The inflatable duct sealing system (IDSS) is a wraparound, inflatable mastic-lined bladder used to seal empty or filled cable ducts. When inflated to proper air pressure, the IDSS seals the duct, guaranteeing a reliable water block. The IDSS can be installed in cable ducts in manhole, vaults, buildings, cabinets, or anywhere a reliable duct seal is needed. Application include ducts containing multiple copper, coax, or fiber cable, customer premise cable entrance, and air pipes. IDSS can be installed between -40℃ to 75℃.