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Recessed Medicine Cabinet on Exterior Wall. Insulation behind?

CTSNicholas | Posted in General Questions on

I’m trying to figure the best *if possible* way to install a recessed medicine cabinet on an exterior facing wall. Right now all I can think of is using 2×2 lumber to fir out my 2×6 wall, so that the medicine cabinet still allows 3.5″ of room for insulation behind it. (Assuming 3.5″ deep cabinet, and 2×6 + 2×2 = 7″ depth)

I would only use 2×2’s for the width of the vanity, and then return to standard 2×6 thickness so my linen cabinet would have the extra space, and the jog in the wall would not be noticeable with the cabinet fit in there.

I live in Zone 5, and am worried about the loss of insulation on top of the fact that the room is a humid environment. Would I be okay by using 3.5″ of rigid foamboard directly behind the box I build for the medicine cabinet, and then continuing my mineral wool in the rest of the wall cavities? With mineral wool I have to use the smart membrane, and wonder how I would detail this medicine cabinet “hole” compared to the standard studs? 3.5″ of XPS foam would not be vapor permeable, so I am concerned all the way around with my osb sheathing being able to dry if it needs to, while the wood in the cavity would only breath towards the medicine cabinet.

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  1. user-1072251 | | #1

    I'd go with the 2x2's.

  2. Expert Member
    Dana Dorsett | | #2

    If you have back-ventilated siding (say vinyl, or anything with at least 1/4" of rainscreen gap between the sheathing and siding) there shouldn't be a problem with stuffing 3-3.5" of foam behind the cabinet, sealing the foam to the OSB with a heavy bead of polyurethane caulk or foam-board construction adhesive to guarantee air never convects behind it, then sealing around it with can-foam.

    If you go that route use rigid polyiso instead of XPS. Polyiso will be both higher performance (~R21 @ 3.5") and greener.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Would I be okay by using 3.5 inches of rigid foamboard directly behind the box I build for the medicine cabinet, and then continuing my mineral wool in the rest of the wall cavities?"

    A. It would be OK, except for the lower R-value.

    If you want to have the full R-value of your wall insulation, consider installing a surface-mounted medicine cabinet. There are two advantages to this type of cabinet:

    1. The mirror is closer to your face, and so it provides a better image (closer view) for shaving or makeup.

    2. You don't have to reach as far to get the bottle of aspirin or Ibuprophen.

    -- Martin Holladay

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