There is a 2 inch vent channel under the roof sheathing created by attaching rigid foam pieces to the sides of rafters in contact with the underside of the sheathing.

I'm going to assume that 2" shims were added to the edge of the rafters to create the air gap and that you have ~2" of polyiso between the rafters. If so, I'd use good tape to air seal the polyiso to the rafters and then add cellulose between the rafters (it absorbs moisture better than fiberglass) to get to R49. If not, first add more polyiso get to 2" and then add fiber fill + interior side EPS to get the final R value you want.

Thanks for the responses. I really appreciate the help. The foil faced polyiso is 1" thick. My understanding is that it is not thick enough to control condensation on the interior. I am thinking that spraying closed cell and filling with unfaced R30 batts might be the best choice. One inch of closed cell is enough to eliminate the dew point on interior foil or do I need R10 of closed cell? Would it be possible to just use spray foam and eliminate the batts? I worry that the interior foil could pull away from the rigid with spray foam weight. Is that a concern?

Thanks you guys are great.

I mistyped there is 8 1/4 inches of joist depth below the one inch poly iso.

With R6 in foam you can safely add about R25 without interior vapor retarders. Compressing a standard density R30 batt into 8.25 inches will be something like R27-R28 ish, which isn't super-risky.  Code prescriptives imply you need ~20% of the total R to be foam in that stackup, and you'd be at R6/R34 which is 18%.  With a kraft facer on the interior side  as the vapor retarder (about 0.5 perms when dry, 5 perms when nearly saturated) and air-tight gypsum you'd be fine.



An R30C high-density "cathedral ceiling" batt  is manufactured at 8.25", and if you installed one of those you'd be at R36 for center-cavity R, but only 17% of R value in foam instead of the code-prescribed 20% for dew point control, which isn't super risky, but would require an interior side vapor retarder to fully meet code.   Fully caulking the sheet rock to the framing as you put it up then painting it with "vapor barrier" latex would be perfectly fine here, and a lot cheaper than smart vapor retarders like MemBrain.

But what ever you do, DON'T put up polyethyene sheeting (not even 4-mil) or foil vapor barriers- it's safer to go with too little than with too much. Air-tight gypsum and standard latex ceiling paint comes in at about 3-5 perms, which isn't terrible. Air-tighness is far more critical than vapor permeance.

If you stapled up netting and blew in cellulose at about 2.5-3.2lbs density you'd also be at about R30, and the cellulose itself will wick the moisture during condensation events (without loss of function), and would be just fine even without vapor retardent paints. That doesn't meet the letter of code, but codes are very un-nuanced and blocky for clarity. With 17% of the total R on the exterior cellulose would not need anything tighter than standard latex paint despite the very low permeance of foil-faced polyiso.

The cheapest solution is standard R30s with kraft facers, which gets you 90% of the way to the IRC 2009 R38 code minimum, which is just fine for a retrofit.

If you want to take it all the way to R49 you can get there with unfaced R30C batts or 3lb cellulose or 1.8lb density Spider/Optima/L77 fiberglas, putting 3" of unfaced Type-II EPS on the interior side between the rafters and sheet rock. At 3" Type-II EPS runs about 1 perm or slightly less, and thus qualifies as a class-II vapor retarder, but isn't so vapor tight that it creates a moisture trap.  But you give up 3" of room height (probably not a problem, but potentially an issue if it interferes with window or door casings where the ceiling meets the walls.)