Do it Poly 1/2 In. x 20 ft. Gray Foam Caulk Saver Backer Rod features resilient poly foam that provides an efficient fill or seal for large openings. This moisture-resistant backer rod helps save on expensive caulks and sealants and is designed to last for years. It is i for filling cement cracks, meeting joints, aluminum siding panels, and door and window frames, as well as sealing foundation bases. Model Number: C22HDI