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Community and Q&A

Basement slab moisture barrier

Matty McDermott | Posted in General Questions on

I am finishing our 800 sq.ft. basement in Portland, OR (zone 4c). I am planning to lay a combination of carpet and linoleum and want to reduce the chances of mold from moisture that might seep up through the concrete slab, which varies in thickness from 2″ – 4″ in some spots. I have noticed a couple 6″ diameter spots on the slab that seem moist during the rainy season, but no pooling of water. I have taken several steps to minimize moisture in the basement, including exterior grading of soil, exterior french drain as well as interior french drain for redundancy in spots that were problematic in the past. My concern is dealing with the small amount of moisture that is less obvious, yet can still cause mold.

I am wondering if I should consider any sort of moisture barrier between the concrete slab and the carpet / linoleum? I wondered if some sort of plastic tarping, rubber matting, or paint-on product might be helpful, or if it might create problems that I did not anticipate.

It is important to know that any product used to solve this potential problem will need to be thin, as overall height in the basement is a concern – appx. 6’4″.

Please advise.


Matt McDermott
Portland, OR

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  1. GBA Editor
    Martin Holladay | | #1

    It is always a bad idea to install carpeting in a basement, unless you are certain that there is a continuous layer of rigid foam insulation under the slab. The reason has to do with physics: during much of the year, the concrete will be cold enough to be a condensing surface for moisture. Moisture build-up and mold under the carpeting is inevitable without foam insulation under the concrete.

    The best solution to your problem is to install at least 1 inch (2 inches would be better) of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCons. If you don't want to lose the height, and you are willing to give up on the carpeting, you could try installing a product like DRIcore or Delta-FL.

    For more information, see:

    Finishing a Basement Floor.

    Green Basement Renovation.

    The Stay-Dry, No-Mold Finished Basement.

    The High Cost of Deep-Energy Retrofits.

  2. Expert Member
    Dana Dorsett | | #2

    The deep subsoil temps in Portland OR are about 55F. ( )The mean outdoor dew points in summer are also about 55F ( ), so with a carpet overlaying the slab you're in a ZERO MARGIN condition. But even R2 (1/2" EPS) would be sufficient dew point mitigation, provided it was a thin rug, but an inch or more would give you free rein, except for the head-banger issues.

    The fact that you have visibly damp patches in the slab means you have a high flux of ground moisture coming through the slab, probably year-round. Before you put ANYTHING over the slab it's worth dosing it with a good masonry sealer, which slows the capillary draw through the slab, and it's worth putting down a layer of poly sheeting too.

    But the fact that you have only 6'4" means the whole thing is sub-code as is, and if you want to really do the thing right, you'll need to demo the slab and excavate, and put a layer of gravel, to allow free drainage to your (new) interior side french drains & sumps. (Not cheap, I know.)

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