How to seal crawl spaces

This vapor barrier system started with an optional piece of plastic down the perimeter walls, sealed with a continuous bead of polyurethane sealant. Then the floor piece was overlapped on top of that up the wall and the seam was taped. Rigid foam board insulation was then applied to the walls with another continuous bead of sealant at the top.

Conventional wisdom used to be that crawl space vents were a good idea − open the vents in the warm months to let the moisture out and close them when it’s cold to keep the space warm. But building scientists have studied crawl spaces and all the research now says “get rid of the vents and totally seal crawl spaces from the elements – including the dirt beneath.”

The problem is that vents in crawl spaces actually perform in exactly the opposite way. They act as vacuums by sucking in exterior moisture during the warmer months, producing musty smells, mold spores and other contaminants in the space. And when warm air in the home rises, replacement air gets drawn up from the lowest point in the home, so those crawl space contaminants then get distributed throughout the house. This natural air movement is known as the “stack effect.”

One of the most important steps to ensuring dry, odorless crawl spaces is to manage the outside moisture. Create good drainage around the foundation by sloping soil away from the home at least ½-inch per foot. Ensure all gutters and downspouts are working properly and diverting rain water far enough away from the edge of the house.

Sealing crawl spaces is relatively simple. Following are the basic steps to seal crawl spaces, but regional building codes vary. Always check with your local code enforcement agency about exact details for this type of job – especially when it comes to fire barriers and termite control.

Full concrete crawl spaces

To seal a crawl space with a cement slab floor cover the vents from the inside with rigid foam board insulation, sealing around the entire perimeter of the insulation with expanding foam or a durable and flexible polyurethane sealant.

From outside, make sure no water can infiltrate the vent assembly and work its way behind the insulation. As long as the concrete slab and knee walls were poured correctly and are in sound condition, the space should be properly sealed.

In this crawl space, rigid foam board that qualified as a vapor barrier was already applied to the perimeter walls. To seal it, a vapor barrier was applied along the floor and up the walls. A continuous bead of polyurethane sealant and/or tape and scraps of wood with concrete anchors completed the job.

Dirt floor crawl spaces

Sealing crawl spaces with dirt floors is only a bit more complicated. In most cases, all you need is some 6-mil plastic sheeting, rigid foam board insulation, heavy-duty tape and polyurethane sealant. If conditions require it, you can use a heavier-duty vapor barrier or multiple layers of plastic. If the crawl space gets any kind of regular traffic such as for storage or mechanical maintenance, use a heavy-duty vapor barrier with multiple plies and fiber reinforcement in the fabric.

Begin by covering the floor with the plastic. Overlap any seams by at least 3 inches and completely tape them. Bring the plastic up the walls about 6 inches and fasten with a continuous bead of polyurethane sealant and/or tape. If the walls are chalky, you may need to drill pilot holes and use a concrete anchor in addition to the sealant to attach the plastic to the wall.

You can also use bricks or heavy rocks to keep the plastic in place as you work. Some people stake the plastic down with a landscaping stakes. A few holes typically won’t effect vapor retardation, and the holes allow any water that may accumulate on top of the barrier to drain away.

Cover the exposed foundation walls with rigid, moisture-proof insulation board. Some rigid foam boards also qualify as vapor barriers, in which case you don’t need to cover the walls with plastic. Fasten by applying a continuous bead of polyurethane sealant along the top edge of the concrete walls and pressing the board into it. You usually won’t need any sealant at the bottom.

Insulate the rim joist using rigid insulation, expanding foam insulation and caulk. This will ensure a warm, dry crawl space for decades.