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Climate zone 4 double stud wall vapor barrier

DaveHope | Posted in Energy Efficiency and Durability on

Hello,

I am in the process of designing and then building a timber frame cottage for my property located outside Victoria, BC (climate zone 4 or 4c depending on which map you check).  So mild and damp, and given that I am in the woods, damper than average for the area.

I am thinking that a double studded 2×4 wall with staggered studs, using rockwool comfort bats(3 layers), will get me in the R38 range (exterior is 3/4 board and batten fir on vertical strapping with horizontal purlins).

The head scratcher is where to put the vapor barrier.  If possible, I would like to put it on the inside of the inner stud/insulation layer, and use tar paper/felt as the moisture barrier on the external plywood sheeting.
 
I feel that this would keep the condensation point away from the vapor barrier, and allow drying in both directions.

Any feedback to say I’m on track or way off base is appreciated.

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Replies

  1. Jon R | | #1

    I'd be conservative (consider it 4C) and follow the recommendations below. Always air seal well (preferably both sides of the wall) and test for leaks. Design with rain screens and overhangs.

    https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

  2. Expert Member
    Malcolm Taylor | | #2

    Hi Dave, I probably quite close to you - out by French Beach.

    By "the inside of the inner... layer", I take it you mean the exterior side of the inner studs - or roughly 1/3rd of the way through the wall. That would work fine, and meet the code.

    If like most builders here you are planning on using poly, you can detail it as a combined air and vapour-barrier. The location means very few penetrations compared to the usual one on the inside of the wall, leaves it much less vulnerable to damage over time, but does mean a different sequence of construction.

    Usually the poly gets installed after all the framing is complete and the insulation installed. That won't be possible in your case. You need to think through how and when it gets put up, how you will access the perimeter to seal it, and how it is made continuous with the poly on the ceiling.

    Those decisions are dependant on how the two walls are built. There are a few variations. Sometimes the outer-wall extends down to the foundation to cover the rim-joist of the floor. If it is a two story house, it also sometimes is continuous over the second floor rim-joist and of course this is complicated somewhat by how it all relates to the timber framing.

    I would draw a section through the walls and use a pen to establish the continuity of the poly, at the same time thinking of how it gets installed. All a long-winded way of saying; Your idea works, but is a bit more complicated than just putting it on the interior.

    1. DaveHope | | #3

      Thanks for the responses,

      Jon, I understand the need to air seal well at the air barrier, which in this case will be the vapor barrier as well, however, by both sides, do you mean the extreme outside and inside of the wall? Wouldn't this prevent the wall from being able to breath/dry in either direction?

      The 6mil poly I was planning to use would be on the outside of the inner insulated wall, and on the outside of the external plywood sheeting would be a high perm barrier, (probably felt), to allow the wall to dry to the outside. There is also going to be a rain screen with a minimum 3/4 air gap and then fir board and batten.

      Malcom, you're dealing with an even worse moisture/damp challenge than I am!

      I appreciate the difficulty I'm setting myself up with as far as how to get the poly in place...made worse by the timber frame. Fortunately this is a single story build, except for the gable ends, so it won't be too bad putting the studded insulation walls in from the outside. and affixing the poly to the outside of the first wall in place(inner one).

      Do you know if the code requires the treated sill plate to be one continuous piece, or can I have to completely separate 2x4 sill plates, both j-bolted, with a gap between for additional insulation going right to the foam moisture strip over the concrete.

      1. Jon R | | #5

        A close enough way to get two air barriers in your case would be the poly and then also tape the exterior plywood. Air barrier does not mean vapor barrier and so outward drying via vapor diffusion still happens.

      2. Expert Member
        Malcolm Taylor | | #6

        Dave,

        You can use separate sill-plates, and they don't need to be pressure-treated if you separate them from the concrete with sill-sealer. From a code perspective only the outer wall which helps provide the necessary shear resistance needs anchor-bolts. You could use the same method of attachment for the inner one as you are using for interior partitions.

        Your proposed wall sounds like a really well performing one for this climate.

  3. DaveHope | | #4

    Forgot to mention that the floor/foundation.....is insulated stem wall foundation with insulated slab floor as the finished floor.

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