I'm working on the design of our home which will be ICF walls flat ceilings on 2/3rd of the living space with a cathedral ceiling down the center. Picture a gull wing roof with 3:12 side pitches and 12:12 pitch in the center. Creating an unvented roof is looking prohibitively expensive since 2/3rd of the roof has a flat ceiling and can be blown. This leads me to a vented ceiling assembly. I've seen pre made baffles like SmartBaffle which look good. I've also seen folks who build baffles with 1" XPS foam board or polyiso foam board. I'm confused about condensation in the insulation against a baffle made of foam board. Is that not vapor impermeable enough to be the condensation plane? Being in zone 4A, 1" of insulation would not be adequate to prevent that condensation. A premade baffle caulked in place with R38 would insulate but are there better ways to do this? Would flashing 2" of foam against the baffle and then batt be better? I'm wondering if even 2" of closed cell foam would be enough to prevent condensation in an R38 assembly. I might be over thinking this.... Thanks!

Foil faced polyiso used as a baffle would have a potential condensation issue if the R-valuve of the polyiso isn't sufficient for dew point control at the interior side facer. This is climate dependent- in US climate zones 1-3 an inch of polyiso would be enough, but in zones 4 & higher it grows rapidly.

In zone 4A the R-value of the polyiso would have to be at least 30% of the total R to have adequate dew point control. That means at least 3" (R18) polyiso baffles over R38 HD batts for about R56 total to beat IRC 2012 code-min. That would usually be cheaper (and a heluva lot greener) than spraying 2" of closed cell foam on the underside of 1" polyiso.

If the goal is R38 total (sub IRC code, but complies with some local codes) 2" polyiso (R12) and R28 batts works (barely), but you'd need at least 2.5" for R30s.

If using 1/2-1" unfaced XPS the R-value hardly matters, since the material itself is semi-permeable. Thinner is more permeable than thicker. Half-inch unfaced XPS is about as vapor permeable as standard latex paint and is sufficiently rigid to make it a better choice than 1" for this application.

Unfaced 1.5lbs density (Type-II) EPS is about 2x as vapor permeable as XPS at any given thickness, and usually quite a bit cheaper too. With type-II EPS even up to 3" (R12.6) does not present a significant moisture trap.

Where possible it's better to design out or minimize the amount of XPS or closed cell spray polyurethane, due to the environmentally damaging blowing agents used. Both polyiso and EPS are blown with pentane, which is a lot more benign. The HFC134a used for blowing XPS has a global warming potential ~1400x CO2, and the HFC245fa use for closed cell polyureathane runs ~1000X CO2, compared to the pentane at ~7x CO2. Also, the majority of the pentane used for blowing EPS and polyiso is recaptured at the factory, and burned on site for process heat, converting it to CO2 & water.