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Waterproofing a Stem Wall

2008gt500 | Posted in General Questions on

Hi all, the general building around here is typically slab on grade.  The other option is they pour a concrete footer in the dirt with a block wall, no waterproofing, and no foundation drain.  It seems to work fine, but I want to give myself an ounce of protection.

I was going to do ICF stem wall below grade, but the whole termite thing sent me back to block.  Detail is attached.

My vapor barrier under the slab is 6 mil poly, which I plan to continue down the inside of the stem wall and over the fastfoot membrane.  Inside of the stem wall will be filled, so slab is about 15″ above grade.  My question is what to do on the outside.

Could I just use the same 6 mil poly up to grade level and let the stem wall breathe above that?

I looked into a lot of the waterproofing products, but seems like such a waste for a problem I likely won’t have.

Looking for options/opinions.  The property is waterfront which is why I’m trying to add some protection, but the lake level has been pretty steady for 50 years so maybe I’m just overthinking it.

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  1. user-2310254 | | #1


    Concrete doesn't need to breathe and doesn't care about being wet.

    This article ( along with the links in the sidebar contain lots of useful information.

    Can you go taller? If this is a crawlspace, 15 inches is not a lot of headroom.

    If it's waterfront property, I would consider installing flood vents in the foundation.

  2. Expert Member


    Being waterfront your concerns about water are a bit different than typical site. I don't think damp-proofing on the exterior buys you much, or exterior perimeter drains. What will make a difference is a 6" layer of free draining gravel under the slab to stop moisture wicking up through capillary action.

    Keeping in mind I'm not that familiar with what works in your warm, humid climate, or how things are typically built there, a few comments on your section:

    - You may want to include a layer of foam either under the slab, or on the interior of your stem walls. The foam around the interior would help insulate the portion of the stem wall which is above grade. Foam under the slab would help alleviate possible condensation.
    - It's unusual to see the floor slab bearing on the exterior walls. It means the slab becomes a thermal bridge to the exterior, and makes it more susceptible to cracking due to differential settlement.

  3. 2008gt500 | | #3

    Thanks for the replies guys.

    Steve, the interior will be backfilled, so no crawlspace.

    Malcolm, I will look into insulating the interior of the wall. Originally I had planned on doing ICF below grade with the slab floating inside, but the termite issues got me moving in another direction. Your point about the thermal bridge might have sparked an idea for me too.

    I was trying to mimic one of Lstiburek's BSI articles. BSI-037 detail attached.

    1. Expert Member
      MALCOLM TAYLOR | | #4

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