An exceptionally lightweight material, an excellent shock absorber and an effective insulator.  Styrofoam is made mostly of air, meaning it is a poor conductor of heat, but an excellent convector. It traps the air in small pockets, blocking the flow of heat energy. This reduces both conduction and convection and makes styrofoam a good insulator. Styrofoam sheets are placed in wall cavities to keep the inside of buildings warm. It traps air and reduces the transfer of heat energy, keeping heat inside the building.