Phenolic (phenol-formaldehyde) foam was somewhat popular years ago as rigid foam board insulation. It is currently available only as a foamed-in-place insulation.
Phenolic foamed-in-place insulation has a R-4.8 value per inch of thickness and uses air as the foaming agent. One major disadvantage of phenolic foam is that it can shrink up to 2% after curing. This shrinkage makes it less popular today.
Article source: The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE). For the most up-to-date information please visit the EERE website.