I’m looking for advice on how to deal with installing ventilation baffles around an insulated metal chimney penetration in a cathedral ceiling.  Climate zone 5.  Rafters are 8″ deep.

The finished assembly will be (from outside in):
– Corrugated metal roof
– Triflex underlayment
– 1″ rough pine sheathing
– 2″ air space
– Smartbaffle corrugated plastic baffles
– 5.5″ Roxul batts
– 4″ foil-faced polyiso on inside of rafters (taped)
– 1×4 strapping
– 1/2 drywall

The rafter bay in question is 20″ wide, with the insulated metal chimney (8″ in diameter) going up the middle of it, therefore leaving 6″ on either side of the chimney.  There are about 4 feet of roofing between the chimney and peak of the roof (ie. it’s up near the top of the roof).

There are 2 possible scenarios I see for detailing the ventilation channels, and I’d love advice on which is best:

1) Use the plastic Smartbaffles up to ~6″ on either side of the chimney (which requires 2″ clearance to combustibles), and use sheet metal for the baffle around the chimney (caulked to the chimney with heat-safe caulking).  My question here is whether the 6″ of free airflow on either side of the chimney is going to be enough to allow the venting assembly to function properly.  I suppose there will also be some venting out the 2″ gap in the roofing around the chimney where it will vent into the chimney flashing “boot”.  

2) Treat this rafter bay as an unvented roof assembly and use 2″ of closed cell spray foam on the underside of the sheathing instead of the air gap (the rest of the assembly remains the same, except I would block around the chimney so no spray-foam was close).

I’d honestly prefer to do the vented channels so I don’t have to deal with spray foam, but if that is risky I don’t want to screw things up at this stage in the game.

Thanks!