I did a remodel last year to a converted garage that included rigid foam on top of the old sloped garage floor and then a new level slab, new walls, sheetrock, bathroom, etc. It was a 25x25 space. It had old fiberglass batts above the flat ceiling that I was able to keep in place with string during the installation of a new ceiling. So this last weekend I went up and installed baffles in every rafter cavity leading to the enclosed soffit, sealed up every penetration with expanding foam and caulk, and finally built a plywood dam around the attic access hatch about 16" above the sheetrock. The old batts have settled to about 5" deep and the plan is to bring that up to an R-38 with blown in fiberglass. I have several issues with cellulose so don't use it. I got a bid for 503$ to add the additional insulation for the 625 SF. When the same company did the same job over the rest of the house they added 11" of fiberglass for a total depth of 16-17". Made a big difference. I thought the price was pretty good especially when you take 30% credit from it. Trouble is they are 6 weeks out! I can wait.