Sashco Gray Pre-Caulking Backer Rod, 1 in. x 50 ft. makes professional joints that last by helping to fill in and insulate large caulk joints while allowing the caulking to absorb more movement. The caulking adheres to the sides of the joint but not to the filler rope, enabling the caulking to stretch and compress without tearing or pulling away. This feature extends the life and effectiveness of the caulk. Made of closed cell poly foam, it a minimum R value of 2.5 per inch, providing excellent insulation and durability for your sealing projects. Model Number: