Hybrid cut-and-cobble foam board with batts in upstairs bathroom exterior walls?
I have a house in Columbus, Ohio (Zone 5) that was built in the early 70s, and I’m in the throes of a complete upstairs bathroom remodel. I’ve gutted the space down to studs (even replaced the subfloor), but I’ve opted to keep the cast iron tub. Well, I’m ready to insulate, but as I read the Q&A exchanges on this site (and several others), I realize that my options are limited. The original insulation schematic included black fiber board sheathing on exterior, unfaced fiberglass batts between 2×4 studs, and a 4mm layer of plastic as a vapor barrier over the whole thing. I tore it all out because much of the once yellow fiberglass under the plastic was now black (be it from mold, air passage, water damage, who knows). The problems I face in replacing the old insulation are
1) The lip on the cast iron tub is really only wide enough to accommodate a layer of Durock cement board and tile, so interior XPS sheathing is out.
2) My original black fiber board exterior sheathing is damaged in places and pulled away from studs in others.
3) I live in Ohio, which has three hot months a year and three very cold ones.
4) Blow-in foam is too expensive for my DIY limited budget.
5) I have one sloped “combo” wall in the space (exterior above the slope line, interior with plumbing and vent pipes below the slope line).
and 6) My stud cavities are only 2×4
So, all this said, it looks like the best I can do is a hybrid application consisting of cut and cobbled 1″ XPS foam board attached to back side of old black fiberboard sheathing (BETWEEN studs) and caulked, unfaced R-8 or R-11 on top of the foam board, and finally, cement board surrounding the tub (and mold-resistant drywall everywhere else). No plastic vapor barrier inside.
So, my question: Is this do-able? Now, I realize that because of the cut-n-cobble, I’ll have thermal bridging through the 2×4 studs, but I had this anyway with the old insulation, and just replacing the old batts and plastic poly with new won’t change that. Right?
I also realize, as Martin Holloday has said in previous posts, that installing XPS between studs is probably a waste of good foam board. But I really don’t know of any other way to work around my existing limitations just short of breaking up and removing the cast iron tub (so that I can interior sheath over the stud walls).
Oh, and BTW: I addressed all of the potential air leak problems when I gutted the room, so I’m hoping that if the black on my old insulation was because of decades of air seeping, this shouldn’t be an issue any more.
My concern is mainly that by installing the XPS between the studs to the back of the exterior sheathing, I’ll be hampering the room’s ability to dry to the outside. Guess the room would now dry to the inside, which I could aid by installing a stronger fan.
Oh, one last variable: The space DOES have a window over the tub.
I’d appreciate any help that the good folk on this forum might give me. I’m a total newb to the insulation game, but I want to get this install right before I seal the walls and turn the project over to the plumbers and tilers for Phase 2.
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