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Community and Q&A

Thoughts on this wall design?

canada_deck | Posted in Energy Efficiency and Durability on
Intended for:
– Climate zone 5 located near the ocean (in Canada)
– Assume heating for 6 months of the year with outside temps as low as 14 F but more commonly near 33 F
– Assume the use of air conditioning for 3 months of the year with outside temps reaching as high as 92 F
– Assume a lot of rain and long periods of damp weather including fog (mildew and moss grows on everything).   There are 2-3 months of warm dry weather in the summer.
Proposed system (starting at the outside):

– Composite siding (A product like PVC shingles that won’t absorb any water)
– A 3/4″ rain screen with furring strips vented on top and bottom

– A single rockwool rigid insulation board
– Liquid applied membrane
– Plywood
– 6″ stud wall filled with rockwool batt insulation
– Polyethylene sheet
– A single rockwool rigid insulation board
– Drywall
My thinking:
– Vapor barrier is close to the inside of the wall to minimize vapor diffusion in the winter months into the wall as is customary in these parts
– A cladding that won’t absorb a lot of water to minimize the amount of vapor driven into the building when the sun hits the wall after a rainstorm 
– Exterior layer of rockwall helps to minimize risk of moisture accumulation on the inside-face of the sheathing during cold days in the winter
– Interior layer of rockwall helps to minimize risk of moisture accumulation on the outside-face of the polyethylene sheet during use of AC in the summer
Things I don’t like about the system: 
– I worry about the exterior sheet of rockwall becoming damp and staying damp
– Interior sheet may become a pain for installing plugs and light switches
– Flashings may be complicated
– Cost

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  1. GBA Editor
    Martin Holladay | | #1

    It's all about airtightness, not insulation material choices. Pay attention to airtightness, and it will work just fine.

    As a side note: In most of North America, builders don't use interior polyethylene any more (to allow walls to dry inward during the summer). MemBrain would be a better choice than interior poly -- but I know you have to talk about these issues with a Canadian building inspector, and the fight is often not worth battling.

    1. Trevor_Lambert | | #3

      As far as building inspector fights go, this one is pretty cut and dried, and probably well worth having. My experience with the building inspector in regards to our vapour control layer (7/16" OSB), was pretty painless. And he was quite a pain in a lot of other areas, so I don't think he was just lax. As long as you demonstrate that the vapour control layer meets the definition of the code requirements, and you can lead the inspector to that info, you should be fine. From what I can tell, MemBrain looks like poly anyway. I doubt most inspectors would even bat an eye.

      As for these concerns:
      – I worry about the exterior sheet of rockwall becoming damp and staying damp
      As long as the siding has reasonable coverage, the rock wool should have no problem drying out faster than it gets wet, given your rain screen. If you are worried about it getting damp from humidity in the air rather than direct water contact, I'd say that's a non-issue.
      – Interior sheet may become a pain for installing plugs and light switches
      Any air barrier/vapour retarder you install is going to require pretty much the same level of detail around those things. The only way around this would be to keep all the electrical inside the conditioned envelope via a service channel.

      1. canada_deck | | #5

        Re: "My experience with the building inspector in regards to our vapour control layer (7/16" OSB).."
        You are saying that you were using 7/16" OSB as the vapour control layer? Where in the assembly was it placed?

        1. Trevor_Lambert | | #9

          Yes. From inside, the layers were drywall, 1.7" service channel on 24" centers, 7/16" OSB, 16" dense packed cellulose, 1/2" fibreboard, Typar Metrowrap, 1.5" rain screen, Hardy Panel.

          I wasn't suggesting you actually use OSB, I was just illustrating a case where an off-label product was approved with relative ease. A product purposefully designed as a vapour retarder, like MemBrain or the like, should be a slam dunk. The inspector will likely either know his stuff, and recognize it for what it is, or he won't have a clue and just assume it's PE.

      2. Expert Member
        MALCOLM TAYLOR | | #8

        Interior rigid insulation complicates a few things.

        - Standard electrical boxes are designed to mount directly to studs with a spacer holding them flush with 1/2" drywall.
        -Boarding and keeping the drywall sheets coplanar is a lot harder when the walls are covered.
        - The standard corner framing and framing at intersecting interior walls won't work with interior insulation.
        - Solid backing for trim and hardware is now at least 1 1/2" back from the finished surface. What nailer sinks brads that long?

        1. Expert Member
          BILL WICHERS | | #10

          If you go over to commercial 4” square boxes, you can use “mud rings” which are available in depths from 1/4” up to around 2”+. I’ve used those in 3 hour wall assemblies (3 layers of 5/8” type X on both sides of the wall) before with no trouble. Using the mud rings gives much more flexibility when working with thick wall materials. If you seal up the box with foil tape prior to install time, you can caulk a vapor barrier to the flat face part of the mud ring too. You can air seal fairly well working only from one side of the wall this way.

          Oh, and Do your electrician a favor and use 2-1/8” “deep” boxes too. Lots easier to work with.


          1. Expert Member
            MALCOLM TAYLOR | | #12


            Good advice as usual. Both the interior and exterior rigid insulation are easily doable, but both require more work and attention than standard wall assemblies. Including one might make sense, but I'm struggling to see the benefit of including two with all the attendant finicky-ness. What does the interior insulation do that the exterior doesn't, remembering that this is being built in climate zone 5?

          2. canada_deck | | #14

            Thanks Malcom and Bill. Malcom, the idea with the interior insulation was to eliminate any risk of moisture accumulation on the inside of the wall given the vapor barrier on the inside edge of the wall.

          3. Expert Member
            BILL WICHERS | | #18

            Canada, in general you shouldn’t have moisture accumulation on the inside (indoor) side of the wall since that is usually the warmer surface. Condensation only occurs on cold surfaces. That’s why exterior insulation can help keep sheathing warmer, and thus, drier.

            I agree with Malcolm that the interior rigid insulation really isn’t adding anything of benefit. You’d be better off just putting more rigid insulation on the outside of the wall and leaving the inside alone.


    2. canada_deck | | #13

      Thanks Martin. It is interesting to see the differences in codes on each side of the border.

  2. BrianPontolilo | | #2

    There's nothing wrong with using PVC siding as far as your assembly is concerned. But I'm not a fan of using unnecessary plastics. Part of the reason for using the rain screen detail is to allow your siding (and paint, if applicable) to dry and last. So, you have other options there.

    1. canada_deck | | #15

      Yes good point. I am looking for something that will not absorb any water and that is hard to find with natural products.

  3. Expert Member

    I'd bump up the exterior insulation, and eliminate the interior board. Having both complicates the assembly without providing a lot of additional benefit.

    1. canada_deck | | #6

      Would you also move the vapour barrier to the exterior sheathing? The purpose of the interior one was to avoid moisture accumulation on the inside of the wall during AC given that there is a vapour barrier right behind the drywall.

      1. Expert Member
        MALCOLM TAYLOR | | #7

        The code precludes having the vapour barrier on the exterior. It has to be close enough to the interior to not become a condensing surface.

        I don't think adding interior insulation is an effective way to avoid the problems of an interior vapour barrier during the AC season. Much simpler is to use a variable-perm membrane rather than p0ly. However, I've never heard of walls having a problem with an interior VB in the PNW. It may not be ideal, but the condensation issues you hear about in the south don't seem to be present here.

        The overwhelming issues here governing wall and roof design are bulk water intrusion, which is most effectively dealt with by flashing and rain-screen design, and that the climate doesn't provide a long dry season to rid assemblies of accumulated moisture, so aiding drying with effective ventilation of the assemblies is important. Inward solar vapour drive comes a lot lower on the list of concerns.

        1. canada_deck | | #17

          Re: "aiding drying with effective ventilation of the assemblies is important."

          Has anyone gone so far as to consider active ventilation (a fan)?

          1. Expert Member
            MALCOLM TAYLOR | | #19

            Moisture accumulations and damage in wall and roof assemblies in the PNW aren't a problem if they are designed and built properly. Walls need a good air-barrier and a rain-screen with careful detailing of the cladding and flashing. The standard trussed-roof with large overhangs, good air-sealing and ventilation only sees cosmetic mold problems. If these things are in place there isn't any reason to add more layers of complexity to the building.

  4. rockies63 | | #11

    I am using a similar wall design for my cabin in the Canadian Rockies and I'm wondering which product you are using for the liquid applied membrane? Also, how are you detailing your door and window openings to accommodate the rigid rockwool panel on the exterior? How are you venting your rainscreen?

    There have been several comments regarding the vapor barrier's type and location in this thread but not much on the interior layer of rigid rockwool board. I'm guessing it's there to help prevent thermal bridging through the wall studs? If that's the only reason then wouldn't it be better to add a thicker rigid insulation board to the exterior where the cold is? By omitting the interior board you'll really simplify the interior detailing.

    1. canada_deck | | #16

      Hi Scott,
      I'm too early in the design phase to know which product for the liquid membrane or the flashing details.
      The idea with the interior layer of rigid rockwool was to prevent moisture accumulation on the inside of the wall against the vapor barrier when AC is running in the house. It does sound like I can probably skip that.

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