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Wanted: vapor barrier coating for existing basement floor

Ben Rush | Posted in Green Products and Materials on

I’ve read a lot recently (mostly here) on vapor barriers, vapor retarders, and vapor diffusion, and eventually my eyes glazed over.
One take-away was that vapor barriers DO matter for basement floors.
My brother is buying an old brick house in zone 5. The concrete floor has been patched in a few places, but is in good condition. The seller has a dehumidifier running, and admits that it smells musty sometimes- like after a rain- if the dehumidifier isn’t running.
We were thinking of painting the basement floor- just to reduce dustiness/ grittiness, and to make it easier to mop. Now I’m thinking this would be a good opportunity to add a vapor barrier coating to the top of the basement floor. Obviously there’s no polyethylene under it, and I doubt it’s even on gravel. I bet the concrete floor was poured directly on the soil.
Other info: built approx. 1900; solid brick walls above grade; shallow basement- approx. 3′-6″ below grade; the below grade walls are no thicker than the brick walls above- but their materials have not yet been identified, due to an interior parging coat. The seller drilled thru the parging coat a little- to identify the material beneath, but it wasn’t conclusive. The color and wall thickness are about right for poured concrete, but I didn’t think that was common in the Chicago ‘burbs back then. There is some efflorescence on the below grade, unidentified, parged walls- but not a lot.
Later, I’d like to look into a vapor barrier coating for the below grade walls too- but I’m concerned that it might cause the parging coat to delaminate from the wall- so right now I’m focused on the floor.
Does anyone have any recommendations for a vapor barrier (or maybe vapor retarder) coating for this old concrete floor? Ease of installation is key. I’d like to avoid a complicated surface prep regimen. Of course, it needs to be durable under normal basement use. It also needs to be paintable (or to be attractive by itself). Lastly (and at the risk of being run out of town on a rail), short-term VOCs are not my biggest concern. We’re not especially sensitive to them, and we can ventilate during the work. In this case, I’d accept a temporary increase in VOCs, for a permanent decrease in humidity and mold.
I welcome any advice on materials and techniques.

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  1. GBA Editor
    Martin Holladay | | #1

    There is no such thing.

    If you want a vapor barrier, install a layer of rigid foam or polyethylene, followed by a layer of plywood or a new slab.

    For more information on this issue, see Does Your Floor Have A Vapor Barrier?

  2. Ben Rush | | #2

    Thanks for the link. Lots of info there.
    What if I dial back my expectations? Would I be able to achieve the results I'm looking for with a vapor retarder coating instead of a vapor barrier coating?

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