Sprayed or foamed-in-place applications of polyurethane insulation are usually cheaper than installing foam boards. These applications also usually perform better since the liquid foam molds itself to all of the surfaces. All closed-cell polyurethane foam insulation made today is produced with a non-CFC (chlorofluorocarbon) gas as the foaming agent. Some polyurethane foam combines with a HCFC gas instead. These types don't insulate as well as insulation made with a CFC gas, but the non-CFC gas is less destructive to the ozone layer. However, these foams still have an aged R-6.5 per inch thickness.  Low-density foams are sprayed into open wall cavities and rapidly expand to seal and fill the cavity.  The liquid foam expands very slowly and thus reduces the chance of damaging the wall from overexpansion. The foam is water-vapor permeable, remains flexible, and is resistant to wicking of moisture. It provides good air sealing and yields about R-3.6 per inch of thickness. It is also fire resistant and won't sustain a flame.