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WRB and exterior Rockwool Insulation

STEPHEN.O.HARDING | Posted in Green Building Techniques on

We are using 2×6 studs, with cavity insulation, plywood sheathing, Siga Majvest as the WRB, then adding 4 inches of Rockwool Comfortboard to the outside before installing the rainscreen furring strips ( which will be 2×4 laid on the flat). ┬áIs there any value (or harm) to installing Tyvek sheeting outboard of the Comfortboard before installing the furring strips?

climate zone 4 Marine in Victoria, B.C. Canada just north of Seattle and Port Angeles.

Thank you

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  1. canada_deck | | #1

    I assume your vapor barrier is on the inside of the wall just behind the drywall? Where is your air barrier? Will it be at the sheathing or the WRB?

    I can't think of many reasons to add an additional WRB so I probably wouldn't bother.

    Are you using pressure treated 2*4s? Are you going to vent that rain screen at the top and bottom or just the bottom?

    With that wall assembly, I imagine you are going to have some crazy details to sort out for receptacles, hose bibs, windows, etc.

    Disclaimer: I'm just a hobbyist.

    1. canada_deck | | #2

      I'm curious about how you are attaching siding. Is the idea that the 2"4s are meaty enough that you will screw those through the 4" of insulation into a stud (e.g. with 8" screws) and will then nail the siding directly into the 2*4s?
      In that case, I wonder if there is an argument for going with cedar instead of PT. I know PT is obviously cheaper but it is sopping wet when you get it from the lumberyard and it can twist and distort as it dries. It's probably not an issue if you have it constrained with screws into the studs and also nails from the siding but something that makes me think. It can also be corrosive to fasteners.

      All of this further discussion is making me think of one thing...
      When we get really foggy conditions (where water is beading up on everything,) if you do have a 1.5" rainscreen with top and bottom venting, I would imagine you might get water beading up on the Rockwool. If you had a layer of Tyvek over the Rockwool, then it would bead up on that and drip down more quickly (presumably the Rockwool would hold more moisture on the surface since it isn't smooth.) Intuitively, it still doesn't feel to me like it's worth the trouble vs other areas you could be investing in. How would you even go about attaching the Tyvek to Rockwool?

      1. Expert Member
        MALCOLM TAYLOR | | #4


        If you are having problems with PT ACQ lumber, find a yard that stocks MCA (often branded Micro-processed Sienna). Most to the ones around me do. It is dry, doesn't affect fasteners, and cleans up well enough I use it for trim like fascias.

        1. canada_deck | | #6

          Interesting. I have used both and have found that the MCA is also wet. I'll pay closer attention on my next project.

  2. Expert Member
    Akos | | #3

    No benefit, just makes your life more complicated as it is pretty hard to hold the house wrap over the MW before the strapping goes on.

    Important part is to figure out your window flashing details and get those right. Windows at the sheathing plane is the simplest but that means exterior jamb extension which adds more work. Outie windows on buck gets around this problem but there is a fair bit of flashing tape origami to seal it.

    P.S. You don't need PT for rain screen strapping. Since it is protected from the elements behind your siding, regular framing lumber is fine.

  3. Expert Member


    The only complication I can see of having the WRB back at the sheathing is flashing the windows and doors. Depending on whether they are innies or outies, the bucks or exterior casing needs to be protected. As Akos said, that is usually done with membranes and flashing tape, but the BCBC requires metal head-flashing as well, which will somehow have to be integrated.

  4. rockies63 | | #7

    I take it you're using 2 layers of 2" thick Comfortboard with the seams offset on the exterior? Four inches is a lot to attach furring strips through, and I don't know why you need 1 1/2" of air gap between the Comfortboard and the siding - you risk a lot of stress on the furring strip screws and potential siding sag.
    Have you looked into using Cascadia clips to attach your furring strips to? They will really help you support your exterior siding.

    There's also the issue of furring out your door and window openings. Have you looked at Thermalbuck?

    The last things to consider are how you're going to detail the bottoms and tops of the rain-screen system, as well as inside and outside exterior corners.

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