The result of having many gaps in it. It is characterized by being lightweight and having low thermal conductivity, and it does not allow water to reach the concrete at all, which protects it from corrosion and cracking. It can be easily applied because it comes in the form of panels that are placed next to each other, making it a good insulator. There are many densities available, so the higher the density, the greater the stiffness. Its types: Extruded polystyrene foam: This type is manufactured by melting polystyrene beads with a specialized melting device, with the necessity of having a blowing agent. It is also characterized by having strong heat resistance, being homogeneous, lightweight, pressure-resistant, and having high density. It is low-cost, so it has been widely used in recent times due to its significant importance in protecting surfaces.