Air sealing unventilated cathedral ceiling wall-to-roof transition – GreenBuildingAdvisor

The layers of my roof assembly are:

80 mil PVC membrane OVER 1/4″ fire protection fiberglass-faced gypsum board OVER tapered EPS over 1/2″ plywood over 16″ OC joists w/insulation batts (and spray foam in some places).

Question: Should I be trying to air seal the edges of this sandwich of materials where it terminates at the top of walls? It seems to me that air from the exterior could leak into and out of the roofing layers at the edges. I’m not sure if it’s a problem or not.

Air from the inside could leak from between the plywood sheathing seams (which I didn’t tape) to the bottom of the EPS layer, through the edges of EPS sheets (EPS is not overlapping; it comes in thick tapered sheets applied from the roof sheathing  to thicknesses up to 6″), through  the edges between gypsum boards. At the edges of the roof, the air can escape through any of these layers.

All the roofer plans to do is screw the edge metal down and then apply heat-welded PVC patches over the screw heads. He does not plan to put sealant between the fiberglass-faced gypsum board and the PVC at the edges, nor does he plan to apply tape or anything to span from the gypsum board/PVC over the EPS to the wall. Of course, WRB will be run along the wall above the termination of the edge metal drip edge.

Attached is a photo of where a low-slope roof meets up with a wall. The roofer has left a flap of PVC going down the wall and will come back later to apply the “Coated Metal Drip Edge” (attached is an image), which is a metal L shape with PVC bonded do it that allows the PVC membrane to be heat-welded to the drip edge.

If air sealing this is a good idea, I can think of some ideas for how to do it. Tape could be applied from the fiberglass (fuzzy horizontal face) over to the vertical wall sheathing after I have applied the WRB (Prosoco Cat 5). I could then ask the roofer to set the edge metal in a thick bead of sealant before screwing it down, maybe silicone or Prosoco FastFlash.