Just a note on this old thread. It's an awesome thread, kudos to the OP.
However, the Prodex website has pants on fire. Think about it: Why would massive quantities of insulation be placed in the walls and outside the walls of energy efficient homes, if you could get R-16 out of a thin sheet? Why are people building double-stud wall houses if you could stack up a few pieces of this stuff and get R-64?
These reflective barriers are good as air barriers (the reason they are so effective in windows), and incredible as radiant barriers in the summer. They are crappy insulators, though, and the companies who make these products need to quit lying.
If you came across this thread wanting to insulate, go read Green Building Advisor, and the Building Science articles. It will change how you see the envelope of your space.
I hate to bump an old thread, but it's showing on a lot of searches, including google page 1 searches.
I personally prefer Reflectix, and you can get it at Home Depot.