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Community and Q&A

Mineral Wool Batts and Exterior Foam

blakeedward | Posted in Energy Efficiency and Durability on

Just wondering what y’all think on the compatibility of mineral wool batts in a wall assembly featuring 5″ of exterior foam.  With rough sawn 2×6 framing 24″ OC.

My hesitation being both the additional compression of the batts intended for a 22.5″ space squeezed into a 22″.  And the 1/2″ gap between the drywall and the batt.

I was hoping for dense pack cellulose, as much of the space is already done this fashion.  But, finding a contractor who is reasonably priced and not booking a year out has proven difficult in rural VT.

Additionally, I’m wondering if folks would recommend an interior air barrier such as Intello, if we do end up going with the batts.  The portion of the house featuring 5″ of foam with cellulose, has no interior air barrier (except perhaps the drywall) because we figured the dense pack would offer enough of a reduction in air movement.

Anyhow, cheers, and thanks in advance.

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  1. Expert Member
    BILL WICHERS | | #1

    Compressing the batt 1/2" to squeeze it into the stud bay won't be a problem aside from some extra work at install time. I've found that "16" on center" is a bit optimistic anyway, so the batts squish a little differently from bay to bay regardless. It's not a big deal as long as you don't deform the batt too much (you don't want a big crease that reduces the effective depth of the batt, for example). Worst case, trim some of the edge off with a bread knife. There is a pretty good knife available inexpensively on Amazon that I've recommended here before. Let me know if you'd like me to post a link again.

    The 1/2" gap between the batt and the drywall isn't ideal, but since it's on the interior side in this wall assembly, I wouldn't really worry about it.

    With 5" of any kind of exterior rigid foam you're in pretty good shape. You don't really "need" a vapor retarder on the interior, but it won't hurt to add one, and a smart vapor retarder like Intello will provide some extra insurance against future moisture issues. Your drywall is the primary air barrier in most cases, just detail it air tight and you'll be fine. Most of the "detail it air tight" part just involves a bead of sealant arount the perimeter wall framing as you put the drywall panels up, so it doesn't really add much effort and it's worth doing. I always detail drywall on exterior walls air tight.


  2. Expert Member


    The walls will work fine with batts or blown insulation of any type, as long as the foam, sheathing, or drywall are detailed as an air-barrier. I don't think you gain much with an interior membrane, but as Bill said it is added insurance.

  3. iwatson | | #3

    If you're having trouble with squeezing them in you can turn them sideways. It means more cuts, but it's easier to accurately crosscut the batts than it is to rip cut a thin slice off the edge. A bread knife will do, as Bill suggests.

  4. dfvellone | | #4

    I also used rough sawn at 24” centers, but had the studs sawn 2x5. The 24” batts fit a little more tightly width wise with the 1/2” less space then they would have with the full 22 1/2, but I think that your full 6” depth will be a benefit in this case. I found that the more narrow stud bay tended to bulge the batt a little more than it would in the standard bay, and your extra inch may accommodate that better.

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