Hi All, 

I’m attempting to construct an unventilated cathedral ceiling in Los Angeles (climate zone 3B). The plan is to use R-30 mineral wool insulation below the roof deck (2×8 rafters) and rigid foam above the roof deck. The assembly will be as follows:
Drywall ceiling
R-30 mineral wool
OSB
Rigid foam insulation
OSB
Screw fastened metal roof

Based on the information I’ve found online and on GBA, am I understanding it correctly that I cannot exceed R-5 for rigid foam? The roofing contractor recommended using 2 layers of rigid foam, which would bring me to R-10. Should I just stick with R-5?

Also, the roofer wants to use radiant barrier OSB above the foam insulation, is there any benefit to this vs. purlins? Finally, I’m a bit confused as to whether the WRB should be installed on the roof deck below the rigid foam, or on the OSB above the rigid foam, or both?

Finally, unfortunately the drywall ceiling will have to have penetrations due to fire sprinklers installed in the roof rafters (not my design, this is how the home was constructed) so a continuous air barrier will not be feasible. Will this cause mold issues due to stagnant air being trapped in the roof assembly?

Thank you all for your help