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Community and Q&A

Floor Slab: Is a Vapor Barrier Needed with Rigid Foam?

jackofalltrades777 | Posted in Green Building Techniques on

If one is using 3″ of rigid foam (EPS) underneath a 4″ slab-on-grade, does one still need to install a vapor barrier?

Here is the build-up from the ground up:

4″ compacted AB
3″ EPS (termite treated)
4″ concrete

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  1. nvman | | #1

    Depends on the building code in your area.
    I did exactly what you are doing and I had to put a vapour barrier on top of the rigid foam.
    I also had to seal it to the concrete with acoustic sealant.
    This was in North Vancouver, BC.

  2. Dana1 | | #2

    At 3" Type II EPS has a permeance of about 1 perm. A sheet of 6 mil poly has a vapor permeance of about 0.05 perms.

    That means if you skip the poly you end up with 20x as much moisture making it from the subsoil to your slab.

    The vapor barrier is pretty cheap insurance, in the grand scheme of things.

  3. Mychief | | #3

    I remember reading somewhere (I know, written does not equate to truth) to put two sheets of poly so the concrete could slide when expanding and contracting. Am I nuts or what?

  4. jackofalltrades777 | | #4

    Does the vapor barrier go ON TOP of the EPS or below the 4" of AB?

    The guy in BC required that it was put on top. Is that a code thing?

    Why so do many concrete guys hate vapor barriers? They said it makes curing the slab much harder.

  5. GBA Editor
    Martin Holladay | | #5

    Peter L,
    Some concrete contractors don't like polyethylene under the slab because it makes it harder to handle concrete with too much water. One obvious solution is to make a better (stronger) mix with the right amount of water -- not too much.

    For more on this topic, see Polyethylene Under Concrete Slabs.

  6. user-2890856 | | #6

    " A slab is a wall is a roof " , someone once said . slab is a wall is a roof

  7. Dana1 | | #7

    It's generally better to put the vapor barrier on top of the EPS, not under, and DEFINITELY not under the AB. When installed at lower layers there is potential for liquid water to accumulate on top of the vapor barrier, that takes a very long time to dry through 1-perm foam, potentially ruining the finish floor in the process.

    With the vapor barrier on top of the EPS and tight to the underside of the slab, any water accumulations that might occur from spillage or flood has a drying path that is orders of magnitude faster.

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