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WRB location with continuous insulation considerations/potential issues – Please help…

OronoWoodworks | Posted in General Questions on

Morning folks,

I am building our families cottage in Ontario, Canada. The walls are 2×6 with r22 batt and osb sheathing, we will be adding 1″ silverboard with taped seams, 1×4 strapping and b&b vinyl. I read another post that said the WRB should go over the continuous insulation if your windows are mounted to that plane. Ours will be mounted to bucks in the same plane as the 1×4.

In the fall we wrapped the whole building in tyvek to protect it for the winter. It is my understanding that silverboard might negate the need for a wrb at all, I’m wondering if anyone can confirm. Secondarily, do I need to remove the existing WRB or is it safe to leave it behind the foam? It also seems as though silverboard might act as the vapour barrier as well? If the town insists we need a normal VB on the interior we will trap moisture I assume. I suppose the solution would be to use Membrain if that is the case.

With all these materials available now it is quite confusing to try and keep up…

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Replies

  1. OronoWoodworks | | #1

    Do I need to add more info? Any suggestions on where I might be able to find more info?

  2. Expert Member
    Akos | | #2

    The WRB is a permeable layer so it doesn't matter if it is there or not, it can stay.

    Detailing the foam as your as the drainage plane for the windows does make sense, make sure to use a tape appropriate for the job. Take a look at the ZIP details on how to flash your windows properly.

    OBC does allow for the foam to be the vapor barrier but you need more than an inch, since this is not the case you need interior poly. This is easy to install, makes the inspectors happy and can work as a the secondary air barrier. If it wasn't silly priced here, Membrain would be a better option for it but hard to justify the cost of that when poly is good enough.

    1. Malcolm_Taylor | | #3

      Akos,

      You have a lot more experience with exterior foam of less than the suggested ratios. What is your feeling about that foam being foil-faced, which precludes drying to the outside?

    2. OronoWoodworks | | #4

      Thanks for the response,

      I have watched quite a few of Matt Rinsingers videos on window install on bucks with continuous insulation and we bought a bunch of 3m 3015 (pretty sure basically same as zip tape) for flashing. I'm pretty comfortable on those details I think.

      I am ok with having to install the poly on the interior, but can/will it trap moisture if the silverboard is taped?

      I just noticed there are 2 silverboard products, I would assume we need to use the permeable silverboard graphite product.

  3. OronoWoodworks | | #5

    So I confirmed that we are indeed getting Silverboard graphite. With it being almost a vapour barrier when taped (1" is too thin to qualify as a vapour barrier per Akos above), is there concern for moisture being trapped between that layer and a traditional interior VB?

    1. Malcolm_Taylor | | #6

      OronoWoodworks,

      What Akos was saying is that the foam is a vapour-barrier (it stops vapour transmission) but not a code approved vapour-barrier in your wall. For it to used that way it needs to be thicker.

      The code requires that vapour-barriers be situated in the wall where they don't collect condensation. The usual way of doing that is to put them close to the interior. If you want to use exterior foam as the vapour-barrier, it needs to be thick enough so that the inside face stays warm enough that condensation doesn't occur. If the foam is too thin for that, you need to use something else (in Ontario that's usually poly) as the code approved vapour-barrier.

      These walls with thin foam and poly, which are widely used in Ontario, rely on a few things to stay safe. Maintaining low indoor RH, a well-sealed air-barrier, and some small amount of drying to the outside. I would suggest switching to 1" unfaced EPS to give you the latter.

      1. OronoWoodworks | | #7

        We no longer have the option to increase foam size without additional problems as we have some framed wall and some ICf wall and it was planed for 1" foam.

        Where does one find unfaced EPS? I'm not sure I would want to go that route as our foam is going to be our drainage plane and i have to assume unfaced changes that unless we add a layer of tyvek. If that's the case it is cheaper to use membrain as the vapour barrier and stick with silverboard graphite XS.

        I just noticed The silverboard page says it has a high level of permeance and allows walls to dry out...I'm guessing the perforated XS version is NOT vapour barrier and suits our purpose perfectly?

        https://www.amvicsystem.com/products/rigid-board-insulation/silverboard-graphite/

        1. Malcolm_Taylor | | #8

          OronoWoodworks,

          Yes, at almost 3 perms the assembly should work well. I'd stick with poly on the inside.
          Good luck with your build!

        2. Expert Member
          Akos | | #9

          +1 on the permeable foam, that is what you want.

          If you do go with faced foam, the generally recommendation with non-permeable foam is to leave the seams untaped as this allows for a bit of diffusion drying through the gaps.

          Also read through this as not many tapes stick well to XPS:
          https://www.greenbuildingadvisor.com/article/return-to-the-backyard-tape-test

          1. OronoWoodworks | | #10

            Wait...I am confused, is the permeable foam I just posted the link to not considered faced?

          2. Expert Member
            Akos | | #11

            Usually most faced foam products are vapor barriers and have very low permeability. The GPS you linked to looks like it is available as perforated, you are good to go with that. The other non perforated version of it is the same as the rest of the faced foam products.

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