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How to retrofit the insulation of an existing basement slab?

buffalokat | Posted in General Questions on

What is the best way to insulate an existing walk out basement slab? When I bought the house I removed the wall to wall carpet and sanded and stained the slab. It is too cold to live with.

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  1. GBA Editor
    Martin Holladay | | #1

    The usual technique is to install 1 or 2 inches 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 TapCon fasteners. (If you are worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it's important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.

    If you don’t want to lose the height required for rigid foam, you could try installing a dimpled subfloor product like Delta-FL. (Note that some similar products, notably DRIcore, have mixed reviews from some builders.)

    For more information on insulating existing basement slabs, see:
    Finishing a Basement Floor;
    Green Basement Renovation; or
    The Stay-Dry, No-Mold Finished Basement.

    If you do this work, it's important to remember that the most critical area is actually the slab perimeter, especially the slab edge facing the walk-out side of your basement. You'll need to excavate the soil away from the foundation on this side of your house to expose the slab edge. Insulate the slab edge with vertical rigid foam. Ideally, this vertical rigid foam will extend down at least 2 feet from the top of the slab. Of course, any exposed foam will need to be protected from physical abuse and UV damage with metal flashing or a similar protective layer.

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