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Community and Q&A

Rockwool ComfortBoard 80

bongo30 | Posted in General Questions on

Hello, we just installed our ComfortBoard mineral wool 1.25” as our exterior insulation. We are in zone 4. Two questions that I have –
(1) has anyone used ComfortBoard on the interior as interior insulation? We have a lot of ComfortBoard 80 left, and unfortunately, cannot return it as it was special order. I’m thinking about using the left overs in our interior walls/ceiling, or maybe basement ceiling. Any concerns with using exterior rigid board on the interior? Not for R-value, but for sound transfer?
(2) also, the contractor installed the rainscreen with horizontal furring strips below and above the windows. This was done so we could secure/nail the window trim. Any concerns with air circulation in those areas? Or is it such a small area that we don’t need to worry about it?

thank you,

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  1. Expert Member


    Feel free to use the comfortboard anywhere. An alternative is that you could list it on marketplace / craigslist / etc. and sell it there, and either recoup some of the investment or use the cash to buy a properly sized batt. The real work is just cutting it up and getting it in place.

    As for (2), I would put some sort of metal flashing at the top of the lower window, so that the flashing was between the first course of siding and the outer surface of the furring strips. Essentially detailing it just like the transition at the base of the wall. There are quite a few ways to do it, but I wouldn't worry about venting that section at the top, or the base of the upper window. If you wanted to, you could use another drip edge there, but that gets kind of flashy looking if you use it all over the place.

    1. bongo30 | | #3

      Thank you! I will discuss flashing at the top of the lower window with the contractor.

  2. Expert Member
    BILL WICHERS | | #2

    Comfortboard is OK to use pretty much anywhere, as long as you don't need it to act as a vapor barrier/retarder. This pretty much means it wouldn't be a good choice on the inside of your rim joist, and you wouldn't want to use it under rafters of a section of roof where you needed to limit moisture migration. It should be fine for the interior or exterior of walls, or in the attic floor of a vented attic.

    I don't think you'll have much issue with those small areas of closed-off rainscreen. If you want to provide for some air movement though, which would certainly not hurt, you could use a router to cut some vertical channels in the horizontal furring strips and then either leave them open, or put in some coravent material. Be careful where you're under the second floor windows though -- you don't want bulk water to sheet off the windows and run into the rain screen gap.


    1. bongo30 | | #4

      Thank you Bill. That was my concern too - that the water would get in from under the 2nd floor window and with the current set up there is no way out for it to escape/exit, and it would just sit on top of the horizontal furring strip at the top of the 1st floor windows.

  3. Expert Member


    If your WRB is back behind the mineral wool, that's where your head-flashing needs to go back to. With a rain-screen cavity it's important to include end-dams, as otherwise water can move laterally along the flashing and end up behind your cladding.

    A horizontal strip of furring under the window is fine, but on top it impedes drainage and ventilation. You are better off just with the vertical ones.

    1. bongo30 | | #6

      Thank you Malcolm, I just went through my pics and looks like we did install our head flashing back to WRB, taped with 4” 3M tape so all is good there. I’m not so sure if end dams were installed, but I’m hoping if the water gets behind the cladding at the window corners it just moves down along the vertical strips until it exits at the bottom of the rainscreen. I’m hoping that’s how it would work.

      I also found this language in Hardie technical bulletin where they approve wider furring strips to accommodate the trim around the window, so hoping we are ok there to. We couldn’t just do vertical strips above the windows as we wouldn't be able to properly secure all the trim.

  4. Jason_K | | #7

    Lana, out of curiousity where in CZ4 are you? I too am CZ4 (middle TN) and have not yet come across a builder who seems familiar with exterior insulation, let alone using mineral wool for that!

    1. bongo30 | | #8

      We are in NY, Long Island. We don’t have a builder, we are the GC so we hired a siding company to install mineral wool, furring strips and siding together. It was extremely difficult to find someone who was willing to install the mineral wool, but now that it’s up we are so happy with it. It’s such a great product, and totally worth it (we also have it under our basement slab but I installed that myself before our concrete contractor poured our slab). The first siding contractor we hired never installed ComfortBoard before and when he arrived at the job site, he demanded that we cut the mineral wool and install it between (!) our furring strips :-) And this was right after he caulked the bottom flanges of my windows which I had to undo :-) So we had to let him go. My advice is try to find someone who is willing to listen, learn and try something new. And ask around. Post here on GBA, maybe there is someone from your area who can recommend someone. We found our current contractor through word-of-mouth. I think there is an older post here on GBA that lists all contractors/consultants by the region. I even emailed (since they are located in Brooklyn, NY not far from us) and asked them which local contractor buys their Tescon Vana tapes in bulk :-) and they were helpful. Do you have any energy raters in your area? They may know someone. Maybe ask your local lumber yard for names of contractors why buy their rigid mineral wool in bulk, and see if they do residential construction. Good luck with your build!

    2. Deleted | | #9


    3. bongo30 | | #10
      1. Jason_K | | #11

        Thanks, I was unaware of this!

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