Are aligned metal studs with knock outs considered communicating stud cavitites?

Good afternoon Bob,

The IBC does not specify what exactly it means by “non-communicating stud cavities” when it references those in 714.3.2 Membrane penetrations:

714.3.2 Membrane penetrations. Membrane penetrations shall comply with Section 714.3.1. Where walls or partitions are required to have a fire-resistance rating, recessed fixtures shall be installed such that the required fire resistance will not be reduced.

Exceptions:

1. Membrane penetrations of maximum 2-hour fire resistance-rated walls and partitions by steel electrical boxes that do not exceed 16 square inches (0.0103 m2) in area, provided the aggregate area of the openings through the membrane does not exceed100 square inches (0.0645 m2) in any 100 square feet (9.29 m2) of wall area. The annular space between the wall membrane and the box shall not exceed 1/8 inch (3.2 mm). Such boxes on opposite sides of the wall or partition shall be separated by one of the following:

1.1. By a horizontal distance of not less than 24 inches (610 mm) where the wall or partition is constructed with individual noncommunicating stud cavities;

1.2. By a horizontal distance of not less than the depth of the wall cavity where the wall cavity is filled with cellulose loose-fill, rockwool or slag mineral wool insulation;

1.3. By solid fireblocking in accordance with

Section 718.2.1;

1.4. By protecting both outlet boxes with listedputty pads; or

1.5. By other listed materials and methods.

A separation distance of 24 inches is acceptable between electrical boxes on opposite sides of a stud wall per exception 1.1 as long as the stud cavities are “non-communicating”. Neither the code, nor the ICC code commentary, nor the ICC reference book on firestopping and dampers provide any clarity regarding whether steel stud walls with punch outs (all steel studs have punch outs) would be considered to create “communicating” stud spaces.
 
To find an answer to that question, one must turn to the original code change proposal that inserted the words “noncommunicating” into the membrane protection section of the IBC. That code change was FS89-07/08, submitted by Bob Eugene of Underwriters Laboratories. That code change proposal can be found here (on page 25): https://www.iccsafe.org/cs/codes/Documents/2007-08cycle/ProposedChanges/V1_FS61-130.pdf

Reading from the Reason Statement for the code change proposal, we can see what was intended to be considered “communicating” vs “noncommunicating”:

 Presumably, this is intended to place boxes on opposite sides of walls or partitions in separate stud cavities. However, Item 1.1 does not differentiate between installations in walls or partitions constructed with individual stud cavities versus those where the adjacent stud cavities are interconnected. An example of the former would be a wall constructed of either wood or steel studs with gypsum board applied directly to the studs. Examples of the later would be walls or partitions constructed using resilient channels, or those using parallel rows of studs or staggered studs.

 The submitted code change was proposed and accepted with the stated underlying assumption that steel stud walls are to be considered the same as wood stud walls when it comes to creating “individual stud cavities”. Thus, steel stud walls are not considered to be “communicating” simply due to the punch outs. However, if a steel stud wall (or wood stud wall) were to be constructed in a manner with staggered or parallel studs, then such a wall would be considered as creating a communicating space, and you would not be able to use exception 1.1 to avoid the need for protection of membrane penetrations.

Regards,

Jaeger Cox
Fire Protection Engineer