A Backer Rod is a round, flexible, closed-cell polyethylene foam with an exterior “skin” used as a backing and thickness control device. It becomes an integral part of the joint into which it is installed and is available in a range of diameters to accommodate joints from 3/8″ to 3/4″. It limits the depth of the sealant (e.g., siliconized acrylic caulk) used to fill the joint, thereby preventing excessive use of sealant and thus promotes proper joint design. Because the sealant will not bond to the “skin” of the rod, it can stretch and recover with joint movement with minimal stress on the points of adhesion to the substrate.