What is the R-value of 2 inches of foam?

The R-value of 2 inches of foam depends entirely on the type of foam, because different foam chemistries have very different thermal resistances per inch. Two inches of closed-cell spray foam, which is the highest R-value spray foam, provides about R-13 to R-14, since closed-cell spray foam is R-6.5 to R-7 per inch. Two inches of polyisocyanurate rigid foam board, which is common for exterior sheathing and roofing, provides about R-12 to R-13, since polyiso is about R-6 to R-6.5 per inch, though its R-value drops slightly in very cold temperatures. Two inches of extruded polystyrene or XPS, the pink or blue board, provides about R-10, since XPS is R-5 per inch. Two inches of expanded polystyrene or EPS, the white beadboard, provides about R-7 to R-8.4 depending on density, since EPS is R-3.6 to R-4.2 per inch. Two inches of open-cell spray foam, which is softer and cheaper, provides only about R-7.4, since it is about R-3.7 per inch. So when someone says 2 inches of foam, you must ask which foam to know if you are getting R-7 or R-14.