NFPA 285 engineering judgments

Review of relevant changes to NFPA 285-2019

Figure 5: NFPA 285 test specimen. Photo courtesy Kingspan Insulation LLC

The 2021 IBC references the 2019 edition of NFPA 285, whereas earlier editions reference the 2012 and earlier editions of NFPA 285. Between 2012 and 2019, substantive technical changes were made to the NFPA 285 test method. As jurisdictions begin to adopt and enforce the 2021 IBC, it becomes necessary for manufacturers, designers, and AHJs to address exterior wall assembly designs based on test data collected in accordance with previous editions of NFPA 285. These technical changes provide an opportunity to discuss a practical example of how data analysis and engineering judgments may provide a suitable compliance solution. For this simple example, the following discussion will focus on technical changes to NFPA 285 regarding requirements for the construction and configuration of test specimens. For the 2019 edition, the NFPA 285 test method contains new, more detailed, and specific content regarding framing systems, joints and seams, and window headers.

Framing system

Prior to the 2019 edition, NFPA 285 test specimen construction requirements limited framing materials of test specimens to steel studs. Increasing use of Type III “pedestal” construction and the option to use fire-retardant-treated wood (FRTW) framing in exterior walls of Type I-IV construction highlighted a disconnect between the IBC and the NFPA 285 test. To address this issue, and to allow manufacturers to validate performance of wood-framed exterior wall assemblies, changes were made in order to specifically permit test specimens consisting of wood framing.

Joints and seams

Editions before 2019 required test specimens contain typical horizontal and/or vertical seams (including backing, caulking, etc.) when joints and seams are used in actual construction, but no other guidance was provided. Experience gained from increasing tests of panelized systems began to show location joints and seams could influence test results as much or more than their presence or absence in the test specimen. Language specifying the placement of vertical and horizontal joints and seams was added to ensure joints and seams are exposed to the full assault of the exterior fire plume, providing a “worst case” test condition.

Window headers

Prior to 2019, NFPA 285 did not provide guidance or requirements regarding the window opening, other than its dimensions and location. However, experience from tests of different types of wall assemblies showed window head designs influence performance during the test. Language specifying certain details of how to close around the window opening and requirements to include window details and drawings in the test report ensure this critical information is included in the test record and clearly communicated to users of the test data when designing wall assemblies for actual construction on buildings.

An example scenario

If a designer is developing a wall assembly based on test data collected in accordance with the 2012 edition of NFPA 285, but the jurisdiction has adopted the 2021 IBC and, by reference, the 2019 edition of NFPA 285. One cannot assume the existing data complies with the 2021 IBC requirements because of the important technical differences between the two editions.

An engineering judgment based on analysis of the existing test data against the 2019 edition of NFPA 285 is a quick way to determine whether the test specimen construction complies with the new limitations and requirements. Also, this analysis and judgment must take place before it is possible to begin considering any other differences between the tested wall assembly and the designer’s assembly intended for use as the basis of design.

In terms of wall framing, prior to the 2019 edition, testing with wood framing was deemed a “modified” test. Now that wood framing is specifically permitted, an evaluation may judge the previous data to be in full compliance with the newest edition of the test method. Guidance regarding extensions of data from steel stud to wood stud framing and vice-versa based on testing experience is still limited but developing.

Reviewing locations of vertical and horizontal joints and seams (e.g. if the assembly contains an MCM rainscreen system) will determine if they comply with the specific locations now required by the test. Figure 5 depicts an NFPA 285 test specimen with dashed lines added to highlight joints in the panel system and a red box outlining the area (not to scale) where a vertical and/or horizontal joint or seam is required: between 0.3 m (1 ft) and 0.9 m (3 ft) above the top of the window and within 0.3 m (1 ft) on either side of the window centerline. In Figure 5, the joint placements of the test specimen fall within the required area.

The review of a window closure will focus on the construction of the window head, jambs, and sill as well as the materials and manner used to close them. For this issue, there is more latitude. Although the newest edition of NFPA 285 includes requirements standardizing the construction and closure of the window opening, there is also the option to close the window opening per details provided by the testing client. The window details must include drawings and the details are included in the test report.

If any test specimen construction details are found to not comply with the requirements of the 2019 edition of NFPA 285, then additional analysis is required.

Such an analysis could entail review of the actual test data and how deviation in test specimen construction could influence results. For example, if the horizontal joint location is not within the required area, then more detailed review of the test specimen construction, thermocouple data and test record, including photographic records of the post-test damage, becomes necessary to determine how results will change if the joint is moved to the area now required by NFPA 285. With any comparative evaluation, it is not always possible to develop a conclusive finding. In that case, retesting in accordance with the current edition of the standard is the most appropriate conclusion.

The design practices and regulatory requirements for today’s buildings are the result of continuous research, observation, and comparative analysis of performance followed by robust review and consensus. There will always be a place for comparative analysis and engineering judgment so long as materials knowledge, engineering, and building science continue to evolve. Talented professionals will continue to search for innovative solutions to current and future challenges and opportunities for the built environment such as climate change, reduced environmental impact, and occupant safety.

Jeffrey H. Greenwald, P.E., CAE, is technical consultant with the North American Modern Building Alliance (NAMBA). In this role, Greenwald supports implementing the NAMBA’s mission, work plans and building codes and standards development. Greenwald is a registered professional engineer in Virginia and earned a master of civil engineering degree from the University of Delaware. Greenwald was awarded the ASTM Alan H. Yorkdale Memorial Award for best paper concerning masonry in 2004 and 2005.


Eric W. Banks is a technical consultant specializing in the development, physical and fire testing, certification, and codes & standards compliance of building products and their applications with an emphasis on foam plastics. Banks has more than 20 years of experience in these areas working with and for both product manufacturers and certification agencies, and he is actively engaged in codes and standards development work.

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