October 12, 2018 | Feature Article

Cavity Wall Standard Being Revised to Reduce Risk

Enhancement addresses prevalent fire risk in high-rise buildings

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

Fig 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 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. (photo: “Grenfell Tower fire, 4:43 a.m.” courtesy of Natalie_Oxford on Twitter)

The Grenfell Tower fire (Fig. 1), for example, in West London on June 14, 2017, which 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 has been blamed for enabling the interior fire to rapidly spread to and engulf the building exterior.

The fire risk posed by combustible ACM and metal composite material (MCM) exterior panels, and the associated assemblies, has gotten the attention of government agencies and tenant groups worldwide. In the U.K., for instance, following intensive investigations, the government 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 safety measures.

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, entitled Grenfell: The Perfect Formula for Tragedy, which analyzes that tragic fire, as well as fires in Dubai and the United States. And, 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 research 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.

Tested 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 desktop assessments. NFPA 285 is a U.S. building code-required test of exterior wall flammability characteristics.

Fig 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 in actual testing. The flames in this test had to be water suppressed within four minutes from the start of the test to avoid exceeding the capacity of the fire products collector (FPC). At right, the panels have been completely consumed by the fire.

However, two wall assemblies that had passed the NFPA 285 test, one the result of physical testing (Fig. 2) and the other by means of desktop assessment, decisively failed the 16-ft. PPT research tests. 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 featured sandwich construction with a thermoplastic core.

Two other ACM wall assemblies that passed NFPA 285 for unrestricted 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 panels.

Fig 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 method of ANSI/FM 4880. At right, the peak flame height reached was approximately 8 ft. (2.4 m). This ACM sample performed well and passed ANSI/FM 4880 for unlimited height.

One ACM assembly (Fig. 3) that passed the 16-ft. PPT for unlimited height, also passed both BS 8414 and NFPA 285. This assembly was built with fire-resistant core ACM panels and no insulation.

FM Approvals updates cavity wall standard

Fig 4 -- The 16-ft parallel panel test method from ANSI/FM 4880 simulates a realistic fire scenario and imparts heat fluxes on the order of 100 kW/m squared to the wall panels. The chart above 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/m squared) and BS 8414 (~75 kW/m squared). The heat flux exposure needs to be realistic and high, such that the metal, joint systems, and insulation behind can be tested for reaction to fire.

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.

The 16-ft. PPT fire test setup is placed under a 5-MW fire products collector, 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 25-ft. and 50-ft. corner fire tests.

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.

Changes to FM 4411 will also include a name change to Approval Standard for Cavity Wall Systems, dropping the previous reference in the title to rainscreens, which are a subset of cavity walls and therefore encompassed in 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.

“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,” notes FM Approvals senior engineer Jill Norcott. ”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 our customers.”

Fig 5 -- Approval Standard 4411 also requires that cavity wall systems be tested using an 8-ft. PPT full-scale fire test apparatus (shown here) to assess the extent of fire propagation and heat release rate of components used within the wall construction.

FM 4411 already included a modified 8-ft. (2.4 m) PPT (Fig. 5) 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.

“Tests such as NFPA 285 are not cavity wall tests,” says FM Approvals senior engineer Mark Tyrol. “FM 4411 addresses the fire exposure from fires that may originate from within the cavity wall itself from hot work, electrical shorts or from fires that originate on the interior of the building and flashover, and 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 as well.”

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

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

According to Phil Smith, vice president and manager of FM Approvals materials group, the revised FM 4411 enables cavity wall manufacturers to stand out in a crowded market on a global scale. “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 they are serious about providing the highest-quality product.

"Cavity wall systems Approved to FM 4411 are going to have a clear advantage in the marketplace as designers, builders and owners aggressively seek out the most fire protective cladding systems available.”