A humidifier assembly is disclosed. The humidifier assembly includes a housing, a heater in the housing, a hollow shell on the housing, a partition wall within the shell, and a liquid water inlet that is unitary with the hollow shell. The heater provides an exposed heated surface for contacting liquid water in a direct heat transfer relationship. The hollow shell and the partition wall together define a humidification chamber. The hollow shell also defines a breathable gas inlet and a humidified, breathable gas outlet, both of which communicate with the humidification chamber. The partition wall includes a hydrophobic filter that permits water vapor but not liquid water to pass therethrough. The hollow shell, the partition, wall and the housing together define a liquid water-tight but water vapor permeable water reservoir between the partition wall and the heater. The liquid water inlet comunicates directly with the water reservoir and provides liquid water that can be vaporized upon contact with the heated surface. Vaporized water passes through the hydrophobic filter into the humidification chamber to humidify the breathable gas.