XPS: Extruded Polystyrene - XPS foam begins with solid polystyrene crystals. The crystals, along with special additives and a blowing agent, are fed into an extruder. Within the extruder the mixture is combined and melted under controlled conditions of high temperature and pressure into a viscous plastic fluid. The hot, thick liquid is then forced in a continuous process through a die. As it emerges from the die it expands to foam, is shaped, cooled, and trimmed to dimension. XPS resistance to heat flow (R-value) is around 4 for every 3 cm thickness of material. XPS boxes are cleanable, reusable and recyclable. XPS is non-toxic and chemically inert (fungi and bacteria cannot live on it).