Consider 2" of styrofoam on the roof joists. It will allow the hot summer air to go up in the chase you have created. Easy stuff to use, cut it with a carpenters saw, use caulk/roof nails with washers to install it. No mold or vector problem like fiber glass. If you want to go with fiber glass go with foil faced, it will reflect light/heat. Doesn't have the flame spread problem like paper faced fiber glass does. If you are working with scissor or regular trusses, etc You might want to consider putting chicken wire across the joists with a staple gun. Start stapling a couple courses of 24" chicken wire with the bonded edges. Then you can start a roll of insulation or bats between the joists, gives it a nice clean look.