Rigid expanded polystyrene (EPS) blocks with cavities to hold concrete are stacked to become stay-in-place forms. Typically two or three courses are filled at a time using 4- to 5-inch clump concrete. EPS forms, left in place, provide good insulation and eliminate air infiltration. Compressive strength may be 50% more than walls whose forms are removed after three days. Concrete can be placed at temperatures as low as 0 degrees F.