This Decoded article was published in Door Security + Safety
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The purpose of a control vestibule is to limit immediate through passage into or out of an area, to reduce air transfer or increase security.

In some buildings, limiting the flow of pedestrian traffic is a method used to reduce air transfer or to provide increased security.  Examples of these locations include clean rooms in laboratories, infection control areas in health care facilities, data centers where secure information is stored, and cash counting rooms in casinos or banks.  Control vestibules are sometimes incorporated into the design to limit immediate through passage into or out of these areas.

A control vestibule is a space with two or more doors in series, arranged so that when one door is open the other door or doors cannot be opened.  Under normal operation, a building occupant opens one door to enter the vestibule, and that action causes the other door(s) to lock until the first door closes.  The building occupant may then open another door to exit the vestibule.  This function is facilitated by electrified hardware – often electromagnetic locks that are controlled by door position switches.  Control vestibules are commonly called interlocks, mantraps, or air locks.

Although the physical layout is similar, a control vestibule should not be confused with a sallyport – this is a term that is typically used in detection and correctional settings.  A sallyport is defined by the International Building Code (IBC) as: A security vestibule with two or more doors or gates where the intended purpose is to prevent continuous and unobstructed passage by allowing the release of only one door or gate at a time.  The IBC allows sallyports to be used in I-3 occupancies (ex. correctional centers, jails, prisons), if there are provisions for egress during an emergency condition.  Sallyports in a detention setting are different from the control vestibules used in other occupancies, because a sallyport typically prevents user passage under normal operation and a control vestibule allows user passage through one door at a time.

The model codes do not currently include prescriptive requirements for control vestibules in use groups other than I-3 – each proposed system must be submitted to the Authority Having Jurisdiction (AHJ) for approval.  Without guidance in the codes to mandate safety overrides and release methods, requirements can vary from one AHJ to the next, or even between projects in the same jurisdiction.  Because of the potential for interlocks to inhibit egress, the operation of these doors during an emergency is critical.

A change addressing control vestibules has been proposed for the 2024 edition of the IBC, and a proposal is in the works for the 2024 edition of NFPA 101 – Life Safety Code.  Until consistent guidelines are established, it will be up to the AHJ to decide how to evaluate a proposed control vestibule.  Some potential considerations may include:

• Use group or occupancy classification: It will likely be easier to ensure the safety of building occupants using a control vestibule in a “trained-traffic” situation, where people are familiar with the operation of the system. For example, a control vestibule for security purposes would be more feasible in a data center than in an educational occupancy.
• Occupant load: Control vestibules are typically used in locations with a low occupant load, such as a laboratory clean room. An AHJ may limit control vestibules to areas with an occupant load below a certain threshold, such as 50 occupants, maximum.
• Fire suppression/detection systems: An AHJ may require the facility to be equipped with an automatic fire detection system and/or automatic sprinkler system. Activation of these systems should release the interlock function of the control vestibule doors, to facilitate immediate egress through the vestibule.
• Power failure: As with other special locking arrangements, loss of power should result in the deactivation of the interlock function of the doors in the control vestibule, to allow free egress.
• Egress-side override: If one door in a control vestibule fails to close, it will prevent the operation of the other doors. To address this potential barrier to egress, an override switch should be provided on the egress side of each door.  This switch would deactivate the interlock and allow immediate egress.  An audible alarm could be incorporated to deter use of the override switch in non-emergency conditions, although this is not crucial for life safety.
• Signage: Instructional signage is recommended, to ensure that building occupants understand how the control vestibule operates under both normal and emergency conditions.
• Number of control vestibules: To have the minimal effect on egress, it is preferable to limit the number of control vestibules that a building occupant may encounter in their egress path.
• UL 294 listing: The model codes require some types of electrified hardware to be listed to UL 294 – Standard for Access Control System Units. This is typically required when the hardware is could affect egress, and this listing may be appropriate for control vestibule components.

Remember, these considerations are not currently included in the model codes but should be addressed when designing a control vestibule that will be submitted to the AHJ for approval.  In some jurisdictions, there may be local code modifications related to this application; it’s also possible that a jurisdiction may prohibit control vestibules completely.  As the 2024 code development cycle continues, we will know whether the next editions of the model codes will address control vestibules, to allow for a more consistent approach to these systems.

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