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Is Vapor Barrier Required for this Wall Assembly?

sid_santo | Posted in General Questions on

Hello All!

Newly registered member here located in Ontario Canada 🇨🇦 Climate Zone 6!

I’m planning to renovate my basement. How’s this wall assembly?

DUROSPAN 250 INSULATION R12.91 + Rockwool R14

The DuroSpan will go against a poured concrete foundation.  Then a steel stud wall assembly.  Rockwool R14 will go into the joist cavities. 

The DuroSpan is 3″.

Links for the DuroSpan:

With this setup will a vapour barrier lead to any issues?

Thanks all!

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  1. Expert Member


    You don't want or need a vapour-barrier on those walls, just make sure to seal the rigid foam against air-leaks.

    1. sid_santo | | #2


      What if the building inspector insists on the vapour barrier?

      Typically they want it on the warm side here.

      Would a vapour barrier be harmful in this case?

      I believe all the materials used in this wall assembly are mold free (eps, steel studs, rockwool).

      1. Expert Member
        MALCOLM TAYLOR | | #4


        Canadian building codes require a vapour-barrier "be installed sufficiently close to the warm side of the insulation to prevent condensation at design conditions" (, and that vapour-barrier can be a variety of materials (not just poly) as long as they are under 1 perm.

        So your foam is a code compliant vapour-barrier, and is located where it won't incur condensation on the warm side. You don't want poly behind the drywall as it will trap moisture.

        One other suggestion: if you are worried that the area may flood at some point, keep your batt insulation up several inches from the floor.

  2. sid_santo | | #3

    Thanks all. My main question is would the vapor barrier cause harm if installed with this assembly?

    Anything facts I can point out to the inspector?

    Is 3" EPS plus polyethylene a double vapor barrier?

    1. Expert Member
      MALCOLM TAYLOR | | #5


      From the article I linked to:

      "Avoid the use of polyethylene. Basement wall systems should never include polyethylene sheeting—neither between the concrete and the foam insulation, nor between the gypsum drywall and the insulation. In these locations, polyethylene can trap moisture, leading to mold or rot."

      1. Deleted | | #7


      2. sid_santo | | #8

        Thanks Malcolm.

        You're suggestions are helpful!

        I've shared this with my GC and he's on board.

        Let's hope we can convince the inspector 😀

        What do you think of the DuroSpan product?

        1. Expert Member
          MALCOLM TAYLOR | | #9


          Used it on my wife's she-shed. Seemed fine.

  3. Expert Member
    Akos | | #6

    Important part there is the code reference in #4. If your inspector insist on it poly, point them to that code section.

    Vapor barrier in this case is not only not needed but can create problems.

    I also see you are looking to build with steel studs. In a basement, if the conditions are bad enough that you need the steel studs as the wood will rot, you have much bigger problems that need to be dealt with first.

    Steel studs are also a pretty big thermal bridge, you loose about 50% of the assembly R value because of it, waste of the R14 batts. I would build with wood studs instead, simpler, cheaper, easier to wire and perform better. Make sure to include a capillary break under the bottom plate and blocking at the top of the wall that also covers over the foam.

    1. sid_santo | | #10

      Thanks Akos!

      I will follow your advice re the building code.

      Re the thermal bridging issue, the GC will be installing a continuous, rigid insulation against the concrete foundation. Then the steel stud wall assembly is installed in front of the rigid foam. Will thermal briding still be an issue?

      The basement is actually quite dry. I've lived in the house for years and have never had standing water.

      We're using steel studs, because they are mold free and do not rot.

      1. Expert Member
        Akos | | #11

        Your stackup with steel studs works to an ~R19 assembly. Swap to wood studs and it is about R25. With the layer of rigid thermal bridging is not as bad but still not the best use of expensive R14 batts. With steel studs, insulate with the cheapest batts you can put in there, it won't budge the assembly R value any.

        In a dry, well insulated and conditioned basement, the RH will never get high enough for mold to grow. If the RH is consistently high, you have bigger problems anyways that steel studs won't fix.

        Also your drywall will be a mess long before any wood studs will have mold on them.

        The better value would be to stick to code min assembly (R6 rigid+R14 batts) but built with wood studs. Takes up less space, cheaper and the energy savings difference is noise.

        Still no vapor barrier though.

        1. sid_santo | | #12

          Thanks so much Akos. I had no idea about thermal bridging and steel studs.

          If you know techniques to reduce thermal bridging with steel stud construction, let me know!

          1. Expert Member
            Akos | | #13

            No way around this. Steel is conductive, so if you want better, you have to use less conductive material.

            There are "better" steel studs that have large punch-out's such as DeltaStud, these are meant for load bearing application so a bit overkill for you. They are also still not as good as a simple wood 2x4.

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