The vents in this crawl space are already sealed, but it is not enough to prevent moisture from building up in the crawl space. There are dozens of places for air carrying moisture to leak in, including the sill plate, rim and band, crawl space door, and any other penetrations in the foundation wall to the outside. Sealing the crawl space does not prevent moisture from building up, as the temperature in the crawl space changes, so does the relative humidity.