Different sizes of EPS panels styrofoam sheets at Gypsum Ceiling Supplies. Styrofoam sheets 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. On the other hand, conductors such as metal are poor insulators because energy flows through them. Glass and air are other examples of good insulators. Styrofoam is 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.