Booth Muirie provides insight into the fire performance of rainscreen cladding systems

John Dunnington, Head of Marketing at architectural cladding specialist Booth Muirie, provides insight into the fire performance of rainscreen cladding systems on buildings over 18m, following the most extensive fire testing the cladding industry has witnessed.

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his rigorous testing of complete systems is providing professionals with the clarity and reassurance they need and gives the architectural and construction communities the opportunity to take a closer look at the detailed performance of rainscreen cladding systems to ensure safer specification.

Rainscreen cladding was thrown into the spotlight as part of the Grenfell investigation and, over a year on from the tragic events, many people are still unsure about, or lack confidence in, rainscreen systems.

The British Standard large-scale test (BS 8414) sets out a standard methodology for assessing the resistance to spread of fire performance of multi-layered wall systems. If the system performs within the limits set out in BR 135 when tested, it is a way of demonstrating that the precise assembly constructed for the test meets with Building Regulations for buildings with storey heights above 18m.

To date, a number of different multi-layered wall assemblies featuring rainscreen cladding systems – which can be supplied by Booth Muirie – have been tested according to the BS 8414 fire test standard.

So far, 17 assemblies featuring Booth Muirie’s panel systems have satisfied the requirements of BR 135. The testing, which is ongoing, has been carried out in conjunction with multiple industry partners and represents the most comprehensive and wide-ranging testing programme of solid aluminium and ACM cladding systems ever undertaken.

The work provides unequivocal information on these systems, providing safer specifications for the future and confidence in those systems that prove compliant with BR 135.

Booth Muirie has shared all findings publicly on its website to give the architectural and construction industries some much-needed clarity going forward, helping professionals to make informed choices and improving safety in buildings over 18m. These findings will help make positive changes across the industry.

The testing has also led to an excellent understanding of the effects that changes in design can have on a wall assembly’s resistance to spread of fire performance.

Many different factors can dramatically affect how a system featuring aluminium or ACM rainscreen panels will perform in the event of a fire.

The evidence that has been gathered to date suggests that the primary determinants of a system’s performance in respect to its resistance to spread of fire are the type of external cladding material used and the cavity barrier detailing.

If the external cladding material is an ACM, then the system’s performance cannot just be delineated by the calorific value of the core. The way this core is bonded to the aluminium surface layers of the composite also has a huge bearing on performance. Insulation type, thickness and cavity size also influence performance, but to a much lesser extent.

Given that there are so many different factors affecting behaviour in fire, the truest indication of real-life performance is always going to be an evidence-based understanding – that is an assembly of specific components that have been fitted in a detailed manner, tested to the BS 8414 test(s) and proven compliant with BR 135.

There are some challenges, however. Multiple assemblies may have to be tested for any one project (and retested for any change in specification at any point) and, with only limited testing facilities available globally, there is currently a testing capacity issue in the industry.

Desktop studies can be an option when based on tested products and systems, helping to demonstrate compliance when the demand for testing exceeds the capacity. However, the way desktop studies are used needs to be strengthened.

Improvements such as having a published register of all approved desktop studies, setting mandatory qualifications for those performing desktop studies, and better prescription of what test data can and cannot be considered in the production of a desktop study, would all help.

The industry has a responsibility to ensure the safety of the modern built environment. Professionals need to take all of the findings available to them and use them to make better informed and safer specifications to restore confidence in the cladding industry.

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