Cathedral ceiling insulation – GreenBuildingAdvisor

I am building an ICF house (Basement and main floor ICF to the level of eaves). It will have a cathedral ceiling with an open half loft. I have read the “Cathedral Ceilings that Work” article what I was thinking about doing is basically a modification of “Assembly #1” as laid out by that article.

What I would like to do is have a complete thermal envelope of the house where the ICF walls, gable end walls, and roof sheathing create the envelope. I read an article about a house being built this way and watched an accompanying YouTube video a while back but I didnt save it and I can not find it for the life of me now. But basically they used Zip Sheathing which came right to the corner of the ICF zero overhang on gable or eave walls. All seams were taped and everything was insulated inside to create a super tight thermal envelope. Then above that 2x4s were placed on edge (with long timberlocks down into the rafters through the zip) to create both an air gap and the overhang framing for proper overhangs.

Another major difference from the “Assembly #1” is that I was thinking of using Dense Pack rather than fiberglass batt.

The third difference is that “Assembly #1″ uses 2x4s gusseted onto the main rafter for a deeper cavity. Rather than do this. I was thinking that if I used a 2×6 and sistered it onto the side of the rafter, (overlapping 1.5”) rather than in line with the rafter with plywood gussets, and used dense pack it would effectively create a thermal break because insulation would be stepped over the edge of the main rafter and behind the add on extension if that makes sense.

Am I missing any reason why this wont work well?

Attaching a drawing for clarity. Drawn approximately to scale where 1 graph block=1 inch