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Community and Q&A

Missing poly under slab

Andrew_BostonArea | Posted in Green Building Techniques on


I am finishing my basement and spent a lot of time here reading about how to insulate it.

Per all my reading, I am planning on putting down one inch of foam on the slab followed by two half-inch layers of plywood with no poly underneath. Based on what I’ve read here, since my house was built in 2008, it should already have poly underneath the slab. In fact, poly was specified to be under the slab on the building plans. My plan was to let the foam take care of stopping most water vapor from coming through and then the dehumidifier in the mini-split to take care of the rest.

However, they have just dug up my slab for the rough bathroom plumbing and… there was no poly! Am I correct in understanding it would be right underneath the slab? Under my slab was just gravel and sand and they have probably dug a foot beneath the bottom of the slab.

Assuming I’m not mistaken and it isn’t supposed to be deeper than that, what do you recommend I do? Stay with the plan even though it is missing? Or should I put down poly on top of the slab?


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  1. gusfhb | | #1

    Astounding that in 2008 they did not use poly. Put it under my first garage slab in 1986.........

    They make vapor barrier adhesives. I used one under wood on a 1969 slab, no poly, and it has been fine for 10 years.

    Tape a square of plastic down to the floor for a period if time to see if you get moisture under it.

  2. natesc | | #2

    A product like Redgard can be painted on to the slab to create a vapor barrier.

  3. Andrew_BostonArea | | #3

    Thanks Keith and Nathan. The person finishing my basement had wanted to use DryLok originally, but I took it out of the plan since I had expected the poly under the slab and I read in a few places that DryLok can actually help mold grow, which I am very allergic to.

    I previously had exercise mats down in the basement and when I pulled them up for this project, they were wet on the underside. Based on all the reading I had done, I thought the water was just condensation from warmer air in the basement against the cold slab, which was what drove me to insulate the slab. But, now that I know that I don't have a vapor barrier, I don't know what the best approach is... is it poly, or a paint-on product? Is DryLok better/worse than Redgard? Or is the 1 inch foam sufficient enough?


  4. Jon_R | | #4

    Joe Lstiburek recommends fully adhered to reduce mold risk. Also make sure that your plywood has much more drying to the interior than wetting from the slab. For example, rubber mats might not provide that, even with a vapor barrier on the concrete.

    1. Andrew_BostonArea | | #5

      Thanks John. Does "fully adhered" mean I should use one of the paint-on products?

      My interior flooring will be mostly vinyl planks and tile, with one room with carpet. How can i make sure there is more drying to the interior?

  5. Andrew_BostonArea | | #6

    Hope everyone had a happy holiday! Any further suggestions on this? Should I let my contractor use DryLok like he wanted to? I've read some stuff that it actually can cause mold? Please let me know as I need to tell him today! Thanks!

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