A surface layer is formed on transfer elements of the rotor of a humidity and heat exchanger rotor, the layer not only being hygroscopic to provide the desired humidity transfer, but also serving as a bonding agent or cement for holding the transfer elements together in the desired rotor configuration. A preferred layer uses as a bonding agent a solution of cellulose acetate or nitrate in acetone, and with lithium chloride dissolved therein to produce the hygroscopic effect.