A ventilator intended for placing in an outer wall has a double-acting axial-flow fan with an inner and an outer impeller blade ring disposed such that the blade rings are each in register with a coaxial channel in a duct placed in a through-opening in the wall. A reversing means is connected to the impeller motor for changing the direction of rotation thereof so that it alternatingly rotates in one or the other direction. A regenerator means is inserted in the channels, the means both absorbing heat and distributing moisture within itself.