CLOSED CELL BACKER ROD Closed cell backer-rod is a resilient polyethylene foam designed for cold-applied joint sealants. Sealants do not adhere to the backer-rod which becomes an integral part of the joint sealing system. Used as a back-up material, it provides the proper shaping of the sealant while permitting it to stretch and recover with joint movement. Backer-rod is extruded to various diameters to accommodate a range of joint widths.