The choices are

(1) Re-roof with 4″ of polyiso insulation above the deck

(2) Re-roof conventionally, then blow in insulation.

This is a California marine zone 3A fog belt 1920’s wood frame and stucco apartment building with a mixed flat and clay tile roof.  The flat attic is crawl-able at 2 to 3.5 feet in height.  The attic is vented.  Unlike many other nearby buildings there’s no sign of moisture or mold issues, not even in closets (a classic problem around here, especially with “cool roofs” and local fog).  The tar and gravel likely keeps things well baked.

The owner pays for steam heat.  The top floors get rather too hot under that roof, but it’s mostly an open the window / heat on cold days climate.

How would one predict the moisture drive loss of the iso insulation, and compare these methods?

R26 iso on top of  vented flat attic.

Or R30 ish blown in to try and do cellulose yet maintain air channels under the roof joists, to keep it a vented assembly.  Yes I see the knob & tube.

Or both?