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Crawl space vapor barrier location

KrisD1957 | Posted in General Questions on

Our home has a crawl space under the living space of the home as well as a partial basement to one side. We had a plastic vapor barrier over the clay floor of the crawl space.

A contractor who did some work on the home advised that this was incorrect, removed the plastic vapor barrier from the clay floor of the crawl space and instead installed insulation along the top of the crawl space (beneath the floor of the home) with a vapor barrier covering this.

Since he has done this, the floors upstairs in our house are cold. Should he have left the vapor barrier covering the clay floor of the crawl space? And if we do cover it again, is it OK to leave the other vapor barrier and the insulation where it is?

Thank you,


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  1. GBA Editor
    Martin Holladay | | #1

    Your contractor made at least three errors. The plastic vapor barrier on the floor of your crawl space was an important membrane to reduce humidity in your crawl space. Removing it was a big mistake.

    Moreover, if you want to install insulation, the place to install it is on your crawl space walls, not your crawl space ceiling.

    Finally, the contractor should never have installed a polyethylene vapor barrier on the underside of your floor joists. That layer of polyethylene is a wrong-side vapor barrier.

    The most important steps you need to take immediately are to remove the polyethylene from the floor joists, and to install a new layer of polyethylene on your crawl space floor. When you get the chance, you should think about insulating your crawl space walls.

    More information on these issues can be found here: Building an Unvented Crawl Space.

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