Where the floor of a crawl space is soil or gravel,a vapor retarder should be installed covering the ground to prevent moisture from entering the crawlspace. The vapor retarder should be a minimum of 6 mil polyethylene sheeting with the joints lapped at least 6-inches and sealed. The edges should extend a minimum of 6 inches up the wall and be sealed to the wall using adhesive caulking. A 55-pound-or-heavier asphalt roll roofing,laid on the floor and up the walls approximately 6 inches can also be used. The joints should lap at least 12 inches and be sealed.