Check the product information sheet in the duct packaging for specific information. These standards are a big step toward ensuring quality and compatibility in duct materials and closure systems. UL 181B covers products approved for use on flexible ducts and connectors. These two groups of products carry one additional designation to indicate if the product is a pressure sensitive tape, a heat activated tape, or a mastic system. See the table below for details.
Mastics provide a greater assurance of an airtight seal.
Mastic is very forgiving and will work even if the surface is a little dirty, wet, greasy, or the installer doesn't have his squeegee handy to press all of the wrinkles out of the tape. This makes it easy for the inspector to verify that the correct closure system is being used for each duct type. Flex Duct 1. UL 181B-M Mastic closure systems, for flexible ducts. Duct Board 1.UL 181A-P Pressure sensitive tape, for duct board.