Yea nick well if you were correct foil faced foamboard both sides would
not have the R rating it has, which is verified, it would be R6 more.
Foil is a Radiant barrier only, it has no R value to speak of, or gee,
wouldn`t the big manufacturers like to be as smart like you and
capitalise on extra performance.

Nick you should go into business, mortage everything, buy 1"
Polyisocyanurate foilfaced foamoard and add R6 to the rating to tell
everyone its R13.2 and sell it, and see what Gov agencys come knocking
to make you prove it.