C.R. LAURENCE EF14B CRL 1/4" Closed Cell Backer Rod - 6400" Case. CRL Closed Cell Backer Rod is a round, flexible, closed cell polyethylene foam with an exterior "skin" used as a backing and thickness control device for elastomeric and other cold-applied sealants. The Backer Rod becomes an integral part of the joint into which it is installed, and therefore is available in a range of diameters to accommodate joints from 3/16" to 1-5/8" (5 to 41 mm). It limits the depth of the sealant, thereby preventing excessive use of the sealant, which 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 being put on the points of adhesion to the substrate..