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Waterproofing both exterior and interior of foundation wall

Jamie B | Posted in General Questions on

Dear GBA community,

I’m at the tail end of a rear basement extension, where the contractor poured 45 linear feet for a foundation wall.

The crew was doing the exterior waterproofing, with an elastomeric liquid applied membrane. Because of very tight space circumstances (this is a row house) not all of the wood forms on the exterior side could be removed, mostly on one side at the bottom of the wall near the footing. Thus, there will be a small percentage of this wall that will exposed to ground water.
I’m worried that ground water at the exposed parts of the concrete, through capillary action will eventually bring in moisture issues. The waterproofing contractor is telling me that it shouldn’t be an issue as I should waterproof the interior concrete face as well. Normally I would call that excessive, but now that I know the exterior is not perfect, it might be a solution.

Taking into consideration of what I’ve read by people like John Straube, where the idea is we want the exterior to be sealed, and the interior to dry to the inside.

Here is my question: if I waterproof or seal both sides of a concrete wall, will this create any issues?

As always, your experience and advice is appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    There are many questions here. When part of the water management details are missing, it's important to know whether other details were skipped, too.

    1. Did the contractor include a capillary break between the footing and the foundation wall? (For more on this issue, see Capillary Breaks Above Footings.)

    2. Did the contractor include a footing drain? (For more on footing drains, see How to Install a Foundation Drain.)

    Assuming these two details were done correctly, the omission you describe may not cause any problems.

    For information on interior waterproof coatings like Thoroseal, UGL Drylok, and Xypex, see Fixing a Wet Basement.

    There is no reason to worry about adverse results from any attempt to install an interior waterproof coating. If the hydrostatic pressure against the outside of your foundation wall is high enough, these products won't be able to keep out the water. But it's a myth that concrete walls need to "breathe" or to dry to the interior, so installing such a product won't hurt the concrete. Damp concrete is strong.

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