Sashco Gray Pre-Caulking Backer Rod, 3/4 in. x 50 ft. makes professional joints that last by helping to fill in and insulate large caulk joints. It allows the caulking to absorb more movement by adhering only to the sides of the joint and not to the filler rope. This unique feature lets the caulking stretch and compress without tearing or pulling away, which extends the life and effectiveness of the caulk. Made from closed cell poly foam, this backer rod a minimum R value of 2.5 per inch, providing reliable insulation and durability for your sealing projects.