Yes, I would certainly be concerned about thermal bridging. As far as vent space above this, I’m really not sure if this would help. It makes sense, but you are suggesting an insulation method that I’ve never seen before. I think your best bet would be to consult with an engineer.

The more the better, of course, but I’m certain that an R32 roof with closed cell foam would outperform an R38 roof with dense pack cellulose.

I’m really leaning towards a hot roof but I need to resolve a few more questions and check our budget… Thanks!

Here is what the code says: R314.5.3 Attics. The thermal barrier specified in Section
314.4 is not required where attic access is required by Section
R807.1 and where the space is entered only for service
of utilities and when the foam plastic insulation is protected
against ignition using one of the following ignition barrier
materials:
1. 1.5-inch-thick (38 mm) mineral fiber insulation;
2. 0.25-inch-thick (6.4 mm) wood structural panels;
3. 0.375-inch (9.5 mm) particleboard;
4. 0.25-inch (6.4 mm) hardboard;
5. 0.375-inch (9.5 mm) gypsum board; or
6. Corrosion-resistant steel having a base metal thickness
of 0.016 inch (0.406 mm).
The above ignition barrier is not required where the foam
plastic insulation has been tested in accordance with Section
R314.6.

My interpretation of R304.5.3 is that you still need to have the foam covered inside the knee walls, but you can use 3/8″ drywall in those locations. If the Minneapolis permit inspector in your area has a different take on that section, please let me know.

- Reuben