agents which may be used in the PIR rigid foam formulations include blowing agents such as water, methylene chloride, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) such as HCFC-141b, HCFC-22 or HCFC-123, pentane andthe like, cell stabilizer surfactants such as organopolysiloxanes, flame retardants such as halogenated organophosphorus compounds and chain extenders such as ethylene g