one thing you could do, which i did, is to coat the metal first
with 3M Rubberized Undercoating. it's a pain the butt to do
(kinda messy, if you're careless where you point the nozzle!! it
sticks to ANYthing!), but keeps the water/condensation from
touching the metal. THEN you can put the insulation in there. a
vapor barrier is a necessity, though.

joel