Edit: simplified this question and posted in Q&A per Martin’s instructions to another user.

I’m in climate zone 3a (Oklahoma) and working out the roofing system of a 2:12 single slope roof on my 1900 sq ft ICF house. It will be an unvented roof, and I’ve read the following (fantastic) articles here on GBA, among others:

“Insulating Low Slope Residential Roofs”

“How to Build an Insulated Cathedral Ceiling”

“Open Cell Spray Foam and Damp Roof Sheathing”

My plan is to install, from top to bottom, standing seam metal panels, synthetic underlayment (permeable), OSB roof deck, closed cell spray foam of 2-3″ (under roof deck) + open cell spray foam to achieve needed R-value of 38+. 

Three questions with this plan!

First, should I actually use a permeable synthetic underlayment? My thinking is that everything below the spray foam will be drying to the interior, while everything above the spray foam would be drying to the exterior. But this also means…water that makes it to the underlayment will go to the OSB. Isn’t it there to prevent this? It seems clear that impermeable above OSB and impermeable below OSB is a recipe for rotting OSB if moisture does collect there. Thoughts?

Second, based on the above, should I tape the seams of the OSB with a Siga Wigluv tape or similar?

Third, thinking through the need for thermal breaks between the OSB and the top of the trusses. OSB will be nailed into the top of the truss, and that OSB will be hot in the summer and cold in the winter. That will transmit into the top of the truss, and then down to the bottom of the truss (rafter) and then into the drywall panels that are attached to it — even though there’s plenty of spray foam around the truss. Is this actually an issue worth worrying about? Does it have any implication for moisture?

Infinite thanks to anyone who can help. Really love and appreciate this site and the countless threads I’ve read to feel confident getting to this point.