A seal for a regenerative air preheater. The seal is comprised of two members that each have a plurality of tabs and slots. The tabs and slots interlock so that the tabs on one member are positioned adjacent the slots on the other member. The tabs have a narrow neck section and a wider sealing section. The wider sealing sections overlay and the narrow neck sections are positioned within a recessed region of the notch on the opposite member so as to interlock the two members. The seal is mounted either to the rotor or to the inner wall of the housing so as to extend through the bypass gap between the rotor and the inner wall of the housing. As the tabs are flexible, the tabs may resiliently deform as a result of the tabs making contact with the sealing surface of the heat exchanger. The air preheater further comprises a reinforcing section that stabilizes the tabs when the tabs resiliently deform.