Any type of underlayment/padding may be used as long as it has a density of 6-8 pounds per cubic foot. Certain underlayment, such as ¼” (6mm) thick cork, or 5mm CeraZorb are recommended for Environ™ Flex Rolls. It is also acceptable to place a vapor barrier, such as a plastic sheet, under the underlayment. Underlayment papers are not compatible with Environ™ Flex Rolls. Underlayment is necessary to prevent Environ™ Flex Rolls from damage from the weight of the floor, furniture, and people. Unavoidable wooden floor movements will not harm Environ™ Flex Rolls. Wooden floors expand and contract with temperature and moisture change. The wooden floor “moves” on top of Environ™ Flex Rolls. This movement will not damage Environ™ Flex Rolls when the installation is done correctly and the underside of the final floor covering is installed evenly. Some underlayment materials also have a noise reducing and insulating effect. When installing over concrete, a ¼” underlayment is mandatory. Any underlayment available in ¼” size is acceptable in most cases. CeraZorb, cork, or ¼” to 3/8” of expanded polystyrene (EPS) with a density of 2 pounds per cubic foot or more, are recommended for installation under Environ™ Flex Rolls. When installing over a concrete slab, choose the underlayment with the highest R-Value.