Materials installed
within ducts and plenums for insulating, sound deadening,
or other purposes shall have a mold, humidity, and erosion-resistant surface where tested in accordance with UL
181. Duct liners in systems operating with air velocities
exceeding 2000 feet per minute (10.16 m/s) shall be fastened
with both adhesive and mechanical fasteners, and
exposed edges shall have approved treatment to withstand
the operating velocity. Where the internal insulation is
capable of being in contact with condensates or other liquids,
the material shall be water-resistant.