Hey Folks
I’m currently working on a building were we recently opened up the rafters to find no insulation in the rafter bays. we explored further and found 3″ blue XPS on the outside of the rafters. 

With the building being located in Winnipeg, MB, i believe a climate zone 7 location, this R15 is not enough/up to current code, and we would like to better it if possible.

To be clear, the assembly is as follows,
drywall
2×10 rafters
3/8 plywood
3″ XPS
7/16OSB (added by roofers in the last couple of years for shingles)

 The options we discussed were as follows
-Dense pack Cellulose, and re-drywall, using the cellulose’s ability to act as a moisture regulating mass for any latent moisture that past the vapour permeable drywall (which will likely crack again on this install)
-2*6 batts of rockwool tight against the sheathing. keeping the dew point at the sheathing above the freezing point. no vapour barrier added
-3 layers of 2″ GPS (graphite EPS), either perforated or solid foil face, havn’t decided. Then all joints and faces taped to rafters with 3m 3015 tape or similar.
-spray foam was considered but the sq/ft of it, makes it somewhat cost prohibitive.

Does anyone have any recommendations or extra reading  I could do on this matter? I have read some of the articles on GBA to do with this situation, but none seem to fit it exactly.

Thanks!