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Community and Q&A

Installing rigid mineral wool on the interior of studs

Douglas Wathen | Posted in Green Building Techniques on

Hello,

We have an old addition on our house that would benefit from additional insulation. It is stripped to the studs at present, from the interior, and the plan is to spray an inch of foam and the remainder of the cavity with cellulose (the combo is for reduced sound transmission). I would like to add additional insulation on the interior surface of the studs. Access to the exterior would be difficult. I am wondering about adding 1″ rigid mineral wool.

Martin posted an article on 3/27/15 that discusses rigid foam on the interior, and makes mention that some builders use mineral wool.

Any experience or advice would be appreciated.

Also, where can we buy 1″ rigid mineral wool. I find 2″ mineral and 1″ fiberglass, but no 1″ mineral.

Thanks,

Doug

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Douglas,
    I advise you to use rigid foam, not mineral wool, for two reasons:

    1. You are unable to find 1-inch-thick mineral wool.

    2. Mineral wool is squishy, and that squishiness will make it hard to install the drywall.

  2. Douglas Wathen | | #2

    Thanks Martin,

    One concern. The spray foam in the cavities will be an air/vapor barrier, so if we put rigid foam on the interior, aren't we creating two barriers? The mineral wool would not be a barrier.

    I know they make rigid mineral wool "boards" but I don't think they have caught on yet in this country, at least not in 1". I found some rigid 1" fiberglass "board" that is intended for sound control but is evidently very stiff.

    Thanks again,

    Doug

  3. Nate G | | #3

    You can get rigid mineral wool boards of any thickness and stiffness, but you may have to go to a lumberyard or building supply house.

  4. GBA Editor
    Martin Holladay | | #4

    Douglas,
    Q. "One concern: The spray foam in the cavities will be an air/vapor barrier, so if we put rigid foam on the interior, aren't we creating two barriers?"

    A. Since you don't plan to sandwich any materials between the spray foam and the rigid foam, there's nothing to worry about.

    If you were sandwiching OSB sheathing susceptible to water-entry damage between exterior rigid foam and interior spray foam, you might have a worry. But this case is different. You're on the interior, so you shouldn't be getting any rain. And there is no OSB sheathing to worry about.

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