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Data Void: Exterior Wall Assemblies

Data Void: Exterior Wall Assemblies

Three years after Grenfell, accurate data on fires involving combustible exterior wall assemblies is still difficult to obtain. What does that mean for the global safety community, and for the people who live and work in thousands of potentially unsafe buildings worldwide?  

BY BIRGITTE MESSERSCHMIDT

Fires involving the façades of high-rise buildings have been covered by the global media for years. In many of these incidents, spectacular fires have spread up the exteriors of buildings at high speed, engulfing the structures in a matter of minutes. Most of these fires produced few if any fatalities, and as a result much of the world failed to realize the hazard presented by combustible exterior wall assemblies.

That changed on the night of June 14, 2017, when a fire at the Grenfell Tower, a residential apartment building in London, destroyed the structure and claimed the lives of 72 people. The fire began in a fourth-floor apartment, traveled outside through a window, and ignited the combustible panels and insulation that covered the building’s exterior walls. The flames travelled almost all the way around the building exterior, spreading the fire to the inside of the building and shortcutting all interior fire compartmentation, trapping scores of residents. The first phase of the official investigation confirmed that a majority of the flame spread was due to the combustible exterior wall assemblies that were added to the building during a deep renovation a few years earlier.

Fires that spread on the outside of buildings, feeding off combustible exterior wall assemblies, used to be very rare. Over the last 30 years, however, the number of these fires has increased dramatically. According to research done at Imperial College in London, the frequency of façade fires in large buildings has increased by seven times in the last three decades. Other researchers have identified 59 fires involving external walls on high-rise buildings between 1990 and 2018, with 36 of these occurring since 2010.

A surprising fact is that the only way researchers know about the increase in these types of fires is from the media—even after events like Grenfell, there is still no coordinated global effort to collect data on these or any other fire incidents. A report published in 2014 by the Fire Protection Research Foundation on the fire hazards of combustible exterior wall assemblies provided a number of insights into the potential dangers of these systems, including extensive flame spread and significant property loss. But even the warnings offered by such a comprehensive review were not enough to inspire policy makers into action.

One result of this inertia is that we do not have detailed data about these fires: the types of exterior assemblies that were used, what kind of fire testing (if any) those assemblies had been subjected to, how the fires started and spread, the types of safety regulations that may have been in place, and more. Without that level of detail, we are unable to make a convincing case for jurisdictions to institute new or more stringent testing methods, or to help create regulations that would lead to unsafe products being removed from the market. Relying on media reports also prevents us from gathering data on the small fires that do not develop into large disasters due to fire safety provisions working as they were intended. The information we are getting is skewed towards disaster and provides few lessons about what works compared to what doesn’t.

If we hope to properly address this significant global threat, this needs to change, and soon. According to the Council on Tall Buildings and Urban Habitat (CTBUH), there are presently 719 buildings taller than 150 meters, or 492 feet, under construction around the world, some of which will most likely include combustible exterior walls. This number will only increase, as rapid urbanization worldwide will see seven out of 10 people living in cities by 2050, according to the World Bank.

THE RISE OF FACADE SYSTEMS

Less than a year before the Grenfell Tower fire, NFPA Journal published an article that contained a foreboding and sadly prescient statement about the threat posed by combustible exterior wall assemblies. In his “International” column for the magazine, Donald Bliss, then the NFPA vice president of Field Operations, wrote that “in every country I’ve visited for NFPA, fire protection professionals are discussing fires involving improperly tested exterior façade assemblies on high-rise buildings. They agree that there is no easy solution to correct this unsafe condition in hundreds, if not thousands, of existing high-rise buildings worldwide.” Bliss went on to say that these fire professionals “also agree that, while it is fortunate that no loss of life has occurred in these fires, it is only a matter of time before an exterior façade fire extends into a building, trapping occupants and firefighters.”

The concern voiced by Bliss and other fire professionals around the world—and the primary reason for the increase in high-rise façade fires—was the introduction of combustible materials into these façade systems. The energy crisis of the 1970s sparked a demand for insulation of buildings to save energy used for heating, and the desire for energy-efficient buildings has increased ever since. One outcome of these efforts is that the thermal performance of exterior walls has improved significantly over the past 50 years. But with the introduction of combustible materials into those exterior assemblies, that performance boost has come at a cost.

NFPA Journal graphic (Photographs: Getty Images)

There are several ways to make an insulated exterior wall. The most frequently used systems include external insulating façade systems and external insulating composite systems—both of which consist of a layer of insulation, a reinforced mesh layer, and a thin coating of exterior material—as well as the ventilated façade system, where an air gap of at least 1 inch between the insulation and the covering panels allows moisture to escape. The insulation used in these systems can be combustible (such as polystyrene or polyurethane) or noncombustible (such as mineral wool or foam glass)—but the price, weight, and thermal performance makes combustible insulation the preferred option in most applications. This choice comes with the inherent challenge of ensuring that it does not become involved in a fire. The covering used in ventilated façades also comes in many versions, from inert natural stone to metal composite panels with combustible cores—again, spanning a vast range of potential risk in the event of fire. The ease with which these systems ignite and spread fire depends on the combustibility of the materials used and how the system is designed to limit ignition and fire spread, such as through the use of fire breaks and other protective measures.

An important aspect that is often overlooked is the quality of the installation of these systems. When using combustible materials, it is critical to ensure that they are protected from ignition and that added protective measures such as fire stopping and barriers are installed correctly. Many installers are often unaware of how minor details can have major impacts on the performance of finished systems when exposed to fire. A highly skilled workforce is essential if we hope to ensure the safety of these complex façade systems.

BUILDING REGULATIONS AND TEST METHODS

In an effort to prevent external fire spread, most building codes and regulations incorporate requirements addressing the fire performance of exterior walls. Controlling external fire spread is done through requirements applied to the facade systems. While the safety objectives of these requirements are similar from country to country, the manner in which they’re carried out can be very different. As with all fire requirements around the world, those related to exterior façades are based on national experience—for example, if a catastrophic fire with external fire spread has occurred—as well as local building tradition.

One approach used in many countries is to apply combustibility and/or flammability requirements to each material used in the façade system—the rationale being that by controlling the performance of each component, the combined system should reflect an appropriate (and appropriately safe) function. The requirements are linked to the perceived hazard for the building and are dependent on the height of the building and its occupancy. A typical example is to require the use of only noncombustible materials if the building exceeds a certain height, which can range from 12 to 50 meters, or 39 to 164 feet, depending on the country.

Another approach is to require testing of the entire façade system, often in a large-scale test to replicate the perceived real behavior of the system. Some countries have chosen to include large-scale testing of façade systems in their requirements, while others have opted to use large-scale façade testing in addition to testing of individual components.

The effort is commendable, but it actually raises an important issue: while these countries are all trying to mitigate the same hazard, there are almost as many tests as there are countries with testing requirements. The 2014 FPRF report, for example, mentions no fewer than 15 variations of these tests from various countries. In addition, similar protocols have also been developed by the International Standards Organization. As with general fire requirements, the various tests that have been developed to assess combustible exterior façades are largely based on local experience and often replicate a scenario seen in a catastrophic fire that occurred in the country. As a result, key differences can exist in the tests, including heat exposure, testing geometry, and criteria for passing the test. The same exterior wall system might get very different results in different tests, one deeming it safe and another unsuitable for its intended purpose. Fire, however, does not recognize geopolitical borders and behaves the same way everywhere. Yet, worldwide, our testing contains no consistent scientific basis that could help eliminate these critical safety differences from country to country.

Some countries allow the use of performance-based design, an engineering approach to fire protection that is based on fire safety goals and objectives, analyses of fire scenarios, and the quantitative assessment of design solutions. This methodology is an incredible tool that allows fire protection engineers to solve complex problems like exterior façade safety—but it requires an expert-level understanding of the systems being analyzed. Unfortunately, the lack of understanding of how different materials interact within combustible exterior wall assemblies during fire makes it difficult for professionals to become experts on this topic.

As a result, in some countries the designs related to combustible exterior wall assemblies apply two assumptions: that façade components will behave similarly in any scenario, and that there is a rule allowing for interpolation or extrapolation between tested façade systems. Research shows that these two assumptions are incorrect. Since a minor change in the material, geometry, or assembly of a façade system can drastically impact its combustibility, it is dangerous to assume that these variations can be used interchangeably in building design without further research.

WHERE IS THE DATA?

These are the kinds of problems that can be traced directly to the absence of accurate data—without it, we can understand neither the true scope of the problem nor the details necessary to create consistent testing requirements and building regulations. That void also results in an ongoing struggle to help policy makers understand the critical link between energy-efficient buildings and fire safety. I once encountered a politician who said to me apologetically that until there were “bodies on the table,” exterior façade safety was simply not an issue that would receive political attention.

So how do we improve our data gathering, and how do we leverage it for maximum effect? The short answer is that it isn’t easy. Even in the US, where I would argue we have the most comprehensive fire data collection anywhere in the world, we cannot obtain the kind of granular data needed to help us better understand this problem. The fire incident data in the US is collected through the National Fire Incident Reporting System (NFIRS). After a fire incident, NFIRS can be used to record data about the item first ignited, the performance of the automatic sprinkler system, and other critical pieces of information. Since NFIRS uses a limited number of choices within each data element, it provides minimal information about complex fire problems such as those discussed here.

A similar shortage of data exists internationally. Researchers from different countries have indicated that incident reports often provide limited information about the type of façade system, the components used in the assembly, and the development of the fire. Another limitation of international data is that different metrics are often recorded by different countries. If data collection is inconsistent between nations, it is impossible to compare the frequency of incidents without a high level of uncertainty.

At an international conference on fire safety of façades in Paris last year, someone asked if we knew whether the façade systems involved in the fires we see around the world had fulfilled local requirements, or if they had passed locally mandated large-scale testing. Sadly, the answer to each question was no. With no consistent way of collecting incident data, it is impossible for us to quantify how different requirements around the world impact the level of risk. With few exceptions, we continue to validate unknown quantities of different building materials with various test methods and often install them with an undertrained workforce bound by minimal regulations. The progression of these practices is seemingly bottlenecked by a lack of knowledge and fire incident data to validate or disprove hypotheses.

It is important to recognize that there is an opportunity to learn from failures, and even successes, and develop strategies based on real-world fire incidents. The education of stakeholders, growth from previous failures, and quantification of this global issue demand much more data than are currently available today. As a champion of building safety, we recognize a need to address this issue, and we look forward to discussing it in an array of upcoming public forums. I have no doubt that we possess the ability to devise new ways to collect and share this information. Until we do, we will continue to try to solve a set of problems we can barely identify.

BIRGITTE MESSERSCHMIDT is director of Applied Research at NFPA. Top photograph: Getty Images

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