Since your problem is with vapors, you may get results with vapor barrier
foam board, or sheet fiberglass. But, the vapor barrier is what's needed.

I've done a bunch of sheet metal work, so I'd be thinking sheet metal cut
to shape, and stuck on with silicone caulk. You may be able to get the same
result with sheet plastic, and easier to cut.

Christopher A. Young
Learn more about Jesus
www.lds.org
..

"Steven Bornfeld" wrote in message
...

We plan to do that (though our contractor seems to think it will take
too many cans--I think he suggested fiberglass insulation, though I'm
sure foam (is it polystyrene?) would seal far better.