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For comparison, most foam panels for sheathing application are worth R3.5 to 7.2 per inch of thickness. So your foam for vinyl might average R-1
but insulation value is a pure bonus for what little it is. If I were doing it for insulation value, I would be more likely to add an inch or 1/2" of foil faced foam instead of or in addition to the meager piece included with the siding.
it doesn't hurt a thiung but there isbn't much to count on for R-Value.