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Roxul semi-rigid Comfort board as fill between double stud walls

cabinflyer | Posted in Green Building Techniques on

Am in the process of starting the interior of my 1500 sq. ft. single level home in Zone 7. 10,000HDD.

The “exterior” stud wall is 2×4 24″OC. We have decided to then have a 3″ cavity, then another 2×4 “inner” stud wall. A smart membrane (Intello Plus) will be on the inside or cavity side of the inner wall. Any needed electrical will be inside the inner 2×4 wall, which allows for a continuous unbroken air barrier, save 6 triple pane windows and 3 insulated doors with very little chances of piercing the membrane in the future with pictures hangings or the like. The walls are 10″ thick with somewhere around R36-39.

We have already put in dense packed cellulose on the exterior stud wall covered with mesh netting.

Question: I am wondering if Roxul Comfort board 80 would be a good choice for the 3″ cavity between the two stud walls? Roxul is available locally in 24×48″ boards, and in the necessary 3″ width. It seems to be on par with the costs of dense pack cellulose, maybe even a little less.

Doing the cavity with rigid Roxul boards allows me to fill the cavity without piercing the Intello membrane.

Any thoughts on this application?



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  1. user-2310254 | | #1


    I will bump this post to move it back to the top. I suspect you could use the Roxel but that more cellulose would be less expensive and more forgiving.

    While you're waiting for more feedback, be sure to read this article:

  2. ethan_TFGStudio | | #2

    This video from Postgreen homes might interest you...

  3. Expert Member

    If you have already dense-packed the exterior walls, how are you going to install a continuous, well-sealed air/vapour barrier on the outside face of the interior stud wall - especially if you fill the cavity with mineral wool boards first?

    Unless I'm missing something this a really un-buildable wall assembly?

  4. cabinflyer | | #4


    The air barrier is stapled to the inside of the wall with enough hanging over to be continued up to the ceiling.

  5. Expert Member

    Hopefully your build will prove me wrong. Standing interior walls once the roof is on is quite difficult. Doing it while keeping the membrane intact will be even more so. You might want to think about cutting your interior studs down a 1/4".

  6. cabinflyer | | #6

    We already planned for this, and are doing pretty much what you are proposing.


  7. Expert Member

    You may well end up teaching an old dog new tricks. I hope you blog the build here on GBA.

    In the midst of all that I have ended up side-tracking you from the question you set out to get answered. If you can get the comfort board at a reasonable price, I can't think of any downside - or benefit to going with dense-pack for the cavity.

  8. KeithH | | #8

    Why use comfortboard instead of comfortbatt?
    Comfortbatt is cheaper by a long shot and smells better and is easier to work with. Comfortboard also doesn't compress nearly as well so I've been meh about single layer installation quality compared to comfortbatt which makes beautiful tight single layer installs. It has the worst properties of rigid and the worst properties of flexible. Comfortbatt at least owns its squishness.

    My suggestion:
    I would make my gap 3.5" for the option to use stock 2x4 products (insulation or framing). BTW, you could probably compress the roxul batts into a 3" space with minimal damage if you had a lot of access. I've done so in a flat framing situation. 3" works ok , 2.75" is a messy wreck, and 2.5" impossible. Your tip up style construction with the intello in place on the backside is probably a problem for using batts though as you can't pre-install them. But you could do tip up with no intello and slide the batts in through the stud openings without too much trouble but you'd have to put your intello on the outer wall interior side before the roxul or on inner wall interior side.

    FWIW if you are just putting electrical wire in the service wall and are putting drywall on the wall, I don't see the point of the service chase. It isn't getting you much.

    If your moisture model can accommodate putting the intello plus on the interior side of the exterior wall, why not do so? Super protected from damage, easy to install, modification to the wall are unlikely to compromise it. If your moisture model can't accommodate that, can it actually handle the double stud wall?

    I'm curious: did you look at exterior larsen trusses?

    BTW, I love Intello Plus and duplex. (get the dispenser tool). But avoid the regular ungrided Intello. It doesn't seem to have the same dimension stability under install tension.


    **DIYer disclaimer

  9. cabinflyer | | #9

    Keith wrote: If your moisture model can accommodate putting the intello plus on the interior side of the exterior wall, why not do so? Super protected from damage, easy to install, modification to the wall are unlikely to compromise it.

    This is actually my first choice and will make my life SOOOOO much easier!!!
    We are having the Intello folks do a moisture modeling now. I'm looking forward to working with this product and hope to have good things to report.


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