I am working on insulating, well....everything, and now it is time to figure out what to do with dormer sections and around the hip and valley rafters. The house is 100 years old and the cavities have no insulation, no holes to the soffits and the rafters meet at a ridge with no other access. "she who must be obeyed" is tired of plaster dust so I am not allowed to strip off the plaster and lathe and spray foam it.

I was thinking of doing as high a density cellulose as I can stuff in there. I suspect it is all 2x4 construction.  Any other ideas?

sorry, but I think spray foam is the clear winner here. Just tell her it has to be done, end of story

Dense packing cellulose into a cathedral ceiling in a Toronto climate is a recipe for roof-deck rot (especially if the roof is shaded or has a north facing pitch) unless you can put at least as much R-value above the roof deck as rigid foam as you have at center-cavity. The same is true if you went with a half-pound foam pour or injection-foam drilled in from the exterior.

If there's room for 2" (3"is better) of iso above the roof deck, go ahead and dense pack the 2x4 cavites it drilling & filling from the exterior, lay down some roofing felt then the iso. If the pitchs of the dormers are is pretty short from soffit to ridge (say a couple meters or less ) you can use 1x furring through-screwed to the original roof deck with pancake head timber screws (eg FastenMaster HeadLok) 24" o.c., and mount an OSB nailer deck on the furring for the exterior felt & shingles, rainscreen style and install a ridge vent, and soffit venting where you can. If you use 25" segments for the furring and stagger them so that there is at least some cross-flow it doesn't take a lot of soffit vent area. If it's longer, use 2x furring for a better venting cross section.

If a vented nailer approach makes the exterior stackup too deep or complex, use just the OSB nailer deck through-screwed 24" o.c. and put up a heavier duty membrane goods like Ice & Water Shield under the shingles instead of 30# felt, no venting.

Just 2" of exterior iso and NO cavity fill would perform better than 3" of closed cell foam sprayed on the underside of the roof deck, but it's still worth installing the cellulose- you'd be able to feel the difference at both the winter & summer extremes.

Damn, now i have now idea what to do. The roof was done 1 year ago and I doubt I will be able to convince "she who must be obeyed" to do anything extravagent (there is beer involved in this response) so I apologize for the spellimng mistakes. I can't do anything that will raise her blood pressure. LOL

Any time you're replacing roofing or siding is an opportunity moment, but you're neither the first nor the last to let this slip by.

Non expanding injection foam cavity fil (eg TriPolymer, CoreFill 500, etc) + vapor retardent latex paint on the interior is probably as good as you're going to do at this point. Unlike slow rise polyurethane pours, with non expanding injection the blowout risk is almost non-existent, and it's fairly vapor-permeable compared to closed cell polyureathane, so the roof deck can still dry (if slowly through vapor-retardent latex). Most injection foams run ~R5/inch, but the thermal bridging of the rafters & ridge reduces the whole-assembly average to about R11-ish despite the ~R18 center-cavity R-value.