Exterior Roof Insulation Climate Zone 5 – GreenBuildingAdvisor

I know this question has been raised before and I have read the articles related to the matter on GBA. That said, there are differing opinions from reputable sources regarding the safe and effective method of exterior roof insulation. 

Our current plan calls for keeping the existing r-19 fiberglass batt insulation on the underside of the roof decking. Installing two staggered seam layers of 2” and 3” polyiso on the exterior (installed over an air tight, vapor open self adhered roof membrane attached to the roof decking). Then, 2”x4” on the flat installed on top of the polyiso (5” @ 28.8 rvalue). Then CDX, underlayment and roof shingles. 

This is a chainsaw retrofit and, obviously, the roof is externally vented. We are not using spray foam on the interior roof bays/underside of the roof decking.

Is this a safe and effective roof assembly for Northern NJ (climate zone 5)?