NOVELTY - The device has edge stripes (12, 14) arranged between diffusion medium layers (4, 6). A set of elongated cords (18) is arranged between the layers and positioned parallel to edges between the edge stripes. The cords are connected with the layers by a thermoplastic adhesive (16) and define multiple flow channels between the layers. The layers, the stripes and the cords are arranged between polymer membranes. The membranes are connected with the layers adjacent to the stripes by the adhesive, where the membranes are not connected with the layers between the stripes.    USE - Water vapor transfer device for humidifying air in a fuel cell i.e. polymer electrolyte membrane (PEM) fuel cell, used as a power source in a vehicle. Can also be used as a stationary electric power plant in a building and an apartment and as a portable power supply in a video camera and a computer.    ADVANTAGE - The design of the device enables efficient and durable leak-free connection of the membranes, and achieves flatness tolerances without influencing water vapor transfer rate. The thermoplastic adhesive connects the edge stripes to the diffusion medium layers and the membranes, so that the overall height of the device is reduced and the need for additional layer of adhesive to be sandwiched between the membranes is eliminated.    DETAILED DESCRIPTION - The diffusion medium layer is formed from glass fiber, glass-based paper, carbon fiber or carbon-based paper. The edge stripes and the cords are made of propylene or glasses. INDEPENDENT CLAIMS are also included for the following:    (1) a method for assembling a water vapor transfer device    (2) a system for assembling a water vapor transfer device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view illustrating an assembling method of a water vapor transfer device.    Diffusion medium layers (4, 6)    Edges of diffusion medium layers (8, 10)    Edge stripes (12, 14)    Thermoplastic adhesive (16)    Elongated cords (18)