Epoxy surfboards have become very popular, since the collapse of Clark Foam in December of 2005. The types of foam used for epoxy construction are EPS (expanded polystyrene foam) and XTR, which is a brand name for extruded polystyrene foam. The XTR brand is manufactured exclusively for Epoxy Pro Manufacturing out of Oceanside, California.
EPS foam is another name for styrofoam, which has been around for years and has been used for insulation, as well as in architectural features in housing construction. EPS foam is more "spongy" than XTR foam, which is closer in appearance to standard polyurethane foam, the most commonly used foam in surfboard construction.
XTR foam is very strong and durable. When combined with fiberglass and epoxy resin, it will outlast a standard polyurethane based surfboard by far, while remaining light and buoyant.