| A moisture resistant closed cell poly foam rod that can be inserted into gaps prior to caulking. Save on caulking materials. Also great for sealing around pipe and vent openings. The backer rod should be at least 1/8" thicker than the gap being filled. Available in 3/8" X 20', 1/2" X 20' and 5/8" X 20' packages. Made by W.J. Dennis Co.