Polystyrene doesn't sound like a green material. In fact, it's a form of plastic. Yet, polystyrene is a fabulous insulator with R-values that range from R-3.8 to R-4.4 per inch of thickness, and so it's considered green because it helps save so much energy [source: U.S. Department of Energy]. Polystyrene insulation comes in rigid foam boards that make it easy to insulate any part of a building, from the roof to the foundation [source: U.S. Department of Energy]. Not only will rigid polystyrene provide your home with good thermal resistance, but it will also add structural integrity to your walls. But you don't have to go with the rigid foam boards; polystyrene insulation also comes in spray foam. And even though it's plastic and takes long time to naturally degrade, it can be recycled.