the crawl space earth will remain over an extended period of time.
In some cases the ground infiltration can not be stopped and therefore the water must be pumped out. This involves a drainage system around the interior of the foundation walls and a collection system called a sump well. An automatic sump pump operates when the collected water level is sufficient to operate the pump motor.
It is usually desirable to finish a crawl space with a plastic vapor barrier that will not support mold growth or allow humidity from the earth into the crawl space. This helps insulate the crawl space and discourages the habitation of insects and vermin. Vapor barriers can end at the wall or be run up the wall and fastened to provide even more protection against moisture infiltration. Some pest control agencies recommend against covering the walls as it complicates their job of inspection and spraying.
There are cases where even after drainage systems and vapor barriers are installed the relative humidity is still above 50 percent. In these cases a Crawl Space Dehumidifier is employed to control the relative humidity.