What are duct liners and what should you know about them? Duct liners are a type of fiberglass internal insulation lining in HVAC and air conveyance systems that tend to break down over time. Often, if the duct systems are not properly cleaned and maintained, the duct liners deteriorate, leaving the remaining fibers to distribute throughout the system and add to the accumulation of particulate debris. Eventually, these fibers are blown into an occupied space and are found as black specks on ...This story is related to the following:ServicesSearch for suppliers of: Duct Cleaning Services