A heat exchanger includes a core adapted to heat air with counter-flowing hot gases. Substantially isothermal hot and cool portions of the core are interconnected by a matrix portion exposed to a temperature gradient. A top sheet is positioned over the core and includes first and second ends and a middle portion between the first and second ends. The middle portion includes a window and compliant ligament portions extending alongside the window and interconnecting the first and second ends. The first and second ends of the top sheet are positioned over the isothermal cool and hot portions of the core, respectively, and the middle portion of the top sheet is positioned over the matrix portion of the core. The compliant ligament portions of the top sheet are deflectable in response to the temperature gradient across the matrix portion of the core to reduce strain on and deflection of the first and second ends of the top sheet.