Is he asking about rigid foam with a fail backer? It's such an old post we'll probably never know, but I'd mount the foil against the roof and use spray on contact glue (applied to the roof and the foil, if you get it on the foam the solvents in the spray may eat the foam. Also you can usae a product called "reflectsit" (I think thats the name) it has foile on both sides with an interior that looks like bubble wrap, Again, applied with contact cement. The R factor isn't that high, but anything you use for inselation is better thatn the bare metal.