A gas-flow heat exchanger comprises a sinuously wound foil strip (11) providing a stack of parallel pockets (12) each of which contains a moulded plastics skeletal insert (13). The insert (13) comprises a flat rectangular frame (14) supporting within it a set of parallel baffles (15) which define a platen of passageways (16) extending between inlet and outlet openings (17 and 18) provided in the frame (14). Webs (20) lie in the medial plane of the frame and support the baffles (15). Holes and rebates (24, 25) are provided in the lower part of the insert to drain water from its interior. The foil (11) may be coated with hydrophilic material on one side.