A heat-exchange body for transferring calories from a first gaseous fluid to a second gaseous fluid, said heat-exchange body comprising a sheet or film folded or bent into a concertina shape defining a first series of bent or folded portions of the sheet or film, a second series of bent or folded portions of the sheet or film, a series of transverse heat-exchange walls (21, 22, 23), which are substantially flat and are located substantially parallel to one another, each extending between a bent or folded portion of the first series and a bent or folded portion of the second series, said folded or bent sheet or film thus forming a first series of channels and a second series of channels, said channels extending transversely between a first longitudinal end edge of the folded or bent sheet or film and a second longitudinal end edge of the folded or bent sheet or film located opposite said first longitudinal end edge, the heat-exchange body being adapted so as to be associated with: - a first longitudinal wall extending along the first longitudinal end edge of the sheet or film, - a second longitudinal wall extending along the second longitudinal end edge of the sheet or film, - a third wall extending along at least one edge or ridge of the bent or folded portions of the first series, and - a fourth wall extending along at least one edge or ridge of the bent or folded portions of the second series, in such a way as to form a first circuit consisting of channels of the first series for a circulation of the first fluid and a second circuit consisting of channels of the second series for a circulation of the second fluid, said heat-exchange body essentially being characterized in that the substantially flat, transverse heat-exchange walls (21, 22, 23) have a thickness of less than 0.5mm, advantageously less than 0.3mm and preferably have a thickness of between 0.05mm and 0.1 mm, in that the transverse heat-exchange walls (21, 22, 23), which are substantially flat and advantageously extend substantially parallel to one another, are made of a material, which is at least resistant to water vapour and water and has an average thermal conductivity of less than 0.25 W/(m.K) and in particular less than 0.20 W/(m.K) in direction of the length of the heat-exchange wall in question, in that the transverse heat-exchange walls (21, 22, 23) have a height of at least 1 cm and a length of at least 3cm, the length/height ratio being greater than 1.5, advantageously less than 5, preferably between 2 and 4 and in particular between 2 and 3; in that the walls (21, 22, 23) each have at least one recess designed to rest against an adjacent wall or a recess of an adjacent wall, in such a way as to form, in the channel (P1, P2) between said adjacent walls, at least one element guiding the fluid circulating in said channel, said element being designed to increase the heat-exchange surface, the length of the path travelled by the gas and/or the turbulence of the flow so as to increase the thermal exchange from one fluid to the other fluid, and in that the flat portions (21, 22) of the adjacent walls defining a channel are at a distance from one another of less than 8mm and advantageously less than 6mm. - a first longitudinal wall extending along the first longitudinal end edge of the sheet or film, - a second longitudinal wall extending along the second longitudinal end edge of the sheet or film, - a third wall extending along at least one edge or ridge of the bent or folded portions of the first series, and - a fourth wall extending along at least one edge or ridge of the bent or folded portions of the second series,