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Combustible Water-Resistive Barriers in Exterior Walls of Type III Construction

October 2023

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This Technical Bulletin establishes acceptable water-resistive barrier (WRB) materials to be used in exterior walls for buildings of Type III construction with heights greater than 40 feet above grade plane.

When exterior walls in a building contain a combustible water-resistive barrier, Section 1402.5 of the California Building Code (CBC) requires that the exterior wall assembly be tested in accordance with NFPA 285 and comply with the acceptance criteria therein. Combustibility is determined in accordance with CBC Section 703.3.

Currently, most of the tested assemblies that satisfy NFPA 285 acceptance criteria use either non-combustible wall framing or fire-retardant-treated (FRT) wood.

It is acceptable to substitute standard wood framing in lieu of the non-combustible or FRT framing specified in tested and listed assemblies. The proposed exterior walls shall also comply with the following:

  • The WRB shall be installed and sandwiched between two non-combustible wall coverings such as gypsum sheathing, exterior cement plaster, non-combustible exterior siding materials, etc. (See Figure 1)
  • When the WRB includes foam plastic insulation, no more than a 2” air gap shall be permitted between the barrier and the non-combustible wall covering. (See Figure 2)

All other aspects of these proposed exterior walls shall be constructed as was originally tested.

Exterior wall assemblies that incorporate combustible water-resistive barriers that have not been tested and listed in accordance with NFPA 285 may be evaluated with a code modification application pursuant to Section §129.0104(a)(6) of the San Diego Municipal Code. In these cases, substantiating fire performance data shall demonstrate that the level of fire safety provided meets or exceeds CBC requirements.  See Information Bulletin 151 for more information.

Figure 1. Sandwiched Barrier

A diagram of a sandwiched barrier

Figure 2. Barrier with Foam Plastic

A diagram of a barrier with foam plastic

Reference Table