Safety-Fire Hazards :
Polystyrene is classified according to DIN4102 as a "B3" product, meaning highly flammable or "Simply Ignited." As a result, while it is an effectual insulator at low temperatures, using is forbidden in any exposed installations in building construction if the material is not flame-retardant.
It should be hidden behind drywall, sheet metal, or concrete.
Foamed polystyrene plastic materials have been coincidentally burned and makes huge fires and losses, for example at the Düsseldorf International Airport, the Channel tunnel (where polystyrene was inside a railcar that caught fire), and the Browns Ferry Nuclear Power Plant (where fire breached a fire retardant and reached the foamed plastic underneath, inside a firestop that had not been tested and certified in accordance with the final installation).