How to Seal a Dirt Crawl Space

This house foundation is sealed with polyethylene sheeting.
This house foundation is sealed with polyethylene sheeting. (Image: Hemera Technologies/ Images)

Unless your home has a full basement or was built on a concrete slab, chances are it has a dirt crawl space. If this is the case, it really should be sealed against ground moisture ingress. Even with proper venting, moisture can build up underneath your home and cause deterioration and warping of structural members, as well as creating conditions that allow termites and other pests to live and breed. Once you seal the dirt floor of your crawl space, the unwanted moisture can dissipate.

Things You'll Need

  • 6-mil polyethylene sheeting
  • Polyethylene tape
  • Utility knife
  • Spray foam insulation
  • Flashlight

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Roll out a length of sheeting 4 feet longer than the house. Cut it off the roll. Cut additional sections until you have enough to cover the width of the house, and an additional 6 to 8 feet.

Crawl under the house. Pull one of the lengths of sheeting along with you. Spread the sheeting out along one side wall. Pull the sheeting approximately 2 feet up each side wall and tape it to the wall. As you work, remove any rocks or debris that may be present.

Pull the next length of sheeting under the house. Spread it out so that it overlaps the preceding piece by at least 6 inches, and tape it to the preceding piece as well as to the end walls. Continue in this manner until you have the entire crawl space floor covered.

Spray a bead of foam insulation around the perimeter of the house inside the crawl space between the top of the foundation wall and the sill plate, which is the horizontal wood runner that the floor joists sit on.

Tips & Warnings

  • Take note of any cautions on the can of spray foam, especially regarding ventilation concerns.
  • Watch out for animals that may be living under the house, including but not limited to spiders, scorpions and snakes.


  • "Insulating, Sealing and Ventilating Your House"; William Perkins Spence; 2006
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