Recessed Medicine Cabinet on Exterior Wall. Insulation behind?
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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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Replies
I'd go with the 2x2's.
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.
Nicholas,
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