Moisture Management of the Crawlspace: The Vapor Barrier

To protect the house from water vapor entering the crawlspace by evaporation of water contained in moist soil, we cover the gravel base with a high performance vapor barrier. The materials are made using virgin high density polyethylene . This bright yellow product is calledStegoWrap. It’s a tough 10 mils thick and very puncture resistant.

Most crawl spaces are not constructed with vapor barriers and consequently are subject to several gallons a day of water evaporating from the damp soil below. This is the reason that most older crawlspaces are vented by building code so that all that water vapor will be removed from under the house by natural air flow through the crawlspace vents. The trouble is that often there are too few vents, they are blocked or poorly sized and positioned such that the area stays damp…not good.

By installing a vapor barrier none of that water vapor ever enters the crawlspace in the first place.

To be effective, a vapor barrier must be continuous so we overlap the seams by a foot or two and use pressure sensitive tape for airtight sealing of all seams and the perimeter attachment to the foundation wall.

We also drill weep holes in the floor drain riser so that if any moisture does happen to collect on top of the vapor barrier it will weep out the drain.

We are now air, water and water vapor tight with a gravel drainage plane below and floor drain above. This meets the EPA’s tough Indoor Air Plus specification for water-managed foundations and crawlspaces. We are ready to insulate the floor.