With a new metal roof you will be good for a long , long,time, So the
roof is actualy your ceiling height, a heated area. how it it finished
now, drywall? And why do you think it is under supported. Yes than
foam would be good but 5.5 " foam is R 30.25? @ R. 5.5 per inch. Well
under minimum code for my area which is zone 5. Maybe an Architect
should be consulted for roof strength. Beams could be staggered to every
other joist to give more depth and get more insulation, but I see roof
hight is a concern . Also the silverfaced foamboard is R 7.2 per inch .
5.5 inches of that will gain you an Extra 5.5 R bringing you to the
minimum codes for most Zone 5 areas. But I believe in alot more. So you
will remove the drywall or ceiling to insulate? The snow will probably
slide but call the roof manufacturer they will know best