Sashco Sealants Pre-Caulking filler rope (Backer Rod). Makes professional joints that last. Helps fill in and insulate large caulk joints and allows the caulking to absorb more movement. Fills the joint so you use less caulk. Fully compatible with all substrates and sealants. Use behind chinking and caulking. Essential for ideal joint design. Caulking adheres to the sides of the joint, but not to the filler rope, which allows the caulking to stretch and compress without tearing or pulling away thus extending the caulks life and effectiveness. Made of closed cell poly foam. Minimum R value of 2. 5/inch. Available in gray color. Measures 3/8-inches diameter by 100-feet length.