A corner seal device for sealing a space defined between respective end portions of an adjacent pair of orthogonal flange members of a conditioned air duct having a generally rectangular cross-section includes a first plate member having a surface dimensioned and configured for placement in face-to-face relation with the respective adjacent upper surface portions of said adjacent flange members; and a leg member extending away from a generally central portion of said plate member and being generally perpendicular thereto, wherein the leg member includes second and third orthogonal plate members having respectively orthogonal surface portions, and wherein the second and third orthogonal plate members are configured and dimensioned to engage corresponding respective adjacent orthogonal inner wall surface portions of the duct when the first plate member is positioned on said flange members.