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Community and Q&A

Concrete moisture content

bski | Posted in Energy Efficiency and Durability on

What should the moisture content in a basement slab be when tested with a moisture meter? I suspect that is where the humidity is coming from in my house, but I did the plastic test and it didn’t condense at all. If I test the slab with a meter it reads anywhere from 12.5% to 8%.

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  1. Expert Member
    Dana Dorsett | | #1

    With concrete it will vary all over the place by season and ground moisture content, and it will still be just fine. Any thing up to and including 100% is OK for the concrete. If you want to put a finish floor above it (or even paint it) it needs to either have a ground vapor barrier under it (preferred), or be sealed to limit the rate of moisture transfer to something the finish floor can tolerate. In many/most cases if there isn't a vapor barrier under the slab you'll need to put one over the slab, and add 0.5-1.5" of rigid EPS (depending on the regional subsoil temps) to keep any subfloor warm & dry enough.)

  2. GBA Editor
    Martin Holladay | | #2

    If you taped a piece of polyethylene onto the concrete floor for a few days, and no water droplets appeared under your plastic, I wouldn't worry.

    That said, it's always a good idea to install a layer of rigid foam on top of a basement slab before installing flooring, as Dana suggested.

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