I have my doubts about it's R Value. I'm assuming that they are making the assumption based on it's reflective properties. Is the cavity you mentioned air sealed? Otherwise it's just a big gap for any temperature air to move through.
Recently I was talking with a home owner who was thinking about installing vinyl siding under the influence of a "contractor" who was also an "engineer". He mentioned one of the perks of vinyl siding to be that it has rigid foam insulation under it which increases the efficiency of the house. He didn't mention, or maybe even understand, several important facts: One, the foam used is so thin that it basically has no value as insulation. Two, it simply lays against the existing siding, which is never flat, and outside air moves quite freely behind it. Three, that the foam is not air sealed to adjacent pieces, so again there is massive air leakage negating any possible value as insulation. And fourth, it's there simply to smooth out any uneven surfaces so the siding isn't bumpy or wavy.
Oooh, Shiny Stuff!