m Ransley errs again:

Styrofoam is R4-4.5" Blue or pink foamboard is R5"

Same thing. All R5 per inch. Open cell expanded polystyrene
(white coffee cup material) is R4 per inch...

Foil faced Polyisocyanurate foamboard is R7.2"

These days, it's often stamped "R6.8" (per inch.)
Add about R3 per foil, if the foils are exposed.

Nick