An apparatus comprising: a flow generator, a controller connected to control the output of the flow generator, a conduit extending from the flow generator to connect with a user interface, an inside of the conduit and an inside of the user interface defining a gases space, a valve at or adjacent the user interface, the valve being switchable between a first mode in which the gases space is significantly open to ambient through the valve and a second mode in which the gases space is not significantly open to ambient through the valve, the controller comprising one or more positive airway pressure support modes in which the controller is adapted to cause the flow generator to deliver pressure support to an airway of a user with the valve in the second mode, and the controller comprising one or more sub-therapeutic modes in which the controller is adapted to cause the flow generator to deliver flow of gases to the user with the valve in the first mode, wherein: the valve is switchable from the first mode to the second mode based on prevailing pressure and/or flow conditions associated with the one or more positive airway pressure support modes, the valve is switchable from the second mode to the first mode based on prevailing pressure and/or flow conditions associated with the one or more sub-therapeutic modes, and the valve comprises a valve member, wherein the valve member occludes between about 50% and about 80% of a cross-sectional area of a flow path from the flow generator to the user interface when the valve is in the first mode. a flow generator, a controller connected to control the output of the flow generator, a conduit extending from the flow generator to connect with a user interface, an inside of the conduit and an inside of the user interface defining a gases space, a valve at or adjacent the user interface, the valve being switchable between a first mode in which the gases space is significantly open to ambient through the valve and a second mode in which the gases space is not significantly open to ambient through the valve, the controller comprising one or more positive airway pressure support modes in which the controller is adapted to cause the flow generator to deliver pressure support to an airway of a user with the valve in the second mode, and the controller comprising one or more sub-therapeutic modes in which the controller is adapted to cause the flow generator to deliver flow of gases to the user with the valve in the first mode, wherein: the valve is switchable from the first mode to the second mode based on prevailing pressure and/or flow conditions associated with the one or more positive airway pressure support modes, the valve is switchable from the second mode to the first mode based on prevailing pressure and/or flow conditions associated with the one or more sub-therapeutic modes, and the valve comprises a valve member, wherein the valve member occludes between about 50% and about 80% of a cross-sectional area of a flow path from the flow generator to the user interface when the valve is in the first mode. the valve is switchable from the first mode to the second mode based on prevailing pressure and/or flow conditions associated with the one or more positive airway pressure support modes, the valve is switchable from the second mode to the first mode based on prevailing pressure and/or flow conditions associated with the one or more sub-therapeutic modes, and the valve comprises a valve member, wherein the valve member occludes between about 50% and about 80% of a cross-sectional area of a flow path from the flow generator to the user interface when the valve is in the first mode.