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Plastic sheet over concrete slab – what about mold?

mrcrisp | Posted in General Questions on

Hi, I will be installing engineered hardwood over a concrete slab on my ground floor (which functions as a basement, part of which is below grade). The floor manufacturer recommends placing a 6-10 mil plastic sheet over the slab to prevent water vapor from damaging the engineered hardwood.

I am concerned about the moisture being “trapped” between the slab and the plastic sheet, and fear that it can create mold on the slab. There is no vapor barrier under the house, since it was built in the 60s. Can anyone please clarify for me why mold would not be a potential concern here? 

 Thank you in advance for calming my fears! I am a first time homeowner and eager to learn.

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  1. Expert Member
    BILL WICHERS | | #1

    Any mold would be contained by the poly along with the moisture, so it wouldn't contaminate your occupied space. The general rule is that as long as the "problematic thing" is contained by some kind of barrier that thing can't penetrate, you'll be OK.


  2. MartinHolladay | | #2

    Mr. Crisp,
    Here's another way of looking at it: soil is moldy. For years, the soil directly under your slab has been full of mold. But apparently, you haven't worried about that fact.

  3. mrcrisp | | #3

    Thank you both, I really appreciate your replies. I was under the impression that mold growth on the concrete slab could lead to structural problems, but it sounds like that's not a concern of occurring under the plastic sheet?

  4. nickdefabrizio | | #4

    I agree with the above. Also. mold needs organic material to grow. When mold grows on glass or concrete it is usually because there is a thin layer of organic material on the surface that supports the mold. The mold does not have "deep roots" like it might in wood. Perhaps if the floor is washed and left to fully dry before the poly is put down there will be less organic material for mold to grow on?

    If you do decide to clean the surface first, I suggest you do not use bleach or mold cleaners-just use mild detergent to clear off the organic material. A few years ago after a flood I used a store bought mold cleaner to clean off a masonry surface. Something was in the masonry (perhaps old cleaning chemical residue or even the chemicals in the concrete itself) that reacted with the mold cleaner chemicals and created a toxic situation and smell that took a while to clear.

  5. Expert Member
    Akos | | #5

    Vinyl flooring (either glue down or click) is pretty common in basements around me in zone 5. Vinyl flooring is a true vapor barrier, so pretty much the same thing as a sheet of poly.

    Generally it works without problems except if there is a lot of moisture under the slab. In this case there will be condensation under the flooring leading to mold. Mold is a very noticeable smell to a lot of people so even though it is bellow the flooring you can still smell it.

    The check soil moisture I would tape down a large square of poly over the concrete and let it be for a while. If you see condensation on the poly, you have moisture issues. To deal with moisture you either need better drainage and/or a vapor barrier coating (ie epoxy or redguard) over the concrete. Poly is not the same as a coating as it is not directly adhered and there is still a small air gap where mold can grow.

  6. jollygreenshortguy | | #6

    Is this a new build? I would encourage you to give the slab as much time as possible to cure before putting the wood floor down. The slab moisture content should be checked before installing the floor. This is not my expertise but from what I've read, ideally it should be between 7-9% and you should not install wood if it exceeds 12%. Also, it's a good idea to store the flooring in the space where it's going to be installed for 2 weeks or so. That way it adjusts to local conditions before installation.

    If the flooring manufacturer recommends a vapor barrier over the slab I'd certainly do it and wouldn't be concerned about mold for the reasons mentioned by others.

    1. Expert Member
      MALCOLM TAYLOR | | #7

      From the OP: "There is no vapor barrier under the house, since it was built in the 60s."

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