Addressing the risk of combustible cladding systems

Figure 1 – On the morning of June 14, 2017, Grenfell Tower is still on fire as hose streams reach only the base of the structure. The fire, which killed 72 and injured 70, began in a fourth-floor apartment and quickly spread up the exterior of the 24-story building. It has been widely reported that aluminum composite material (ACM) cladding was installed on the exterior of Grenfell Tower during a 2015-2016 renovation.

From Dubai’s Torch Tower to Grenfell Tower in the U.K. and from Lacrosse Tower in Melbourne, Australia, to a 28-story high-rise in Shanghai, China, devastating fires involving combustible exterior cladding have resulted in major new initiatives to identify at-risk buildings and improve the safety standards used to evaluate the fire performance of cavity wall systems.

The Grenfell Tower fire (fig. 1), for example, in West London on June 14, 2017, which reportedly killed 72 and injured 70, began in a fourth-floor apartment and quickly spread up the exterior of the 24-story building. The lack of an internal sprinkler system and the use of combustible aluminum composite material (ACM) exterior cladding with a polyethylene (PE) core have been largely blamed for enabling the interior fire to rapidly spread across the building exterior.

The fire risk posed by combustible ACM and metal composite material (MCM) exterior panels is receiving the attention of government agencies and tenant groups worldwide. In the U.K., for instance, following intensive investigations, the government has proposed a ban on the use of combustible cladding materials on high-rise residential buildings, restrictions on the use of desktop combustibility studies, and various other measures to address the risk they pose.

Figure 2 – At left above, this assembly features a polypropylene core ACM with a water/weather resistive barrier (WRB) on a gypsum base. This assembly exactly replicated an assembly that had previously passed NFPA 285 via actual testing. The flames in this test had to be water suppressed within four minutes of the start of the test to avoid exceeding the capacity of the fire products calorimeter (FPC). At right, the wall assemblies have been completely consumed by the fire.

Research shows fire tests for cladding are not equivalent

Major insurers such as FM Global are also continuing to support worldwide efforts to strengthen building codes and standards to reduce the fire risk in both residential and commercial structures. In October 2017, FM Global issued a white paper, titled Grenfell: The Perfect Formula for Tragedy, which analyzes the U.K. fire, as well as fires in Dubai and the United States.

In December 2017, the company released a research report, Evaluation of the Fire Performance of Aluminum Composite Material (ACM) Assemblies using ANSI/FM 4880.

The report evaluates the fire hazards of ACM wall assemblies using the 16-ft. (4.9 m) parallel panel test (PPT) method of ANSI/FM 4880, American National Standard for Evaluating the Fire Performance of Insulated Building Panel Assemblies and Interior Finish Materials.

The purpose of the study was to examine the fire hazards of ACM cladding assemblies using the 16-ft. parallel panel test (16-ft. PPT) method of ANSI/FM 4880, compared to the results with NFPA 285 (National Fire Protection Association) and BS 8414 (British Standard) fire tests.

Figure 3 – At left above, assemblies with fire-resistant core ACM on non-combustible gypsum and a 2-in. (51 mm) air cavity are tested in the 16-ft. high parallel panel test of ANSI/FM 4880. At right, the peak flame height reached was approximately 8 ft. (2.4 m). This ACM wall assembly performed well and passed ANSI/FM 4880 for unlimited height.

In the research conducted by FM Global, tested wall assemblies were constructed with various types of ACM cladding, continuous insulation and water/weather resistant barriers (WRB). Several of the tested assemblies were the equivalent of those that had passed NFPA 285 testing for unrestricted height installation in the U.S., either through actual test or via desktop assessments. NFPA 285 is a U.S. building code-required test of exterior wall flammability performance.

However, two wall assemblies that had passed the NFPA 285 test, one via physical testing and the other by means of desktop assessment, decisively failed the 16-ft. PPT research tests (fig. 2). Both assemblies produced high heat release rates (HRR) with flame heights extending higher than 25 feet (7.6 meters) within four minutes of ignition. Both ACM panels used in the assemblies featured sandwich construction with a thermoplastic core and aluminum skins.

Two other ACM wall assemblies that passed NFPA 285 for unrestricted (or unlimited) height installation in the U.S., failed for unlimited height and passed only for up to 50 ft. (15 m) limited-height installation using ANSI/FM 4880 criteria. These two assemblies used fire-retardant core ACM and combustible polyisocyanurate (PIR) insulation behind the ACM.

One ACM assembly that passed the 16-ft. PPT for unlimited height (fig. 3) also passed both BS 8414 and NFPA 285. This assembly was built with fire-resistant core ACM and non-combustible gypsum and a 2-in. (51 mm) air cavity.

The research report concluded that the NFPA 285 is not robust enough to evaluate ACM wall assemblies. The comparison of fire performance data with BS8414, while very limited (unavailability of assemblies), had comparable results to ANSI/FM 4880.

Figure 4 – The 16-ft. parallel panel test (PPT) from ANSI/FM 4880 simulates a realistic fire scenario and imparts heat fluxes on the order of 100 kW/m2 to the wall panel assemblies. The chart shows that the heat flux exposure of the 16-ft. PPT (ANSI/FM 4880) is higher and more realistic than that provided in the NFPA-285 test (40 kW/m2) and BS 8414 (~75 kW/m2). The heat flux exposure needs to be realistically high, so that the metal, joint systems and insulation behind can be tested for reaction to fire.

FM Approvals updates its cavity wall standard

The FM Global research report confirms the effectiveness of the 16-ft. PPT in evaluating the fire hazard of ACM assemblies. The 16-ft. PPT method simulates a realistic fire scenario and imparts heat fluxes (fig. 4) of the order of 100 kW/m² to the wall panels. The 16-ft. PPT fire scenario is representative of both exterior fires in corner situations and post-flashover fires from the building interior.

Unlike NFPA 285 and BS 8414 fire tests, which rely primarily on temperatures recorded via thermocouples, the 16-ft. PPT fire test setup is placed under a 5-MW fire products calorimeter (fig. 5), compliant with ISO 24473, to measure the HRR and smoke generated during the test. The peak HRR generated during the 16-ft. PPT has been correlated with the test results from large-scale 25-ft. and 50-ft. corner fire tests previously required in ANSI/FM 4880.

The rationale behind this correlation rests on the assertions that:

the radiation view factor between panels in the 16-ft. PPT is similar to that in a corner fire test, and
the peak heat flux in the 16-ft. PPT and corner fire tests are both on the order of 100 kW/m².

The ANSI/FM 4880 Approval criteria of a wall assembly tested with 16-ft. PPT is based on the peak HRR generated during the first 15 minutes of the test. Based on test results, Approval of wall assemblies up to 50 ft. (15.2 m) or unlimited height is provided. The wall assembly fails the 16-ft. PPT if the peak HRR during the test is greater than 1100 kW.

Late last year, following several notable fires involving ACM wall assemblies around the world, FM Approvals decided to enhance its standard covering this type of assembly: FM 4411, Approval Standard for Cavity Walls and Rainscreens—originally issued in August 2016—by adding the 16-ft PPT.

FM 4411 already included a modified 8-ft (2.4 m) PPT that was developed specifically to enable the testing of insulation and other interior components of cavity walls in the same vertical orientation in which they would be used on a building. This test will remain in the standard and, with the addition of the 16-ft PPT for external fire evaluation, will enable one standard to be used to evaluate the complete flammability characteristics of cavity wall systems.

Changes to FM 4411 also include renaming it Approval Standard for Cavity Wall Systems, dropping the previous reference to rainscreens, which are a subset of cavity walls and therefore encompassed by the new title. Because ACM/MCM assemblies include an air gap, which allows condensation and other moisture to be captured and eliminated, they are considered cavity wall systems.

According to Jill Norcott, FM Approvals senior engineer, who oversaw the development of FM 4411, “Our original goal with FM 4411 was to assess the combustibility of the interior of cavity walls which, until then, was not addressed by any other standard on the market. Based on the many fires we have seen recently, including the tragic U.K. fire, we decided to incorporate our 16-foot parallel panel test in the standard. It was already part of ANSI/FM 4880 to evaluate exterior fire performance. By making it part of the FM 4411, we simplify things for customers and ourselves.”

Figure 5 – Unlike NFPA 285 and BS 8414, which rely on thermocouples to record peak temperature, the 16-ft PPT fire test, which is part of the FM 4411 cavity wall standard, is conducted under a 5MW fire products calorimeter (FPC). This method provides a more accurate measure of overall combustion and is used in the Large Burn Laboratory in the FM Global Research Campus in West Glocester, Rhode Island, USA. Unlike most laboratories that primarily use simple thermocouples to measure local temperatures, the FPC enables FM Approvals to assess more charcteristics of the fire including the heat release rate and the smoke generation rate.

A robust standard for cavity wall systems

Tests such as NFPA 285 and BS 8414 are not designed specifically for cavity wall system testing. FM 4411 addresses the fire exposure from high heat flux fires that may originate from within the cavity wall itself from hot work, electrical shorts or from fires that originate in the interior of the building, generate flashover and spread out of the building through a window or other opening. Now, the standard also incorporates a test specifically for the exterior of the cavity wall systems.

The inclusion of PPT tests for interior and exterior fire performance within FM 4411 means that fire testing does not require costly and time-consuming construction of non-combustible masonry elements in the laboratory. The innovative PPT test apparatus can employ noncombustible outer panels that simulate masonry elements.

In addition to interior and exterior fires tests, FM 4411 includes simulated testing for wind loading, hail resistance, corrosion resistance, manufacturing quality control and an optional noncombustible insulation rating.

The revised FM 4411 enables cavity wall system manufacturers to stand out in a crowded market on a global scale. According to Phil Smith, manager of FM Approvals materials group, “This is still the only standard we know of that specifically evaluates cavity walls, including the composite panels that are under extreme scrutiny right now. By submitting their cavity wall systems for performance testing under FM 4411, manufacturers can demonstrate that they are committed to providing the highest quality product.”

Going forward, cavity wall systems that have been fire-tested to realistically high heat flux levels should have a clear advantage in the marketplace as designers, builders, insurers and owners aggressively seek out the most fire-protective cladding systems available in order to minimize future property losses.

For more information, go to www.fmapprovals.com