In terms of foam R/$ you're usually better off with low-density EPS rather than iso.
On foundation sills and band-joists the ignition barrier requirements are eliminated in local codes, per the IRC, so long as:
* The maximum thickness of the foam plastic shall be 3-1/4 inches
* The density of the foam is between 0.5 & 2.0lbs per cubic foot
* has flame spread index ≤25
* has a smoke developed index ≤450
(Which would be true of most rigid foam products.)