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Community and Q&A

Concrete Slab and Vapor Drive

Sequence_eternal | Posted in General Questions on

In his book Builders Guide to Cold Climates, Joe Lstiburek sais “if the under slab insulation is not a major vapor diffusion retarder, the slab will be able to dry into the ground, even if the ground is saturated. You should use either a polyethylene vapor diffusion retarder under a slab or under slab insulation. It is unnecessary to use both.” because an insulated slab is warmer than the ground, and water vapor flows from warm to cold. He refers to the thousands of pounds of water remaining in concrete after construction and that this needs to dry somewhere even if you prevent water from getting into the concrete from the outside. However, with code requiring polyethylene sheeting under the slab, does this not force the slab to dry to the interior? Then, what flooring materials allow this drying, or how long do you need to wait for a slab to dry before it is ready for interior finish to be applied? I am specifically curious about the vapor permeability of polished concrete floors treated with resin, or tile applied over the concrete. It seems both of these would prevent drying to the interior, but with polyethylene under the slab, the water in the concrete would have nowhere to go.

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  1. Expert Member
    PETER Engle | | #1

    Yes, the poly layer under the slab prevents drying to the ground. Joe mentioned that possibility more as a thought experiment that a recommendation, I think. IRL, slabs dry slowly to the interior airspace because while moisture moves from warm to cold, it also moves from high vapor pressure to low vapor pressure. So long as you keep the interior space dehumidified, the net vapor flow is up, not down. It is best not to install any vapor-impermeable materials on top of the slab until it is dried out. This can take a year or more, depending on interior conditions. If you need to install sooner or just want to make sure, use a Calcium Chloride test kit to measure the vapor emission rate.

    1. Sequence_eternal | | #3

      Thank you. I didn’t think of waiting that long, but it makes sense. I will get a calcium chloride test kit for sure.

  2. Expert Member


    Here is my entirely anecdotal experience:

    Exposed slabs are very popular around here, and I've done quite a few myself - including my own house. The common practice is to seal the slab just before the other floor finishes go in - so depending on the speed of the build, between 4 to 6 months after the slab was poured. I've never heard of any of these slabs showing moisture problems on their finishes.

    1. Sequence_eternal | | #4

      Thank you, that amount of time should give the slab a chance to dry. I appreciate your experience (I have none!).

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