Polyurethane is made of isocyanates, polyol resins and other chemicals. A foaming agent is added to cause expansion, creating the foam. Foams can come in the form of rigid boards or sprayed-in insulation.

Currently used more often in commercial buildings than in homes. However, rigid boards are often used as sheathing, underneath the siding of a house or as foundation insulation.
Blowing agents are used to inject foam insulations into attics and wall cavities or onto opened walls and are very effective in hard to reach places. Trained installers are skilled in mixing the necessary ingredients and properly spraying them. Improper installation of foam insulation can leave gaps as well as warp windows and door frames.
Polyurethane is one of the most effective insulators. Most contain 5% recycled (or recovered) materials as recommended in EPA procurement guidelines.
Polyurethane insulation foams have a history of using agents in their manufacture which contain chlorofluorocarbons, or CFCs, and hydrochlorofluorocarbons, or HCFCs. CFCs are chlorine-based chemicals that destroy the earth's protective ozone layer and have been eliminated in the U.S. by the Clean Air Act. Many manufacturers have been shifting to less damaging HCFCs, but HCFCs are to be eliminated by 2020.
Foam insulation manufacturers and chemical producers are working to develop zero-ozone-depletion alternatives. Carbon dioxide (CO2)is a preferred choice and is already being used by some manufacturers.
Polyurethanes are flammable. They burn rapidly and are known to emit cyanide gas and carbon monoxide when burning. As a result, they are banned in some areas of the U.S. Exposure during manufacturing can cause asthma and other symptoms.
Generally considered fairly inert once cured.