An apparatus for humidifying a flow of breathable gas includes a water reservoir and a water reservoir dock forming a cavity structured and arranged to receive the water reservoir in an operative position. The water reservoir comprises a reservoir base including a cavity structured to hold a volume of water, the reservoir base including a main body and a thermally conductive portion provided to the main body. The thermally conductive portion comprises a combined layered arrangement including a metal plate and a thin film, the thin film comprising a non-metallic material and including a wall thickness of less than about 1 mm. The thin film is adapted to form at least a bottom interior surface of the water reservoir exposed to the volume of water, and the metal plate is adapted to form a bottom exterior surface of the water reservoir.