One thing I did was use 6" roxul between joists covered with vapour barrier (not pretty) then installed a drop ceiling with 2'x4' 2" rigid fiberglass with cloth glued on the room side for asthetics. This worked very very well. I also double wrapped with
bubble foil and taped any ductwork. Try to find fabric that is fire retardant and make sure you have some type of cold air return in this room if it is air tight like mine.
This might not be a popular method but it worked great and very cost effective. cheers