Cathedral ceiling insulation
You are going to have a hard time doing this and getting very much insulation value. As Rocko says you need to have ventilation above the insulation, open at the bottom and the top to the outside. This only leaves you say 4" of space to insulate. If you want exposed rafters, you have even less. For a 1" exposed rafter and using 1/2" drywall in between the rafters with a 1/2" stop to hold the drywall in and trim the edge, you're down to 2". You could put spacers between the insulation and the deck to screw the drywall to, this would save you 1/2", but at best (4") you're gonna get r-20 or so with rigid ins.
Where are you (hot/cold?) Are you doing this with a permit/code?
Perhaps you'd be better off adding some 'faux' beams to the inside after all the drywalling is done?
Tell us more about the situation: size of room/building; use of; pitch of roof; height of walls; location (geographical) etc.