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I am looking for a good solution to damp basement floors in old homes

GBA Editor | Posted in Green Products and Materials on

I am looking for a good solution to damp basement floors in old homes that have no vapor barrier below the slab, and which are being super-insulated and air sealed.

I know to start with exterior improvements (drainage and grading), but that seldom cures the damp slab situation. And in my current case, I have super-insulated the house, which makes the damp/dank basement all the more concerning. We have considered installing an exhaust fan or ERV (RH controlled), but would rather solve the problem at its source (with a coating of some sort over the existing slab I hope). The least toxic the better.


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  1. GBA Editor
    Martin Holladay | | #1

    If you want your house to be superinsulated, the slab should be insulated as well. That means you should install XPS foam on top of your slab. The XPS foam should be covered by more concrete or a plywood subfloor.

    The details will depend on how bad your moisture problem is.

    If you ever have standing water, you should install a dimple mat under the XPS foam, and a French drain at the interior perimeter of the basement. The French drain should be connected to a sump that drains (with a sump pump) to daylight or the municipal sewer, if permitted.

    If you never have standing water, you can probably get away with installing 6-mil poly over the existing slab, then a continuous layer of XPS foam, and then a plywood subfloor.

  2. Steve Konstantino | | #2

    Finding the source of the water is important. Most wet basements are the result of improper exterior drainage from weather. Roof overhangs, gutters, perimeter grade pitch, exterior foundation drains are all things to consider. If possible, stop the water before it comes in. Then you just need to deal with humidity levels.

    I agree that you need to insulate but where and how will depend on the wetness and what you're plans are for the basement. Adding layers of rigid foam and plywood will make your ceiling height lower. Whether you are finishing the basement or not may impact your decision.

    The manufacture of XPS foam is not the best for the environment.
    Another option may be SlabShield which is 1/2" thick closed-cell polyethylene with an aluminum layer in the middle. This product is made with some recycled content. You can find it here:

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