Hybrid insulation in interior wall cavities.
Two story Garage/Studio building, 2 x 6 framing, Zone 6/6A, Zip code 12461, NY.
I’ve decided on the following interior insulation assembly:
Latex paint on the interior side of outer wall sheathing, 2″ of polyiso foam board, 3 1/2″ of mineral wool batting, Certainteed MemBrain (or similar) behind 5/8″ fire code drywall. There is no exterior insulation on this structure, and the siding product is a T-111 type, 4 x 8 sheeting which doubles as the sheathing.
My question: How do I decide whether to use foil-faced, or fiber-faced polyiso foam boards, and what adhesive would you recommend to attach the foam board to the sheathing given that I plan to cut the foam board 1/2″ light on all 4 sides, then spray foam that 1/2″ space for an air-tight seal.
Thanks in advance.
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I don't see that painting the interior side of the exterior sheathing gains you anything here. It looks like you're planning on using 2" polyiso cut to fit between the studs (what we like to call "cut'n'cobble") foamed in place with canned foam, then 3.5" mineral wool to fill the remainder of the 2x6 studwall cavity space. Membrain over the interior side of the studs, then 5/8" drywall on the interior. This would give you about R28 in the stud cavities, but would do nothing to reduce the thermal bridging of the studs.
Aside from the paint issue I mentioned earlier, I don't see a problem here, but I also think the additional step of using polyiso isn't gaining you much for the extra material cost and labor involved. You could get to R23 with regular mineral wool batts for 2x6 walls. Putting the polyiso on the exterior would address the thermal issues, and you'd end up at about R36 for the wall, and due to the reduced thermal bridging it would perform MUCH better. You could still use the T111 exterior siding, by either using let-in bracing for racking resistance, or using a layer of structural sheathing attached directly to the studs, then the polyiso, then the T111.
Bill
Bill, I neglected to mention a couple of things. This was a pre-fab from an Amish company out of PA, so the building is up and I'm not going to modify the exterior in any way for now. My reason for the polyiso foam board is it would allow me to get some insulation in the walls for the winter without having to deal with running all my electric. I could then, with a couple of plug in heaters, get some heat in there so I could get some projects done, run my wiring and rough in boxes, etc., at my own pace, then add the mineral wool layer. The latex paint layer was suggested by someone else on this forum as an additional vapor barrier/retarder.
So far no one has offered an answer regarding my question about foil-faced vs. fiber-faced foam polyiso foam board.
Perhaps the person who suggested the latex paint had one or the other type of foam board facing (foil or fiber) in mind, but it wasn't made clear to me .
What do you think?
Also, I'm a retired contractor/historic preservation craftsman and at this point at 76 I can't afford to do the whole shebang at once. But I can cut and fit that rigid foam in fairly quickly, then as time and money permits finish up things in a month or so. It's the only way I can do it and with all my stuff moved in on both floors I couldn't bring in a foam company to give me 2" in the wall because there's no way I'm moving all this stuff outside.
Whatever R value I end up with in the walls will be fine, after all it's a workshop.
I'm ready to start as soon as someone advises me on the correct facing on the polyiso for the wall assembly I've outlined.
Ah, understood. The paint layer doesn't really offer much here, but I don't think it's going to hurt, either. If you use foil faced polyiso, the paint doesn't do anything, since the foil facing is a vapor BARRIER. If you use fiber faced polyiso, the paint acts as a bit of a vapor RETARDER, slowing moisture ingress from outdoors during the cooling season, but also slowing drying during the winter heating season. I don't think it will really hurt you here, but I don't think it helps any, either. I don't see any issue with either type of polyiso. You could use EPS to save some money though, at a slightly reduced R value (about R8.4 for 2" instead of R13 for polyiso).
Bill
Thank you Bill. With an uninsulated concrete slab I'll spring for the polyiso boards. I'll compare cost as to the different facing choices and decide one way or another. Either way I'll look for some kind of adhesive to center the boards left/right, up/down until I can spray foam them in place. Maybe something I can apply with a roller for expediency.
Thanks again, and if you think of anything else, please reply.
Al Tirella