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Vapor barrier under concrete floor

Christa Campbell | Posted in General Questions on

We are building a home with a suspended 4” concrete floor. The subfloor is 1-1/8” plywood and will be covered with 1” rigid foam prior to the pour. We plan on taping the seams of the rigid foam. We are installing radiant floor tubing, with fasteners that will penetrate the foam and attach to the subfloor. The floor is over a conditioned crawl space, which will have its own HRV system. The ground of the crawl space will be covered with a 10mil vapor barrier. There will be batt insulation between the floor joists. We are wondering if there is any reason to lay down a vapor barrier between the subfloor and the rigid foam, given that the vapor barrier will be penetrated repeatedly by the fasteners.
Thanks –

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  1. GBA Editor
    Martin Holladay | | #1

    The answer to your question is no, you don't need polyethylene. By the way -- the 1-inck-thick foam is already a vapor retarder.

    If your crawl space is conditioned, I hope that means that you are planning to insulate the crawl space walls.

    I'm not a fan of fiberglass batts between floor joists above crawl spaces. I would be inclined to advise you to skip the fiberglass batts. If you want more R-value under your hydronic tubing, just install thicker rigid foam under your slab.

  2. Christa Campbell | | #2

    We plan to insulate the framed portion of the stem walls. We were not planning to insulate the cement portion. What do you recommend?
    Given that the sub floor is already installed, we don’t have the option of putting down more than 1” of rigid foam without decreasing the cement thickness. We’re wondering why fiberglass batts between the floor joists aren’t your cup of tea.
    Also, is there any benefit to taping the rigid foam before we cover it with concrete?

  3. GBA Editor
    Martin Holladay | | #3

    To create a conditioned crawl space, you need to insulate the entire wall, on all four sides. The concrete walls definitely need to be insulated. For information on how this work is done, see Building an Unvented Crawl Space.

    As long as your crawl space stays dry, the fiberglass will probably work. The main problems occur when the crawl space is damp. Even in a dry crawl space, however, it's common to see fiberglass batts fall down over time. You need to come up with a durable way to keep the batts in place.

    If I were you, I would tape the seams of the rigid foam before placing the concrete. This will minimize bleed water from leaking between the sheets of foam.

  4. Christa Campbell | | #4

    How would you attach the rigid foam to the concrete walls? The article you referred us to doesn't give that detail. Thanks -

  5. GBA Editor
    Martin Holladay | | #5

    More information on attaching rigid foam to concrete walls can be found here: How to Insulate a Basement Wall.

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