An expiratory limb for a breathing circuit includes an enclosing wall defining a flexible singular expiratory flow passage between an inlet and an outlet and bounding the flexible singular expiratory flow passage and ambient air. The expiratory limb includes one or more regions of the enclosing wall that made from a material that allows passage of water vapor without allowing passage of liquid water, thereby forming a water vapor flow path from the flexible singular expiratory flow passage to the ambient air through the material. The expiratory limb is configured to allow diffusion of water vapor along the flexible singular expiratory flow passage and dry the flow of expiratory gases between the inlet and the outlet.