Moving from a standard attic to a single plane ceiling creates more challenges from an insulation perspective
1. 2 inch air baffles - Likely will need to be custom built and air sealed as described in this article from green building advisor
2. Air sealing is very important - a continuous layer of rigid foam on the underside is recommended.
Building Science Corporation approach for house in Minneapolis:
More information from Martin about insulated cathedral ceilings.
Still not sure how to put in the "fluffy" insulation and keep it from settling from the top (where the stack effect is high). Maybe fill the cavity in multiple phases? Leave the top accessible to allow for settling?