Systems, methods, and devices for humidifying a breathing gas are presented. The system includes a source of pressurized breathing gas, a vapor transfer unit external to the source of pressurized breathing gas, a first gas tube connecting the source of pressurized breathing gas to the gas inlet of the vapor transfer unit and having a first length, a liquid supply having a heater that heats liquid, a first liquid tube coupling the liquid supply to the liquid inlet of the vapor transfer unit, and a second gas tube having a second length and connecting the gas outlet to a patient interface. The first length is greater than the second length. The vapor transfer unit includes a gas passage, a liquid passage, and a membrane separating the gas passage and the liquid passage. The membrane is positioned to transfer vapor from the liquid passage to the gas passage.