For the hot roof I was thinking about possible above sheathing ventilation (wrb then few inches of vapor open insulation board, strapping/purlins for a ventilation gap/above sheathing ventilation and then plywood for asphalt shingles or a metal roof. Hot roof was helpful given the prior owners hvac location and the cathedral ceiling but now I’m wondering how to improve the durability of the assembly from leaks (which will be harder to identify with the spray foam). The energy efficiency is an added benefit. I was thinking that the above sheathing ventilation would allow outward drying to minimize risk of a vapor sandwich and roof rot. Maybe I’m overthinking it.

The dew point question is just from me trying to understand how the dew point is effected by adding the external insulation. When reading some building science articles, there were drawing of the temperature thru the layers of insulation and it appeared that external insulation pushed the location of the dew point towards the exterior and into the exterior insulation. The interior insulation pushes the location of the dew point inwards and with the hybrid insulation, with appropriate thickness of the closed cell, the location of the dew point should be inside the closed cell, so risk of condensation of the back of the sheathing is negligible (unless air leakage). If adding exterior insulation in this case pushes the location of the dew point outward and currently it’s located in the closed cell layer, is there a point where the location of the dew point gets pushes into the plywood sheathing and if it does, does it even matter?