Patricia Skinner, Reichel, Claudette H.
A great deal of moisture can move from the ground into and through a foundation and floor system, which can lead to decay, mold under flooring and high indoor humidity. To prevent these problems, keep rainwater away from the foundation with a 5% slope (6 inches/10 feet) of the grade around the house, prevent materials from absorbing water, drain groundwater with underground drainage before it gets to the foundation and allow the moisture in a foundation to escape.
If the crawl space under the house is not completely open (house on piers), it’s helpful to cover the ground with plastic sheeting to block moisture evaporation from the ground into the enclosed crawl space. The ground level inside the crawl space should be higher than the outside ground level to prevent rainwater entry and ponding. Crawl space openings are needed for access, flood insurance requirements and/or to satisfy existing code requirements, but they are not an adequate means of moisture control. Research is underway to confirm the moisture performance of alternative floor insulation systems.