GregS wrote:

I looked very closely at the foam sheeting at the home building center.
The R value is definately added with the foil backed foams.

What would lead you to believe that? The FTC says that's illegal.o

Polyisocyanurate often has foil, unlike Styrofoam, but its bulk R-value is
higher and the foil helps the foam avoid outgassing which lowers the R-value.

Nick