While the air spaces do not have the R-values claimed by the manufacturer of that insulation, foil-faced insulation products do provide additional benefit in that they resist radiant heat transfer. This benefit is hard to quantify in terms of R-value, because R-value is the resistance to conductive heat transfer.
A claim of R-16 in this application is unfounded. Any foil faced product will provide similar radiant heat blocking benefit (not R-16) but I think you need to use a product with real R-value (like fiberglass batt) in addition to a radiant barrier. Foil-faced batt is the best product for this application.