A seal around the perimeter of an air passageway which passes between adjacent plenums in an air-handling system includes a first channel containing a sealant which is located in the first plenum and extends around the periphery of the passageway. The first channel partially overlies the sidewall of the plenum and partially extends past the exposed edge of the plenum walls. A second channel, which is similarly located in the second plenum, also contains a sealant. A first blade projects from the wall of the first plenum into the sealant in the first channel, and a second blade projects from the wall of the second plenum into the sealant in the second channel. A third blade, which extends into the sealant in both the first and second channels, is located inwardly of the edges of the channel walls that form the perimeter of the passageway.