Shurtape SF-685 Shurflex Indoor/Outdoor Mastic On a Roll is used for air-tight and moisture-free sealing of rigid and flexible air duct systems. Used primarily to seal sheet metal duct in both indoor and outdoor applications and used as a permanent, water resistant sealer. Use duct mastic to mend poorly sealed connections; improve indoor air quality (since fewer pollutants circulating); lower utility bills; and burn fewer fossil fuels. UL 723 classified. Features include: waterproof backing ; stays supple and forms a permanent bond to substrate ; aluminum foil backing may be painted with any paint recommended for aluminum ; contains no asbestos, CFCs or PCBs .