m Ransley errs again:

... R6 added with 2 sheets of foil does not exist anywhere in
reality or at any store on this planet, or it would be sold as
R 12.8* polyiso

It can work that way in the real world, but it's illegal to advertise R12.8,
according to US federal regs, because the R-value depends on the installation
conditions, which is confusing to the general public. Altho we can measure and
calculate and advertise the "system R-value," if we have the merest grasp of
physics :-)

Nick