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havelock wool insulation outside WRB

ultra_archie | Posted in General Questions on

Hi all,

I was curious if anyone had any thoughts about using havelock wool batts as an insulation layer outside of the WRB. The application is over a CLT wall so we could apply WRB to the CLT, then attach 2×4 battens at 16oc with 2″ havelock batts between. Then could apply our wood siding over the same battens. Detail attached.

I supposed another option could be to move the WRB to the outside of the battens which would keep the wool inside the barrier and more protected. Didn’t know then if that would be negating the air gap behind the wood siding and the rain screen would no longer function properly. Also included this detail.

Please let me know if anyone has any thoughts on this.
thank you!

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  1. Expert Member
    Michael Maines | | #1

    Are you showing a vertical or horizontal section? In either case, I'd suggest looking at using a Swinburne truss:

  2. ultra_archie | | #2


    Thanks for the reference article, that was very helpful. Wondering if maybe a rock wool batt would be the way to go similar to what's described there. Seems like Havelock wool would perform just as well in terms of vapor permeability, just wonder about its ability to stand up to the abuse of being on the exterior side of the barrier.

    The section shown is a plan view, so a horizontal section. Sorry, I should have clarified that in my original post.

    thanks again for the insight!

  3. Expert Member


    A few random thoughts:

    - I don't think it makes any significant difference where you put the WRB. I would decide that based on which plane your windows and doors will be on.

    - Are you in a climate zone where R-12 wall insulation makes sense, or meets code?

    - If you are using 2" x4"s as strapping, why not just extend the floor below out beyond the CLT walls and build a conventional stud wall, rather than hanging everything off the CLT.

    - One of the main advantages of exterior insulation is it's continuity. Wouldn't it be better to use mineral wool boards and strapping so it was not interrupted?

  4. ultra_archie | | #4


    Thanks for the feedback.
    - I'm planning to put the windows/doors at the plane of the CLT so thinking it makes most sense for the WRB to be directly over the CLT panels.
    - Yes, I'm down in the Piedmont of NC and actually our code doesn't even require any insulation over the CLT since it qualifies as a thermal mass with r5 integral to the material (I did confirm that with inspector). Thinking I would just apply a layer of exterior to be safe.
    - Long story, but already have the CLT on hand and want to use it all up. If I were to extend the floor and build a 2x4 (or 2x6) wall then I wouldn't need to use the CLT wall panels
    - I've thought about the mineral wool boards (rock wool comfort board) as an option. I could do 1.5" of that pretty easily and avoid the need for the outriggers. Maybe that's the better option.


    1. Expert Member
      MALCOLM TAYLOR | | #5


      A CLT structure with mineral wool continuous exterior insulation, and strapping to both support the cladding and provide a rain-screen gap sounds like a pretty good wall to me.

      1. ultra_archie | | #6


        I think you're right. I'm really starting to think that might be the way to go. The reason I was hesitant on mineral wool at first is because in this guide - - it shows a chart on page 18 where it talks about insulation types that says mineral wool is not great for the hot humid south (using Atlanta as its example).
        I can't figure out why it doesn't recommend mineral wool would be good since it's so vapor permeable, but maybe I shouldn't let that sway me.

        Thanks again for your feedback. Super helpful.

        1. Expert Member
          MALCOLM TAYLOR | | #7


          That's a good guide. I'll admit I didn't read it exhaustively, but wasn't the caution about permeable exterior insulation made in the context of walls with reservoir cladding and no rain-screen? Elsewhere it said: "Laboratory research and hygrothermal modeling have shown that CLT walls, with vapor-permeable exterior insulation and a vapor-permeable interior finish, are expected to perform well in a hot, humid climate"

          Good luck with your build!

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