I have a 12/12 pitch cathedral ceiling.  Rafters are 12″ I joist with 10″ open cell spray foam in them as well as ventilation baffles under the roof sheathing.  At the peak of my ceiling, approx.  four feet down from the Ridge beam, I have created a flat ceiling so I could run mechanical to the area below.  In this flat section I also have placed free air transfer vents to let air move out and through that chase.  To aid this I also dedicated an ERV exhaust vent in there to rid it of that bad air that will be up there.   So, my question comes down to ceiling material and more importantly mold prevention.  I have car siding that I will install and from the articles I’ve read so far it sounds like I’ll need to put sheetrock up first to prevent air transfer and a fire barrier.  However, as far as air transfer is concerned, my attempt with this design was to create continuous air flow between the spray foam and car siding preventing any kind of moisture build up. Would this be accurate? And if so, would I still need a vapor barrier between spray foam and finished ceiling?  Climate zone 4, btw.  And Thank you in advance 

Jeff