In closed-cell styrofoam (and other plastic foams), each air cell is discrete and not connected with any other cell. In open-cell styrofoam (and other plastic foams), the air cells interconnect, like a bath sponge. Clearly, closed-cell is better for flotation purposes. My Epic's compartments are like any other standard hardshell kayak's; it's just that adding additional flotation within adds another layer of safety in case the hull is holed or develops some other leak. It is often suggested that kayakers add additional flotation to their boats via airbags (NRS sells these) or air-filled gear bags, if going on a trip. Belt AND suspenders!