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Community and Q&A

Applying exterior foam to stem wall — water proofing

John Huber | Posted in General Questions on

I am building a slab foundation with a stem wall and 4″ of rigid EPS applied to the outside. I’ve read various GBA advice about fasteners/adhesive to install the foam, and about how to coat/finish the foam so that it looks nice. But I remain confused about a couple of things.

(1) Do I need to seal the concrete before installing the foam. A product like STYRO Industries Grey FlexCoat Brush on Foundation Coating has instructions for putting the sealer on the outside of the foam ( Does this eliminate the need to seal the foundation itself? Just glue the foam to the foundation and then apply the FlexCoat? Seems like water could get in through the top during the construction process?

(2) If it makes more sense to seal the concrete first, is there a recommended product that won’t affect the adhesives (so I can still glue the foam to the stem wall)?



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  1. GBA Editor
    Martin Holladay | | #1

    If your slab is above grade, and there is no crawl space or basement, and all of your other details (crushed stone, polyethylene) are installed correctly, there is no need to install dampproofing or waterproofing on your concrete stem walls.

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