GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

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?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  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. ethant | | #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!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |