So I’m building a Tiny House with a Hybrid unvented roof and wall assembly.   Several inches of foam board on the exterior of plywood sheathing.  Looking at the hybrid assemblies online, I see that they call for air-permeable insulation inside (especially below the roof deck) and no vapor barrier on the inside.  

Question #1.  Does the insulation towards the interior have to be “Air permeable”?  As in, does air actually need to be able to flow around and through the insulation, or is the prescription for vapor permeability?  In other words, could I install 3.5 inches of open-cell spray foam against the sheathing from the inside (a vapor-permeable air barrier at that thickness)?

Question #2.  If I have the proper ratio of exterior to interior insulation for my climate zone (6), is a vapor retarder required?  I get that I cannot have a vapor “barrier” (i.e. Class 1) because I need drying to the inside, but I can’t seem to find information as to whether a vapor retarder is required (other than by code officials).  If it is required, I guess that would answer question 1 as well.