[Greenbuilding] using rigid foam (PUR/Polyisocyanurate) on exterior of foundation?

[Reuben Deumling]

I'm curious if there is a reason *not* to use 3" rigid foam of this variety
outside a foundation wall? The stuff I'm looking at is R-19.
I'm thinking EPS is the more common material, but am not sure.

Thanks.

Reuben Deumling