Do I need a vapor barrier in my crawl space?

Short answer: Yes, almost always. The DOE says crawl spaces should have a polyethylene ground cover regardless of whether the crawlspace is vented or unvented, seams lapped 6 to 12 inches and sealed, run at least 6 inches up the walls and piers [1].

What it does

The DOE: the ground cover serves as a capillary break and vapor barrier to keep ground moisture and soil gases from entering the crawlspace [1].

What the code says

For an unvented crawl space, Ohio's residential code (Section 408.3, effective March 1, 2024) calls for exposed earth to be covered with a continuous Class I vapor retarder, joints overlapped 6 inches and sealed or taped, and edges run at least 6 inches up the stem wall and sealed [2]. The Department of Energy says a plastic ground cover should be installed regardless of whether the crawlspace is vented or unvented [1].

What we install

A cleanout first, then new plastic over the ground, seams overlapped and taped, sealed around every post and pier and run up the walls. Then the walls or the floor above get insulated. The DOE notes that standing water, mold or rotten wood should be addressed before air sealing and insulating the crawlspace [3]; we do not do that kind of repair, and if we find it, we tell you before any work starts. Crawl space cleanout, vapor barrier and insulation.

More questions

How much should the plastic overlap?

The 2021 IRC R408.3 calls for 6 inches, sealed or taped, for unvented crawl spaces; the DOE recommends 6 to 12 inches.