Basement – existing vapor barrier over concrete
Hey all:
I’m about to start finishing my basement (MA, Climate Zone 5) and have a wall assembly conundrum. We installed an interior perimeter drain and sump pumps a few years back. As part of that system, a vapor barrier was installed over the concrete walls, fastened to the wall and sealed at its perimeters. It ties into a plastic flange at the wall-slab edge joint. The aim is to encapsulate the concrete wall and direct any moisture on walls down into the perimeter drain below the slab. (Our walls have never had any moisture issues, but the vapor barrier was included as part of the system.)
Based on what I’ve read here and elsewhere, I’ve always thought I’d need to remove this prior to installation. The rationale against poly in basements is clear (though this is talked about when installed on the warm side at the interior face of studs, preventing drying to the interior.)
This is the product currently on my foundation wall: https://www.grateproducts.com/moisture-control/thermal-shield-vapor-barrier.html . It’s a two-layer plastic sheet (0.014 perm) with about 1/8″ of insulative material inside, it may have some nominal R-value and could have slightly less potential as a condensing surface than the bare concrete.
Thanks all!
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Replies
Hi Mike,
I'm curious why you had the perimeter drain installed if the basement hasn't had any water issues. With a perimeter drain, the idea is that any water that enters through the basement wall can make its way down the wall to the drain and sump and be pumped out of the house. So, if you take the existing vapor barrier down, you'll need a dimple mat or similar product to create that drainage plane before installing the new rigid foam. Do an excellent job air sealing the new rigid foam before you build your stud walls. Have you read this: How to Insulate a Basement Wall?
Thanks for the reply Brian, (huge fan of your work on the FHB podcast, by the way.) The origin of the perimeter drain was due to water coming up at the slab edge, and it's been dry ever since the drain was installed. Thankfully, the walls have always been dry.
Here's where I landed with my builder: keeping the vapor barrier over the foundation wall tied into the drain, installing the rigid foam over that. Filling any voids with spray foam, taping seams before studs and sheetrock.
Thanks again for the input!
Mike