Two different ways I think you could do it...using standard foam baffels and fiberglass insulation? Or could use radiant barrier (comes in different R value)?
If using the baffels staple them right up tight on the roof to the side of the truss and then add regular insulation. Or if using the radiant barrier roll it out across the truss and staple to the bottom side of the truss.
I in now way am a professional or know what Im doing but those two options seem practical. Might not be a bad idea to contact the supplier of the roof and ask for their opinion
I had my new metal/synthetic coated roofing put on right over my conventional roof that had just been installed 6 months earlier. Tyvek in between the original roof and the
new metal/synthetic coated roof. No sweating, big time noise reduction and great insulation.
We roofed every building on my property in this manner and it worked out great.
Never care about what other people believe, be strong and walk in the way of the Lord.