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Community and Q&A

Moisture Control in this Wall Assembly

Hawnes | Posted in General Questions on


I am working on a build in North Vancouver and would like some advice about this wall structure. The goal would be passive house:

1.5″ Roxul comfortboard (R5)
2x6x16″ Roxul comfortbatt (R22)
2x4x16″ insulated service wall (R14)

The house wrap would be a SIGA WRB and Air barrier.

My main concern with this would be water getting into the external wall and with the internal service wall not being able to dry.

Also with the 1.5″ external insulation being so thin, wouldn’t there be condensation issues?

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  1. Expert Member


    You may find this useful:

    At first glance it looks like you have much more insulation than is necessary to meet Passive House standards for Vancouver.

    I don't see anything in your proposed stack-up the would limit drying, but you haven't included all the code required components so it's hard to say. What is the sheathing? How is the rain-screen cavity constructed? What are you using for a vapour-barrier, and where is it located in the wall?

  2. Hawnes | | #2

    Hi Malcolm, Thanks for that, I read through

    And what my builder suggested is this assembly on page 28 (screenshot below) with a 2x4 service wall.

  3. Hawnes | | #3

    I'm worried about water getting into the exterior wall and not having an easy time to dry out due to the exterior insulation. Plywood is required for seismic stuff I think, so can't replace it with gypsum board.

    What do you think?

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Unlike rigid foam, the exterior Comfortboard lets the wall dry easily to the outside, and with the air/vapour-barrier on the inside of the 2"x6" wall the service cavity can dry to the inside. I wouldn't have any worries about building that wall in Vancouver. It will perform very well. I wish I had it on my house.

  4. Hawnes | | #5

    Ok, great thanks, I'm reassured then!

  5. Hawnes | | #6

    What do you think about a wall like Dan Kolbert's wall, would that work better in my climate? Would it be cheaper and easier to build?

    1. Expert Member
      MALCOLM TAYLOR | | #7


      You mean a double wall full of dense-packed cellulose?

  6. Hawnes | | #8


    1. Expert Member
      MALCOLM TAYLOR | | #9

      If you can find reliable dense-pack installers near you then sure, but whatever you fill the cavities with, I'm not sure there is much benefit to the exterior insulation. A double stud wall in Vancouver's climate already has enough R-value and can be thickened if you want more. The rain-screen cavity makes it very resilient without the added complexity exterior mineral wool brings.

      What I particularly like about your original wall is the placement of the Air/vapour barrier. Being well away from the interior, it is well protected, with very few penetrations.

      1. Hawnes | | #10

        "without the added complexity exterior mineral wool brings"

        Thanks for replying. Do you mean that exterior wool isn't needed?

        I'm wondering if there is a way to save on cost, as in:

        the exterior wall would be dense pack cellulose and,
        the interior wall would be mineral wool.

        And dense pack cellulose has its hygroscopic and buffering capacity making the wall more durable?

        1. Expert Member
          MALCOLM TAYLOR | | #13


          Yes. If you include a rain-screen, that will be enough to mitigate the potential problems of moisture accumulation in the sheathing without exterior insulation.

          The wall you describe sounds great, although I'm a bit unclear how the two walls would intersect. If the poly is on the inner face of the outer wall, are the two walls touching, or is there a gap between them, and if so what is it filled with?

          1. Hawnes | | #14


            My builder suggested a 1" gap, I'm not sure what it would be filled with haha. Could it just be filled with mineral wool from the inside?

  7. alex_coe | | #11

    Your original plan wouldn't have high enough ratio of exterior insulation to eliminate condensation risk at the sheathing. At R42 nominal in Vancouver I also agree it would be into the realm of diminishing returns.

    You could do advance framed 2x6 @ 24"OC with exterior R8 to prevent cold sheathing condensation, and sheathing as your air barrier with taped seams, which would allow you to do painted drywall as code compliant vapor retarder. I'm not certain this meets Passivehaus, but I think it may. I think the simplicity, performance, thickness, and cost balance are great. You could easily add a 1.5" insulated service cavity by simply strapping horizontal 2x2's over the inner framing, which would also give you the option to add a redundant smart vapor retarder protected from penetrations. I'd just go cheap with Certainteed Membrain in this context.

    In my own build I've opted for simple Tyvek, as I felt the premium WRB seemed irrelevant with exterior 2" Rockwool R8 a 3/4" rain screen gap.

    Without the exterior insulation I'd want to do a ProClima or Siga WRB though.

    Going back to your original design, I think another preferable option in terms of cost/performance/thickness would be 2x4 double stud with exterior R8. With double stud walls people often will cite exterior insulation as unnecessary, but I really like the idea of wrapping the building with 2" of vapor open R8 rockwool.

    1. Expert Member
      MALCOLM TAYLOR | | #12


      The ratio of exterior insulation is only important when it is foam, not mineral wool, which allows adequate outward drying. Here is what Martin wrote:

      "Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"

      1. Hawnes | | #15

        Hi Tom and Malcolm,

        Yea this is my concern about the thickness of the exterior insulation. And I have read and been told what Malcolm quoted about its vapor-permeableness.

        I am definitely looking for ways to save cost and what you describe Tom, is what I've seen in passivehauses around Vancouver, but my builder really likes the double stud idea I posted above and I can see how it would work really well.

        My ultimate worry though is the problem you describe Tom.

        What to do? I don't know haha.

        1. Hawnes | | #16

          I do like your idea of the R8 exterior and 2x4 double stud wall. Then that double stud wall can just be filled with cellulose. hmmmm.

      2. Jon_R | | #17

        > the ratio of exterior insulation is only important when it is foam
        > You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement

        Both general statements simply aren't true. If you apply only a little exterior mineral wool and have a poor interior to exterior perm ratio, the wall is likely to fail. Add some more mineral wool and it won't.

        If and only if you have a wall that works based on perm ratios can you add exterior mineral wool without worrying about how much. And even with this, you may have to worry about the R-value to reach a required total wall R value.

        1. Hawnes | | #18

          Hi Jon,

          I found one of your responses in this thread:

          "I suggest that ~8 perms on the interior side, < 1 perm on the exterior side (eg Zip sheathing), 1" exterior mineral wool and no interior side air barrier (eg just taped Zip) would be quite risky in a cold climate, even with rain-screen and cellulose.

          Use CDX, <= 1 interior side dry perms and an interior side air barrier and I agree with you."

          Your perm ratios here don't apply to this wall right?

          As my perms are greater on the exterior?

        2. Expert Member
          MALCOLM TAYLOR | | #19


          That the thickness of mineral wool doesn't pose the same problem as foam is made in the context that inadequately thick foam can be the cause of sheathing moisture build-up, not that exterior insulation is being used to cure problems in a wall that would otherwise fail. You are talking about an entirely different issue.

          Knowing that Canadian building codes require an interior vapour-barrier, and the Vancouver Building Code requires either Plywood or OSB as sheathing, I can't see how the interior to exterior perm ratio could end up being a problem. It's in that context, and in the context of the OP's proposed wall (which is safe without any type of exterior insulation) that it's entirely fair to agree with Martin's quote.

          Although it's impossible to know without running all the proposed building assemblies though the Passive House software, I think it' a safe bet to say it's highly unlikely exterior insulation of any type would be necessary or of much benefit to a double stud wall in Vancouver's climate. It simply isn't warranted.

          1. Jon_R | | #20

            My point stands. As a general comment, it's often false, potentially leading to some serious problems. It shouldn't be repeated without qualifiers such as "for your wall exactly as proposed and in your climate it doesn't much matter ....".

          2. Expert Member
            MALCOLM TAYLOR | | #21


            Fair enough. I should have included the source for context, which was Martin's article discussing the minimum thickness of foam sheathing.

            I'd be interested in what you think of the OP's proposed double-wall in Vancouver with or without exterior mineral wool.

          3. Jon_R | | #22

            For a double wall, I'd exceed the Table 2A recommendations here. Ie, make sure the perm ratio is right.

            Since it doesn't decrease exterior perms and it warms the sheathing some, mineral wool would make it a more robust wall. But for the Hawnes case, it shouldn't be necessary and so I'd skip the expense - we agree on this.

  8. Hawnes | | #23

    Hi Jon and Malcolm, thanks for the great discussion, I learned a lot.

    Sorry, I'm just a layperson trying to understand all this. I have a question.

    @Jon R,

    My understanding of the wall assembly I presented above would place the air barrier on the outside of the internal wall (the poly) and so based on your recommendation the exterior would need a high permanence to be safe.

    Also, if I could afford the external insulation of mineral wool, would it be beneficial to have?

  9. Hawnes | | #24

    So based on Table 2A what does my exterior permanence need to be? For Class I it says Marine 4 is 0.5, which doesn't seem high to me.

    1. Jon_R | | #25

      It says .5, so say > 1 perm to be conservative. With plywood, you are well over that (a good thing, making exterior insulation unnecessary). Switch to Zip (sometimes < 1 perm) and I'd want it to be more conservative - adding exterior mineral wool is one way to do this.

    2. Expert Member
      MALCOLM TAYLOR | | #26


      As both Jon and I (and table 2a) agree, the double wall you propose without exterior mineral wool is very safe and robust. In your climate a bigger threat for walls than interior moisture is bulk water intrusion. That's where I'd spend my efforts.

      I'd also clarify what your goals are around levels of insulation. ROI? Comfort? Gaining Passive House accreditation? Being clear about those will be a big help determining what assemblies make sense, and where it's worth putting your emphasis. Spending the money you saved on better windows, doors and roofing will yield appreciable gains, that the exterior insulation doesn't.

      1. Hawnes | | #27


        Thanks so much. Really helped clarify everything.


        Thank you as well. Really helpful.
        My goal is mainly a healthy and comfortable home for my kids and with this comes the benefit that I get to tinker around with a home that is built like a machine!

        Based on the discussion here I will discuss removing the exterior insulation and spend more on windows and such.

        With regards to bulk water what would be other methods? Like larger overhangs?

        1. Expert Member
          MALCOLM TAYLOR | | #31


          Definitely larger overhangs help, especially ensuring exterior doors are always covered by a deep overhang quite close to the head. Choose appropriate, high quality materials for the siding (no open-cladding, or problematic things like stucco, adhered stone or other reservoir claddings). Detail openings and transitions between materials carefully, and make sure the base of walls are protected from excessive splash-back at grade.

          1. qofmiwok | | #35

            LOL, you just knocked out pretty much all siding materials except wood. Fortunately in reality those are fine as they are detailed well.

          2. Expert Member
            MALCOLM TAYLOR | | #36

            No the types of siding that are problematic in the PNW is pretty narrow. It's just the ones I listed, and it's not much of an imposition considering no one uses stucco here anymore, and adhered stone or brick are almost always limited to just accents, while open-claddings don't meet BC's rain-screen code requirements.

            You've still got wood, metal, vinyl, composites, cement-board, and I'm sure lots of others I'm not remembering.

            You can make just about anything work in any climate, but the success of those gymnastics doesn't mean there aren't things which are more appropriate or less risky than others. For the PNW a list of things to avoid would include flat roofs and lack of overhangs as two obvious examples.

        2. Jon_R | | #32

          +1 on the often under-valued larger overhangs. And hip roofs that make them more effective.

          1. Hawnes | | #33

            Great, thanks!
            We will be having large overhangs and hip roofs.

          2. Hawnes | | #34


            Would a sunroom against the wall or a greenhouse against the house damage the wall?

      2. alex_coe | | #28

        Having the sheathing and WRB tucked away behind the exterior rockwool and rain screen.. you would effectively solve everything (no sheathing condensation, drying, water intrusion) and with less thermal bridging. If installing the exterior insulation and rain screen at the same time it's just a matter of using longer (4" #10) screws and adding a layer of 2" thick batts as you go. Seems worth the price of admission?

        With R7.5 exterior, to my recollection the code permits painted gypsum (Class III) as an acceptable vapor control layer, saving material & labor cost.

        1. Expert Member
          MALCOLM TAYLOR | | #30


          By "code" I think you are probably referring to the IRC. Hawnes is building in Vancouver which uses its own modified version of the BC Building Code - and which requires an interior vapour-barrier, not a vapour-retarder.

          A wall with exterior insulation isn't just distinguished by the length of the screws needed for the rain-screen strapping. It's uses completely different materials (you don't need any screws for a conventional rain-screen, 1"x4"s, window and door bucks, or expensive mineral wool), completely different detailing of openings, and flashing - and represents significantly more labour. It's not just a matter of "adding the layer as you go along"

          More significantly, as we have discussed repeatedly, it's an open question as to whether it solves anything. The sheathing is already safe from condensation, the wall drys perfectly well into the cavity, and bulk water intrusion is best dealt with by proper detailing, and selection of appropriate materials for the cladding and rain-screen.

  10. Hawnes | | #29

    Where would gypsum go? In place of the plywood? Or the poly?

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