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Redoing a bathroom where one of the shower walls is an exterior wall. Framing is done with 2x6 and I would like to replace the current insulation pictured with XPS foam board. Due to my location, the best I can get my hands on is 2” thick with an R10 value.

My plan is to double up since I have room and worth it in my opinion. Main question is should I put in a vapor barrier? House was built in 1984 and no vapor barrier anywhere else.

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XPS does not need vapour barrier. I tape mine to the studs using technical tape (red vapour barrier tape). Fill gaps with expanding foam.

If you are getting this inspected, check first: climate and jurisdiction matters. As with vapour control and inspections: "opinions vary on the facts" if you get my drift.

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  • Should I worry about having the XPS foam board all the way against the exterior sheathing? I plan on leaving a small air gap between my two foam boards since that will also "help" insulate/buffer. But wondering if I should also leave some breathing space for the sheathing Aug 2 '20 at 21:28
  • The sheething breathes outward. I am not sure what you mean with "air gap between my two foam boards". Are you layering? I have not left a gap. I had it against concrete, and then filled the remaining space with fiber glass insulation batting. The fibre glass then breathes into the house, and this works if the XPS is thick enough for your climate to prevent condensation on its inside surface.
    – P2000
    Aug 3 '20 at 0:45
  • I have 5 1/2 inches of space between my sheathing and the finished wall. If I double up two 2" foam boards, that leaves me with about 1 1/2" of space to deal with. Aug 3 '20 at 2:49
  • Ah, I see. As you perhaps know, the required R value is defined in your national code and climate zone, and may differentiate between above/below grade. You have 2xR10 plus a bit (I think R3) for the drywall and sheathing. Is that enough for your area? In any case I would fill it with fibre glass. Compress a thicker batt, e.g. a 3.5in down to 1.5in to get even more R than you would with 1.5in uncompressed ("peeled") fiber glass. You can also drop down to the cheaper EPS.
    – P2000
    Aug 3 '20 at 4:22
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The 4 inches of foam board acts as the vapor barrier. Just make sure you use spray foam to fill the gaps around the foam board install. Air gap is not necessary.

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