In accordance with embodiments of this disclosure, a gas humidifier system includes a reservoir of liquid with at least one selectively permeable tube at least partially submerged within the liquid. A gas stream may be fed into the selectively permeable tube that passes through the liquid reservoir. The selectively permeable tube may be substantially impermeable to the gas flowing therethrough but permeable to vapor (e.g., water vapor) entering the selectively permeable tube from the liquid. Thus, vapor from the liquid reservoir can enter the selectively permeable tube and humidify the gas flowing therethrough.