The blue waterproof core is made from waterproof extruded polystyrene foam, reinforced with fiberglass mesh on both sides and coated with synthetic polymer resin mortar which fully embeds the mesh layer resulting in the panel's strength and the ability to develop highest adhesion strength to virtually any type of thin-set mortar. It's CFC-free. 100% waterproof due to its extruded polystyrene (closed cell) foam core 100% mold and mildew proof. High insulating properties due to its extruded polystyrene core giving tile or stone finish surfaces a warmer touch.