Best venting/insulation option for 2/12 monoslope roof assembly – GreenBuildingAdvisor

Zone 4A. I am building a 36×42 ICF house with a 2/12 mono-slope roof that will face North. My initial plan was to use 14′ TJIs for the rafters to span halfway to a load bearing wall running the length of the house. It would be an unvented assembly topped with sheathing, roofing membrane, 1″ vertical furring strips and 1″ horizontal furring strips to attach a standing seam metal roof. The interior would have drywall and paint to allow vapor diffusion.

After learning more of vapor control, I am considering 24″ trusses that are able to span without the need for the load bearing wall. It would be filled with 18″ of Rockwool and have 6″ of vented area bef0re the roof sheathing and standing seam roof. It looks like a smart vapor retarder secured by furring strips for electrical would be needed prior to drywall and paint. The draw back is the height this assembly adds to the wall.

My main goal is durability and a minimum of R49

1. Which is a more forgiving structure for vapor/condensation?

2. Would I need to vent under the Standing Seam with the truss assembly?

3. Would a 2/12 roof vent enough with top and bottom eave vents?

4. Any recommendations for the assembly?

Thanks in advance,

Robert