Disclosed is a device to reduce conduit rainout, a valve arrangement, a humidifier, a respiratory PAP device and a method of reducing rainout in a conduit of a respiratory treatment apparatus. The disclosure notes that condensation or rainout is a common problem with humidification systems as humidified air may cool on its path along the conduit from the humidifier to the patient or can occur when the system is switched off. The device comprises a breathable gas conduit, an atmosphere access port (160) and a vapour barrier (150). The gas conduit has an input aperture (102) and an output aperture (104). The gas conduit is configured as a channel for movement of humidified gas between the input (102) and output (104) apertures. The atmosphere access port (160) is disposed between the input aperture (102) and output (104) aperture. The vapour barrier (150) is located in the channel between the input (102) and output (104) apertures of the gas conduit. The vapour barrier (150) is operable to selectively permit and prevent vapour to pass from the input aperture (102) to the output aperture (104). The atmosphere access port (160) is configured to allow communication between atmosphere and the output aperture (104) when the vapour barrier (150) selectively prevents humidified gas from passing to the output aperture (104).
