I’m in the middle of a DIY project of sealing up and addition insulation to my attic. A few the guys here have been helpful with some good suggestions. I’m at a point where I need some more help. The ‘great room’ of our house has a ‘partial cathedral ceiling’… at the exterior walls, the ceiling run up along the roof rafters and the center ceiling is flat, as if it was installed on rafter ties (see sketch). The remaining 80% of the house has a flat ceiling with attic space above. The angled cathedral part is about 8 feet long and currently has FG batts between the ceiling and roof deck. My original thought was to remove the batts and add a multilayer XPS sandwich between the rafters to get the most R value possible in the space available. I was going to keep the sandwich thickness just shorter than the space to allow for venting. But after further inspecting of the rafters I found that the spacing between them can vary up to one inch. I can probably conclude that they are not parallel either. This would mean the installation of these long XPS sandwiches (sliding them down the between the rafters, ceiling and roof deck) would be difficult and I would probably end up with air gaps on the sides of sandwiches… After some more investigation on FHB, I saw where some authors suggest that ‘shed roofs’ don’t need to be vented… that asphalt shingles get more damage from UV rays than by high attic/roof deck temperatures. Since I’m located in the mid-Atlantic, ice-dams are not an issue… my second thought was to just fill all the empty space with the blown in loose fill insulation. The remaining 80% of the attic will be vented. Any suggestions or other ideas would be appreciated. I figure I’m going through all this work to improve the thermal envelope of the house, I should try to the best job I can. Thanks!