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4C marine wall assembly

user-1047633 | Posted in General Questions on

Hi All,

I have a building envelope vapour management question regarding new construction in 4C (marine, Vancouver BC).

I have a sunken slab and used slightly higher than usual stem walls to get the foundation above grade (18-24″ above slab). my 2×6 wall sits on this and i require either a low-height furrout of the concrete wall or I could just go full height with furrout. I was planning on using closed cell spray foam at the concrete below, up and over the vapour barrier at warm side of wall to continue the plane and provide required insulation. See for sketch.

I was planning on going with the full height 2×4 furrout so that I can use this as a service wall – run all of my plumbing, electrical, etc in this plane and minimize the penetrations through the vapour retarder, minimize thermal bridging through studs, etc. This is more or less in line with a double wall system described by BSI:

So my two questions are: what is the correct location of the vapour retarder and does that interior furrout need to be insulated (aside from at the face of the concrete)? For vapour retarder, placing it between the two walls would put it right around that 2/3 point. Placing it on the interior face of the furrout, right behind GWB means it would take more of a beating over time, kill my “service walls” and could trap moisture if proper paint, etc is not used. If I keep the 2×4 insulation out, the vapour barrier is closer to the warm side, but I have the odd air cavity between the actual space and retarder.

As an aside, what builders seem to commonly do here in BC is use siding or cladding w/ “drainscreen” as required by building code (vertical furring strips and min 1/2″ cavity between exterior finish and building paper), with building paper or tyvek over plywood. The thing I haven’t really seen before is that they usually leave a horizontal gap in the plywood (I’d say 1/2-1″ wide, which I assume is to let moisture escape to the exterior more easily) and put holes in any dead cavities over windows, etc. Typical framing under plywood is 2×6 (R22 batt) w/ 6mil vapour barrier (as in Class I) on insider of stud and GWB directly to vapour barrier. This forum mentions the BC condition that I’ve describing: .

This is similar to figure 12 shown here in BSI:, but the Class I barrier is used as a recommendation in ANY zone and my exterior system will exceed 1perm transmittance (plywood & gap, Tvyek Homewrap @ 30perms). The intention of both BSI and local practice is to leave the building able to pass vapour easily to the exterior and to cut off vapour transmittance (entirely?) at the interior. How about the VB plane though – is the vapour barrier better off in wall or at face of wall? Any other thoughts? Thanks!


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  1. GBA Editor
    Martin Holladay | | #1

    Vancouver, BC is a world of its own. You guys often drill big holes in your sheathing and leave deliberate gaps. Suffice it to say that a lot of these Vancouver practices are based on tips passed from one ill-educated builder to another, not building science.

    Moreover, the Canadian building code is 15 years behind the times when it comes to requiring interior polyethylene.

    1. Don't drill holes in your sheathing.
    2. Don't use interior poly. Use MemBrain if necessary to keep the building inspector happy.
    3. In the case of your suggested wall, more insulation is always better than less, so fill the 2x4 wall with insulation if you want to. Leave it empty if you prefer to have somewhere to run your utilites.
    5. Exterior rigid foam would improve the performance of your wall.
    6. Your slab is below grade. That's not ideal. I hope you have good drainage and waterproofing details. If I were building the house, I'd fill the foundation with compacted gravel, and raise the slab above grade.

  2. user-1047633 | | #2

    Thanks Martin.

    Do the plywood gaps become problematic as the wall will be only as good as the 30 perms of the HomeWrap? Do I need to be worried about permeance becoming too high here?

    And if no interior poly, do you mean use a Class II retarder? I've been leaning towards MemBrain as it'll allow at least some vapour transmittance. That, or I've been looking into finish coatings that can manage vapour for me.

    Any thoughts on vapour barrier at middle vs inside of wall?

    And as for the slab - we have clear crush below the slab and filled the excavation for new sewer/storm with the same. Perimeter drain also covered w/ gravel and fabric to protect against silt blockage. The slab height was necessary to achieve the clients' ceiling within the building height restrictions.

    Thanks for the response!

  3. GBA Editor
    Martin Holladay | | #3

    Plenty of people have built walls with relatively airtight plywood sheathing. More and more builders are taping the plywood so that the plywood becomes the wall's principle air barrier. As long as your wall is well designed, the permeance of the plywood does not represent a problem.

    Yes, a Class II vapor retarder is less risky than a Class I vapor retarder, especially if the building will ever include air conditioning.

    If you plan to use MemBrain, I see no problem placing it in the location shown in your sketch.

  4. user-988403 | | #4

    Even though I am not a big fan of spray foam I would suggest to fill (or partly fill) the inside wall with close cell spray foam. That would take care of the vapor barrier, act as a air barrier and improve the insulation value. That would be a safe and easy approach. If you (like me) want to avoid foam wherever possible I would suggest to insulate both cavities with cellulose or Roxul (as you are proposing), move the vapor barrier to the inside (vapor barriers don`t have to be 100% tight). I totally agree with Martin to use Membrain over Poly for the VB. If you do so you have to designate a airbarrier though and detail it carefully. This could either be done with the airtight drywall approach (as often suggested here) or how I prefer it with taped sheathing (in your case the outside Plywood). Whatever you do I highly suggest to use the inside Stud wall to improve your R-Value. It would be a waste of space and resources if you don`t

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