Insulation for unvented roof, mixed climate with metal roof – GreenBuildingAdvisor

I’ve gotten myself all turned around and I’m hoping y’all can help me out. Really want to make sure to get this detail right and not have it be something I regret down the road with mold/rot.

Building in the Raleigh, NC area. Walls are zip r9 with 2×6 studs. Rafters are 2x8s. Attempting to be pretty airtight… aiming for 0.6 ACH50… but spec’d HVAC system with 1 ACH50 calcs. I borrowed this “modified monopoly framing” idea from https://www.youtube.com/watch?v=sYDT17A6Cr4&t=352s and in hindsight I think I regret it… but it would be a fair amount of work to change at this point as all the framing and taping is done.

I already ordered metal standing seam roofing which is expected to arrive in the next two weeks. Roofer has already put down ice guard across the entire roof. I’ve reached out to the metal supplier to see if they’ve already cut the metal or not, but haven’t heard back yet. If they haven’t cut the metal, I’d consider doing exterior insulation on top of the current roof sheathing.

HVAC contractor has already put a number of rigid metal ducts right up against the rafters in the attic. If absolutely needed, we could likely lower them, but not necessarily an easy option.

With those caveats out of the way… how do I make sure I don’t end up with mold / moisture issues under my roof deck and maximize my roof insulation R value?

Option #1

After talking with ChatGPT, it suggested that the best way to handle my situation was to put 2x4s or 2x6s perpendicularly underneath my rafters with an intello plus smart membrane as a vapor retarder between layers of mineral wool to prevent condensation building up under my zip and rotting it out. The extra furring protects the vapor barrier, minimizes thermal bridging and gives room for more insulation. This sounded reasonable to me until I walked the build over the weekend and realized there are a lot of dormers and weird roof angles in addition to the metal HVAC ducts currently being right against the rafters.

Option #2

Using closed cell foam under the roof deck. I know this isn’t the best environmental choice nor the least expensive, but from a performance and logistics pain do you think I’ll regret it? I could spray closed cell in the full 7.25″ of the rafter bays – ChatGPT tells me that my effective R value will be something like R35 due to thermal bridging. The added risk here is that if water intrusion happens on top of the spray foam, it’ll rot my rafters and roof deck for a long time before I’d know it. Is that likely with zip sheathing, ice guard and a metal roof on top? Would you risk it?

Option #3?

Lastly… I talked to my architect today and she didn’t seem to think I needed to worry about having a vapor retarder at all and that I should go forward with open cell spray foam. Is she right? Could I go ahead with open cell spray foam or mineral wool right under the zip sheathing and not have to worry about mold from condensation forming under the zip when it’s cold outside and warm inside?