Easy for those who know I guess. Want to know if I can, and what I need to do so, to get code approved R30 in the depth of a 2 x 8 rafter.
Let’s not confuse the issue with facts!
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Presume you're talking cathedral ceiling here, as you say rafters rather than ceiling joists.
Spray polyurethane foam, which is ~ R6 per inch, to a depth of at least 5"? That will leave room for venting under the sheathing if it is code required in your area (it is in mine, even if you use spray foam). If it isn't, many here will recommend a non-vented roof.
Or use 1/2" rigid foam on spacers to form a ~ 1.25" venting space, followed by 5.5" of R21.5 mineral fibre batts, followed by 1" of rigid foam (your choice) under the rafters? Unlike the spray foam, this will give you a thermal break as well as the required insulation rating for the bays themselves.
Thanx canuk, I'm a displaced canadian myself down here in North Carolina. Doing a low slope shed roof bump out in the 1 1/2 story 'this old house' we own, and want all the head room (no pun intended) I can get. Have to check if your suggestions will pass code here in NC. Have lots of kin in T.O .and am up there from time to time. Let's not confuse the issue with facts!