I would just use some R19-R30 insulating batts with paper facing and staple the facing to the bottoms of the Joists. If your bays are too wide for 24" batts then you can use thin galvanized wire strung accross the joists about every 16-24" using roofing nailson each joist and wrap the wire around each nail. That creates a wire tray basically for your insulation to lay on.
In walls you can do the same technique to hold the insulation back into the stub bays.
This technique works great if you want to use thicker insulation that your studs or joists are....R13 is 3-1/2" thick and R30 is 10"....you could put R30 in a 2x4 joist or stud bay using the wire to retain it without over compressing it.
it is easier to ask for forgiveness than permission