A humidifier assembly is configured to humidify a pressurized flow of breathable gas from a flow generator of a CPAP unit and includes a base configured to be attached to the flow generator, the base including a recess portion. A water receptacle is configured to be received within the recess portion of the base and includes a floor and a flange around an opening at the top of the water receptacle. A lid is hingedly attached to the base and is configured to pivot between an open position and a closed position. This lid includes a top wall, an outer depending wall, an inner depending wall in the form of a double wall, and an outlet pipe. A lid seal is attached to an underside of the top wall of the lid by way of a tongue and groove structure. A catch is located on the base and configured to lock the lid in the closed position.