Systems, methods, and devices for humidifying a breathing gas using a vapor transfer unit are presented. The method includes providing a first vapor transfer unit having a gas passage and a liquid passage, delivering a liquid to the liquid passage, delivering a gas to the gas passage, humidifying the gas by delivering vapor from the liquid in the liquid passage to the gas in the gas passage, exiting the humidified gas outside the vapor transfer unit at first relative humidity and at a high gas flow rate, and reducing the gas flow rate through the first vapor transfer unit to less than a low gas flow rate, while preventing the relative humidity from exceeding the first relative humidity by more than an acceptable margin.