The DiversiTech® 16491 firestop caulk is U.L. Classified and code approved. It's used to seal penetrations of plumbing, refrigeration, A/C lines, ducts, vents, electrical cable through fire-rated floors, walls and ceilings. It's non-shrinking, non-toxic, and permanently flexible. It blocks fire, gas, smoke, air, and water migration. Use ceramic fiber filler together with putty or caulk for thick penetrations.