[Drywall DIY Guy]

What climate are you in? If you get any winter at all, what can happen is that moisture will condense up in batting type insulation if it placed snug up to the roof, leading to rotting. In a home I bought, this happened to me. Better to snug in 2" or 2.5" insulating rigid foam in between rafters (leaving an air space between foam and roof boards) and then put 1" foil-faced sheet over the faces of the rafters. Tape all edges of the 1" sheet well so as to form a good moisture barrier and keep moisture from condensing on your roof boards. You may even want to put up a plastic vapor barrier over the 1" foam to be on the safe side. Then place drywall or other finish product over all of this fastening to the rafters driving extra long screws through the 1" foam .