Moisture in crawl spaces

If you have condensation on the ductwork and floor joists in your home's crawl space, usually outside air is the source of the moisture. It is often better to close foundation vents in the summer. Make sure that groundwater is not draining into the crawl space and there is plastic sheeting (vapor barrier) greater than 6-mil over the earth to prevent soil moisture from evaporating into the crawl space.