Nick, White Styrofoam is not R5", extruded polystyrene, the pink Owens
Corning or blue Dow is R5" . White Styrofoam board is sold just like R5
extruded polystyrene board, in the same manner, but is inferior in R
value, workability and stability.

Polyisocyanurate foilfaced newly used rating of 6.7"-6.8" is the aged
or mature R value after R 7.2" polyiso outgasses.

Perhaps you could provide us with some detailed mathematicals of your
opinion.