Polyiso along ceiling under truss chords
New build in climate zone 5. My plan is to establish my air barrier on the outside with zip sheathing and liquid flashing, and want to continue the air barrier across my ceiling (below a vented attic with blown cellulose). I thought about continuing the zip panels across the ceiling attached under the truss chords and tape them from the inside. I want to have options for drywall and tongue and groove wood for different rooms. I also thought about running polyiso boards in the same manner, maybe in two layers with staggered and taped seams. Any thoughts on this option? If I plan for an extra thick layer of blown in cellulose for the attic with raised heel trusses, is mitigating the thermal bridging through the truss chords with the polyiso something worth considering? And can the polyiso address the air sealing I’m looking for? I’m also aiming for limited ceiling penetrations, with any electrical or lights being run in decorative sofits or faux beams.
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That's a pretty expensive and complicated air barrier.
With trusses and deep blown insulation the trusses get buried and there is a good thermal break over the structural timbers. For say, R60 cellulose you'll have R50 of cellulose above the the trusses, which makes the additional R12 of polyiso much less important.
There may be an argument for installing a layer of OSB screwed or ring-shanked nailed to the underside of the trusses to manage the dead-weight load of the cellulose at high-R. Given typical 24" o.c. truss spacing you'd be taking 3/4" gypsum to the limits if you were installing R75+ up there. Detailing it as an air barrier isn't difficult usingt the appropriate tapes & caulks (even duct mastic on the interior does a good job).
You don't really need ZIP there, since the OSB is on the conditioned space side of the assembly where it's warm & dry. ZIP is more appropriately used on the exterior, where it's cold and wet some of the time, hot and wet some of the time, and everything in between.
If the OSB isn't needed to support the weight (do the math on that), a less expensive continuous air barrier using broad-sheet goods such as housewrap is good enough, as long as you detail it where all taped seams are directly over the framing. You could also use 6-mil polyethylene (the gold standard in Canada, where it's a code requirement) but you don't really need a vapor barrier in a zone 5 ceiling with a vented attic above the insulation, and leaving it vapor-open enough to dry toward the interior is still a good thing. OSB is a smart vapor retarder, less than 1 perm when bone dry, but 5+ perms under sustained high humidity, which is fine.