Membrane selection for 6″ exterior mineral wool over timberframe roof – GreenBuildingAdvisor

I have a 4.5/12 open gable roof section that is timberframe truss supported with 2×6 t&g decking (exposed on interior). I am putting 6″ of Comfortboard 80 above this, 2×4 vertical purlins on the flat, then 5/8 sheething, then standing seam metal roof. After reading a zilli0n posts and details here there still seems to be a lack of consensus on the best membrane approached.

This house is near Augusta GA, so generally leans towards hot and humid. So far the guidance I have gleaned here SEEMS to be to use a air-seal low perm vapor barrier directly on top of the decking, taking care to seal. Then use a higher perm barrier on top of the sheething. 

Anyone care to comment on that?