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Community and Q&A

Vapor barrier over/ under scribed sleepers on concrete slab

Marc Rubin | Posted in GBA Pro Help on

We have an out of level slab by over 1-1/2″. We plan on gluing and screwing scribed sleepers to the concrete, applying a 6 mil poly vapor barrier over the sleepers and then screwing plywood to the sleepers and installing engineered flooring material. There is foam and a vapor barrier under the slab. My question is if I put another vapor barrier over the sleepers… will this trap moisture since there is one already under the slab? We can’t install EPS or foam board because the floor varies so much and spray foam is cost prohibitive and probably unnecessary since we have foam under the slab.We can install the vapor barrier under the sleepers but this relies on the screws (tapcons) to hold the sleepers and I feel better with an adhesive for added strength and to avoid squeaks.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    It would be helpful to know more.

    1. What is your climate zone or geographical location?

    2. How thick is the rigid foam under the slab, and what type of foam is it?

    3. Is there vertical rigid foam at the slab perimeter?

    4. Is this a slab on grade or a basement slab?

    In general, if you have poly under the slab, you don't need additional poly over the slab.

  2. Marc Rubin | | #2

    Zone 5. 2" foam under the slab. There is no foam on the perimeter. The bozo's who poured the slab used built up wood on top of the foundation walls and then poured the slab to that. So essentially the slab is above the foundation walls. Don't ask me why... they obviously didn't care. It's slab on grade construction.

    So I assume my method would that correct? No need for any vapor barrier?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    There is no need for a vapor barrier above your slab.

    But your house is a good candidate for exterior rigid foam (vertical rigid foam) at the perimeter of the foundation, since the perimeter of your slab is uninsulated.

  4. Steve Knapp CZ 3A Georgia | | #4


    Have you considered having a contractor pump in some self-leveling concrete? It might be easier than your current plan.

  5. Marc Rubin | | #5

    I've considered self leveling compound. Concrete guys here are booked solid. I talked to Mapei and they have a product that would work but the cost is high and I'm unfamiliar with the process since we have to cover the entire floor. Martin,,, the slab is new so what about all the moisture released from the slab while curing. Wouldn't that wick up? Would another layer of poly over the sleepers be detrimental ?

  6. User avatar GBA Editor
    Martin Holladay | | #6

    It's always a good idea to let a slab cure before installing finish flooring. How long to wait -- and how dry the concrete needs to be -- depends to some extent on the finish flooring you plan to install.

    If you have specific concerns on this issue, contact the manufacturer of the finish flooring you intend to install, and ask about their requirements for installation over a concrete slab.

  7. Marc Rubin | | #7

    Will do... but do you believe there is an issue if we put poly over the sleepers?

  8. User avatar
    Dana Dorsett | | #8

    Why put the vapor barrier over the sleeper rather than between the sleepers and concrete? Trapping moisture-susceptible sleepers under the vapor barrier subjects them to any residual moisture in the concrete. With the sleepers above the concrete they dry toward the interior. Both the foam board & concrete tolerate moisture well, so it doesn't matter if they remain in the moisture trap.

    If you really insist on glue the sleepers to the concrete, putting vapor barrier on the concrete between the sleepers, and/or a capillary break between the sleepers and plywood limits the rate at which moisture can pass between the concrete and plywood, but it seems like a lot of trouble.

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