Image courtesy DuPont Building Innovations
NFPA 285 provides a method for evaluating fire propagation of exterior wall assemblies for post-flashover fires of interior origin. The specific characteristics evaluated include the ability of the wall assembly to resist vertical and lateral flame propagation over the interior and the exterior surfaces and the cavity of the assembly and resistance to vertical flame propagation within the combustible components (NFPA 285, 2019). To achieve this, the method uses a full-scale, two-story (two room) test facility with a movable frame (Figure 1). The two-story wall assembly that will undergo testing is constructed on the test frame and moved and mounted onto the test apparatus. As required by the standard, the test assembly includes a specifically sized window opening. Two gas burners are used to sustain fire. One is placed inside the first-story test room (test room burner), and the window gas burner is fixed outboard of the wall opening.
The primary performance characteristics evaluated by NFPA 285 are the capability of the entire test wall assembly to resist the following:
- flame propagation over the exterior face of the wall assembly;
- vertical flame propagation within the combustible core components from one story to the next;
- vertical flame propagation over the interior (room side surface of the panels from one story to the next); and
- lateral flame propagation from the compartment of fire origin to adjacent spaces.
The test’s performance requirements are assessed both through visual observations and temperature data obtained during the 30-minute burn (plus an observation period) via thermocouples located throughout the wall assembly.
Compliance with NFPA 285
The most straightforward way to demonstrate a wall assembly complies with NFPA 285 is an existing or new test report. In this scenario (where a test report is used), the components of the specified wall assembly and configuration must be the same as the materials and configuration tested at the accredited laboratory. Product manufacturers, including ISO insulation manufacturers, have conducted thousands of fire tests under NFPA 285 requirements. However, given the number of materials that are used in modern construction and the variability in configurations, it would be impossible to test every possible exterior wall configuration.
Therefore, similar to many other applications (e.g. structural) under the code, IBC recognizes the ability to use engineering practices to evaluate substitute materials or different assembly configurations. An engineering judgment is a professional analysis letter issued by a competent expert with relevant training, licensing, and/or experience in the fire engineering field. Engineering judgments must be based on at least one baseline tested or approved assembly. The author of the engineering judgment will then use fire test data to compare the substitute material to the material tested in the baseline assembly and evaluate how the substitute material will perform when combined with the other components of the evaluated assembly. Like the test itself, the engineering judgment is concerned with how the overall assembly will perform as compared to the baseline assembly. In instances when other aspect of the assembly differ (e.g. air gap within the assembly), the evaluator may take other considerations, data, and past experience into account. Ultimately, the engineering judgment is intended to answer the question: “Does the variation, change, or substitution provide a wall assembly that exhibits the same or similar fire performance as the based NFPA 285 test(s)?”
In the case of ISO insulation, an engineering judgment may be used to qualify the use of a thinner insulation board as compared to a thicker one that was part of the baseline assembly. Engineering judgments may also allow for the substitution of other materials that comprise the wall assembly (e.g. exterior wall covering, WRB). It is important to note under IBC’s model code language, the authority having jurisdiction (AHJ) has the authority to accept (e.g. “acceptable to the code official”) the engineering judgment for a specific project.
Conclusion
Today’s fire safety codes and standards have evolved in response to failures and successes of the past. The codes and standards protecting buildings and occupants are continually reviewed and updated as necessary. It is important for design professionals and other key stakeholders in the construction industry to be familiar with the requirements for fire safety and performance and energy efficiency that apply to exterior wall assemblies using ISO ci or other products. Building professionals should always consult the local code requirements and/or verify said requirements with local officials. With this knowledge, a wide variety of materials can be used to construct safe and high-performing exterior wall systems.
Marcin Pazera, PhD, is the technical director for Polyisocyanurate Insulation Manufacturers Association (PIMA). Pazera coordinates all technical-related activities at PIMA and serves as the primary technical liaison to organizations involved in the development of building standards. He holds a doctoral degree in mechanical engineering from Syracuse University. He has expertise in building enclosure and product manufacturing encompassed-research, testing, product conception and development, and computer modeling/analysis.
