Breaking the thermal bridge on the interior
I had a thought about a possible wall construction that I wanted to gauge with everyone. The primary reason I thought of it was for ease of building for a traditional builder – especially flashing complications and building out around windows if there was exterior rigid insulation. It involves using polyiso on the interior of a 2×6 stud wall. In my mind it allows me to use a thinner layer of rigid insulation with higher r value per inch, placing it on the warm side so it retains its r value in Michigan winters (climate zone 5). And it addresses the thermal bridging issue. The wall would be sheathed with zip system panels, with liquid flashing for the joints (establishing the air barrier on the outside), with a rainscreen of traditional furring strips or something like this Keene Driwall system:
https://youtu.be/-8apmbLOKiw?t=92
http://www.keenebuilding.com/products/building-envelope/walls-and-siding/driwall-rainscreen-10mm
It would be insulated between the studs with dense pack mineral wool or roxul batts. On the interior would be 1.5 inches of polyiso on the walls and ceiling followed by a 2×3 service cavity along the walls and ceiling. I could then use a combination of drywall or tongue and groove wood paneling for the interior walls.
If I went with rigid exterior insulation, I would’ve opted for rigid mineral wool, but with three gable ends in our design, and a vented attic, I would’ve had to continue the insulation up into these areas for continuity reasons in the exterior siding, not for needed insulation value. I can avoid this with doing it on the inside.
I know this sacrifices interior space, and that it is unconventional, with all the recommendations saying to add rigid insulation on the exterior. I just want to know if this is possible, and if it sounds any easier as far as buildability for a traditional builder. Admittedly, I’m still brainstorming here. Thanks again.
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Putting the foam on the interior of the framing does not thermally break the band joists or subfloor, so it's not exactly equal.
But if you're going ahead with the 2x3 service chase you might as well go with 2x4 /R15 on the structural wall, add 1.5-2" of polyiso taped & sealed, and insulate the 2x3 wall with split R23s. The polyiso will still stay in it's higher-performance range, and you'll have a thinner, same or higher performance wall than the 2x6/R23 + 1.5" polyiso + 2x3 service chase described.
At a 25% framing fraction (typical for 16" o.c. spacing) the rainscreen mesh + ZIP + 2x6/R23 wall layer comes in at ~R15 whole-wall, the 1.5" polyiso adds another R10, and the empty service chase + interior gypsum another R2, counting air films. That's an R27-ish wall.
At the same 25" framing fraction the rainscreen mesh + ZIP + 2x4 / R15 comes in at about R10 whole-wall, the 1.5" polyiso adds another R10, and the compressed split-batts in the 2x3 wall + gypsum & air films comes in between R7-R8. Same performance, 2" thinner wall. The first condensing surface is the interior face of the polyiso, which has R25 to the exterior side of that facer, and about R10-R11 to the interior side, which means it'll stay well above the indoor dew point in a zone-5 climate. You won't have condensation events even if the t&g layer isn't air tight, but it doesn't hurt to put a layer of housewrap or something under the t & g detailed as an air barrier.
Alternatively, you could put 1.5" edge strips of polyiso on the 2x4 framing and install R23 rock wool, with a layer of smart vapor retarder between the R23s& polyiso strips and the service chase cavity, which would allow the thing to dry in either direction, using 75% less foam. The hit in whole-wall R would be pretty small, less than R1.
Just saw these older posts that sounded similar to mine:
https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/38446/polyiso-interior
https://www.greenbuildingadvisor.com/articles/dept/musings/walls-interior-rigid-foam
Still am curious if there are any thoughts on my proposal.
Thanks Dana. Very helpful information. The last suggestion you made at the very end, does this resemble what you're describing?
http://m.finehomebuilding.com/how-to/articles/breaking-the-thermal-bridge.aspx
With my proposed air barrier being in the exterior with the sealed zip system, I might just skip the service chase, use the extra thick mineral wool batts in the 2x6 walls with 2" polyiso strips on the studs. Followed by drywall. Any smart vapor retarder required here? How does this setup sound?