I am undergoing an extensive renovation that involves adding a second story to an old house (zone 4a) that includes multiple additions including a 20′ x 30′ stone house which will be opened up to a single great room with the attic removed to expose the large timber cross ties and old rafters with wooden pegs.

Part of the second story will bear down on the peak of the roof of the stone house and I am working with an engineer who is advising me on reinforcing the structure with trusses, composed of 2X8 rafters that will sit on top of the old standing seam roof and connect to the cross ties. One side of the roof will be interior to the new second story while the other will be exterior and will need to be insulated. My plan is to use fiberglass batts between the rafters followed by OSB then 4″ of rigid foam on top of that held down my 1″x4″ purlins that will support a new metal roof installed overtop the existing one. 

I have received conflicting advice on the importance of ventilating such a roof and I’m curious if anyone has advice for this particular case. 
I have heard of the paramount importance of creating an airtight ceiling, and I wonder how this might be complicated by using the bottom of the existing standing seam metal as the interior ceiling, with the new rafters and insulation on top of that.