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I am working with a GC on one similar, except it's a straight gable.

At this point we are looking at 5" of Iso above the deck, followed by a nailing (screws actually) deck for the metal roof that would go over the insulation. This leaves the underside of the structure completely exposed.

With the gambrel and the steep slope you will need to consider the fastening carefully.

Insulation screws coming through the inside of the decking may not be desirable.

A two ply inslulation could help.

First the perimeter would need to have blocking.

Then a layer of 3-1/2" insulation. Follow with 2x4 straps (flat way) screwed through the insulation and into the trusses/rafters. The orientation of the straps could be horizontal or vertical, but run continuous rake to rake or drip to ridge.

Then in-fill the straps with a layer of 1-1/2" Iso.

Then deck over the assembly and install the roof.

Vapor barrier to the inside of the roof deck or possibly on top of the deck and under the insulation.

This system could accommodate ventilation above the insulation as well, but would require modified material layout.

Hunter makes a laminated insulation product that could be a consideration.

It is important to address vapor transmission with whatever assembly is considered..
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