A conditioned vestibule, control system, and method are disclosed for use with a cold storage doorway that separates warm and cold sides. A heater warms an air stream discharged across the doorway by the vestibule. A control unit operates the heater to maintain an air stream temperature. The control unit continuously monitors the air stream temperature and operates the heater to maintain the air stream temperature in a non-saturated state based on a humidity ratio. One embodiment calculates first and second temperatures based, respectively, warm and cold side tangent lines to a psychrometric saturation curve and based on the humidity ratio. The tangent lines are based on humidity and temperature sensed on the warm and cold sides. The air stream is warmed to at least the greater of the first and second temperatures. One embodiment controls air flow to adjust the humidity ratio to more efficiently operate the vestibule.