16 May 2018
Nick Jenkins takes a closer look at aluminium and aluminium composite material (ACM) cladding and its resistance to the spreading of fire.
DESPITE SOME understandable reticence, there is increasing clarity within the construction industry about the resistance to spread of fire performance of rainscreen cladding systems on buildings over 18m. In this article I will explore how extensive testing of complete systems is providing professionals with the clarity and reassurance they need.
During my career I have gained insight and understanding and this topic through involvement in an number of tests that have taken place featuring Booth Muirie BML Systems and the specific wall assemblies that it can form part of. I intend to explain to you that the results of these tests show an ever-clearer path ahead for aluminium and ACM rainscreen systems.
Use of ACM
ACM materials are commonly fabricated into panels used to clad buildings, typically in the form of rainscreen systems forming the outer most layer of multi layered wall constructions. ACM is particularly popular as a cladding material because it offers a wealth of options for new-build and renovation projects, bringing practical benefits and huge aesthetic flexibility, as well as being lightweight and available in a vast choice of flawless finishes.
ACM cladding is made up of two skins of aluminium bonded to either side of a core material. The principle of this type of material is that you get a lightweight yet strong material that retains excellent flatness. As well as this, ACM can be formed into various shapes and panels with ease, due to an automatic cutting and routing process and the nature of the material. The core can be made up of different types of materials depending on the type of ACM.
Building safety programme
Following the Grenfell Tower tragedy, the government established a Building Safety Programme with the aim of ensuring high-rise residential buildings are safe, and residents feel safe in them.
ACM was thrown into the spotlight as part of the investigation and, almost a year on from the tragic events in North Kensington, many people in the construction industry are unsure about - or lack confidence in - the way forward for ACM rainscreen systems.
In 2017, screening tests at the Building Research Establishment (BRE) identified whether the different types of ACM as installed meet the limited combustibility requirements of current Building Regulations guidance.
ACM with an unmodified polyethylene filler (category 3 in screening tests) presents a significant hazard on buildings over 18m. This was the kind of ACM used on Grenfell and the obvious starting point for any progression is to avoid the use of this material at all costs. This ACM is commonly suffixed PE.
Cladding with a fire-retardant filler, commonly suffixed FR or Plus (category 2 in screening tests) have cores that contain a mixture of combustible polyethylene and minerals that limit the proliferation of flame and restrict the development of smoke. Such materials are generally classified as B1-s1, d0 according to the EN 13501-1/2 Reaction to Fire Classification. While they have a very low combustibility, they are nonetheless considered to be ‘combustible materials’ as defined in tables A6 and A7 of Appendix A of approved document B.
In addition, cladding with a limited combustibility core, commonly suffixed A2 (category 1 in screening tests) are the safest form of ACM and have cores that primarily consist of minerals that limit the proliferation of flame and restrict the development of smoke. Powder coated aluminium (PPC) and A2 ACM both share the same A2-s1, d0 classification as according to the EN 13501-1/2 Reaction to Fire Classification.
While calorific values of PE cores start at 35 MJ/kg (blatantly very combustible and not far from the calorific value of petrol, at 44.8 MJ/kg) and A2 cores have a calorific value of less than 3 MJ/kg (‘limited combustibility’ as defined in tables A6 and A7 of Appendix A of approved document B), the FR type cores can have a calorific value ranging from 3 MJ/kg to 35 MJ/kg. So, globally categorising FR cored (category 2 in screening tests) ACMs is not truly representative of the fire performance of different brands.
This strengthens large-scale tests as the route to compliance; eliminating, as it does, the variables inherent with: the choice of panel systems; panel geometry; cavity barriers; insulation; the type of ACM (A2 or FR core); the brand of the ACM; and, perhaps most importantly, how all these elements fit together with each other and the rest of the multi layered wall, and ultimately how they perform holistically as a system.
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 when tested to this standard the system performs within the limits set out in BR 135, it is a way of demonstrating that the precise assembly as 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, that can be supplied by Booth Muirie, have been tested according to the BS8414 fire test standard.
At the present time there are 17 assemblies featuring Booth Muirie’s panel systems that have satisfied the requirements of BR 135. The testing, which is ongoing, has been carried out in conjunction with multiple industry partners, and represents by far the most comprehensive and wide-ranging testing programme of solid aluminium and ACM cladding systems that has ever been undertaken.
We have undertaken this work to provide unequivocal information on these systems, giving us confidence on what to advise the industry and to support the provision of safer specifications.
We have gained an excellent understanding of the effects that changes in design can have on a wall assembly’s resistance to spread of fire performance through our extensive testing, and we made the decision to share the findings publically on our website to support the industry in making positive changes to improve safety.
The evidence that we have gathered to date suggests that the main determinants of a systems performance are the type of external cladding material, the panel system and the robustness of 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 not to the same extent.
Given that there are so many different factors and the subtlety of changes that can dramatically affect how a whole system featuring aluminium or ACM rainscreen panels will perform in the event of a fire, the truest indication of performance is that of an assembly of specific components that has been fitted in a detailed manner, tested to the BS8414 test(s) and proven compliant with BR135. It is however an expensive route; multiple assemblies may have to be tested for any one project and there are limited testing facilities so there is currently a capacity issue in the industry associated with this route. This is where Desktop Studies come in. These can be a viable option when based on tested products and systems, helping to demonstrate compliance when the demand for testing exceeds the capacity.
It is very difficult to predict how a fire will act and react in reality. In my view, despite cost and time implications, the way to be certain of how an entire system will perform in a real-life fire situation is to gain an evidence-based understanding of the system; built to specification, BS8414-tested and proven to meet the requirements of BR135.
Nick Jenkins, Executive Director at architectural cladding specialist Booth Muirie. For more information, visit www.boothmuirie.co.uk/technical/fire-performance/