As a result of the many voids within it. It is characterized by being lightweight, low in thermal conductivity, and not leaking water, preventing it from reaching the concrete, which protects it from corrosion and cracking. It can be easily applied because it comes in the form of boards placed next to each other, therefore it is characterized as a good insulator. It has many densities, so the higher the density, the greater the hardness. Its types: Extruded Polystyrene Foam: This type is manufactured by melting polystyrene granules in a special melting device, with the necessity of a blowing agent. It is also characterized by its strong ability to resist heat, being homogeneous, lightweight, pressure-resistant, and of high density. Low price, therefore it has been used extensively recently due to its great importance in protecting roofs. Price: 35