A vestibule is an elegant architectural feature of a building entrance that can also be a practical energy saver. The double set of doors reduces a portion of the building load attributed to the envelope: exfiltration and infiltration. These are, respectively, the passage of conditioned air out of the building and the movement of unconditioned air into the envelope. Busy building entrances can lose a steady stream of valuable energy, approaching losses from a door open to the exterior all the time.
A double set of vestibule doors reduces the flow of air through an entrance. Automatic operation prevents a clear passage for airflow since one set of doors is always closed. Also, the volume of air trapped between the portals acts as a buffer to the transfer of heat through the vestibule. When there is no traffic, the trapped volume of air is an effective insulator that increases the thermal resistance of the passageway. Otherwise, the glass vestibule can still be a source of high energy loss, either if the foyer does not have double doors or if the doors are not operated to minimize outside air movement into the building.
Another option—for buildings with busy entrances but no vestibule—is to resize the exterior doors, making them smaller to reduce infiltration.
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Excerpt from Energy Conservation Projects.