Vestibules 2007

The following Bulletin is a supplement to the Client Bulletin issued in April 2006 on the subject of Vestibules related to the 2003 International Energy Conservation Code.  

Certain provisions in the 2003 and 2006 editions of the International Energy Conservation Code (IECC) seem to require an air lock type entry vestibule, even on single story commercial and retail projects, without exception or alternative. While this approach is valid and correct using prescriptive design criteria under Chapters 5 and 8 (2006 edition and 2003 edition), both editions of the IECC allow, as an alternative to the prescriptive approach, compliance with ASHRAE Energy Conservation Standard 90.1.  This ASHRAE Standard only requires entry vestibules on commercial buildings 4 stories and more above grade, thereby exempting most retailers from the vestibule requirement under this alternative design approach.

To achieve flexibility and economy for our Clients, CASCO designs in accordance with ASHRAE Standard 90.1, omitting vestibules when requested by the Client.  In submitting such designs for permit, to avoid confusion of the reviewing Code Official regarding vestibule requirements, this issue must be addressed directly in the documents rather than ignored.

Accordingly, CASCO has adopted a policy of adding the following notes to the cover sheet of all projects in which a vestibule is omitted.


  • An air lock vestibule is not included on this project based upon Exception (b) of ASHRAE Standard 90.1 2001, paragraph and ASHRAE Standard 90.1 2004, paragraph  Exception (b) allows omission of air lock vestibules for commercial buildings less than 4 stories above grade.
  • Compliance with IECC 2006 with regard to air lock vestibule omission is based upon Code paragraph 501.1 which permits [exclusive] compliance with ASHRAE Standard 90.1.
  • Compliance with IECC 2003 with regard to air lock vestibule omission is based upon Code Chapter 7 which requires compliance with ASHRAE Standard 90.1.
  • In lieu of the prescriptive compliance under Chapters 5 and 8 of the IECC [2006 and 2003 editions], this project is designed per the Code alternatives to comply with ASHRAE Standard 90.1, and, as such, is hereby submitted without an air lock vestibule.

It is believed this approach will offer a high success rate of avoiding undesired and inefficient vestibules many retail applications.  If you have questions regarding the applicability of this approach to your program, please contact CASCO at 314-821-1100.

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