talking insulation: spray-in polyu vs polyisocyanurate sheet

I have read they make a poly spray foam that is fire retardant for use in enclosed areas were it would be difficult for someone to get out..the polyiso is some good stuff to use is what will be going around my refer unit do to it's fire rating..the only problem i see in using sheet foam is getting a tight seal..i had to use can foam on my bead board to tighten it up...my two cents..