We are planning an addition which will have a low slope roof – 2 1/2” / 12”.  There will be both a shed vaulted roof (over a great room) and an open gable section (8’ ceiling). The small attic will be conditioned.

Rafters are currently speced as 2×12 dimensional.

We are currently looking at spray foam insulation for the roof only. But we are talking about how to possibly do this without spray foam. We are looking at an unvented rigid foam over sheathing, rock wool under design.

Beyond any suggestions in general, the question I have is whether the very low roof pitch might cause any issues with this alternate plan?  And whether either method would be preferable on this low pitch roof?

Also a question on how strict the ratio of external to internal insulation is?  For our 4A location I see 30% external on R49.  Looking at available products it seems that we could easily do R15 XPS foam (3”) and R38 (9.5”) rockwool.  That larger total throws off the external percentage.  Is this a problem?

Also, because of the low pitch, we are talking about a continuous underlayment for the entire deck. It would seem that this should be on the outermost layer of roof sheathing.  (Still planning on taping and staggering all rigid foam layers).

Thanks for any thoughts.