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Community and Q&A

Vapor barrier under basement floor

Providence1979 | Posted in General Questions on

I have a ten year old house that has a 6mil vapor barrier under the slab.  There is about six inches of crushed gravel under that and native sand even lower.  The basement is a walk-out and we’ve never really had any water issues.  We do run a dehumidifier in the summer months.

We are finishing the basement and adding Coretec Pro Plus Vinyl Plank flooring.  The instructions are vague about adding a vapor barrier between the slab and the vinyl flooring.

I was wondering what everybody’s opinion would be about this.  I can see two arguments:

1.  Adding another layer of plastic on top of the slab would help preserve the vinyl flooring and isolate any water vapor.

2.  But I could also see this trapping any normal moisture/vapor between the slab and the plastic, creating it’s own set of problems.

So what do people think?  Should I add another vapor barrier or not?

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  1. Expert Member
    Dana Dorsett | | #1

    Neither vinyl nor concrete are damaged by moisture.

    A redundant vapor barrier on top of the slab doesn't hurt anything, but it doesn't buy you anything either.

    1. Providence1979 | | #2

      Thanks for your reply. It just seems like I am sandwiching the concrete in plastic. I can see that as a bad thing. I worry that it would just trap water/vapor under the slap and start growing mold on any little bits of dust or dirt that might get trapped under there.

  2. GBA Editor
    Martin Holladay | | #3

    I agree with Dana. Either way is fine. If you have poly under the slab, you don't need another layer of poly -- but if you install it, no harm is done.

    Don't worry about what happens under the poly. It doesn't matter.

  3. ethan_TFGStudio | | #4

    I'm going to be that guy: vinyl, except in critical applications where there is no suitable alternative, can never be considered a "green building" material. I believe this should be mentioned here at Green Building Advisor.

  4. Jon_R | | #5

    Unless you believe in perfect air sealing, stick to the recommendations below.

    1. Providence1979 | | #6

      That's a really interesting read. Thanks!

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