Published on June 30th, 2016 | by Henri


Waterproofing and insulating the basement

Q.  My husband and I would like to finish our basement in our 13-year-old home. We would like to frame the space and make a laundry room, game room, and storage/work area.

Our ongoing discussion is always about how to frame, insulate, seal and protect before we actually ‘finish’ the space. There seems to be an array of opinions out there on how to seal and prepare basement walls prior to finishing.

Our house is on sandy soil, and we have had no issues to date of any water infiltrating into the basement from the outside. Our foundation is poured concrete. We have tested for radon, and find we are within safe limits.

We have read about spray foaming basement walls as a way to seal for moisture. Another way we have read about is using foam board to both seal and insulate.

If framing is used with foam board, should a barrier of some type (such as sill seal) be used between the framing and the concrete wall? We don’t know where to begin. Your advice would be greatly appreciated.

A.  Since you are on sandy soil and have had no moisture problems, there is no need to seal concrete walls, but you can do so with a cementitious waterproof coating such as Super ThoroSeal by BASF. It should be available in Ace, TrueValue and other independent hardware stores, as well as Lowe’s and Home Depot.

For other readers who may be thinking that this product would be good to waterproof their block foundations, please be aware that I have warned against waterproofing block walls from inside, as it can create a build-up of water within the blocks’ cores that can result in serious moisture problems in the house.

You mention sill seal against the walls. Sill seal is used between the masonry foundation and the first floor deck, as well as between all floor platforms and the framing above.

Rigid foam insulation is fine to use as insulation, but I would not depend on it as a way to seal the walls against moisture penetration or leakage from outside, if that is your concern.  Rigid foam insulation protects the walls from summer condensation, if properly adhered to them.

You can adhere the rigid insulation with a few dabs of polyurethane caulking to hold it in place until you build the framing.

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