mastic vs tape: confirm the approved sealing system sealing, leakage testing & airside verification mastic vs tape duct sealant foil tape ul 181 seal class use the sealing material approved for that joint, pressure class, duct type, exposure, and project specification; mastic and tape are not automatically interchangeable. the wrong sealing system can peel, crack, fail leakage testing, or violate the project specification. project duct-sealing specification, approved product data, manufacturer instructions for this joint and pressure class, is the approved seal system mastic, listed tape, gasket, or a combination?