The Engel Backpack Cooler features a rugged, tear-resistant 500-denier polyester diamond ripstop outer shell with a waterproof TPE coating for easy cleaning. On the inside, there are two radiant barrier liners of reflective foil - another exclusive Engel feature you won't find on other bags. Sandwiched in-between the foil layers is a 1-inch thick layer of closed-cell foam insulation. While the insulation effectively minimizes heat transfer to maintain cold inner temperatures, the radiant barrier liners reflects the sun's long-wave heat radiation to further enhance the Backpack Cooler's cold-keeping capabilities. Under typical conditions, the Backpack Cooler will hold ice for up to three days.