According to Nick Jenkins, the cladding industry needs to stop second guessing fire performance and take responsibility by acting on test evidence and ensuring compliance
LONG BEFORE the horrors of Grenfell Tower unfolded, questions had been raised about the building regulation compliance and fire safety of buildings over 18m tall with rainscreen cladding systems. One year on from the disaster, there are still major concerns about how the industry is handling fire performance and compliance.
The Grenfell fire highlighted many things to all of us, including the difficulty of predicting how fire will behave. Its speed and ferocity shocked even the most experienced firefighters and safety experts.
You cannot second guess fire and there are so many different factors that can affect how it behaves.
It may seem obvious, but unfortunately that is a lesson still to be learned in the cladding industry as a whole, where there are legitimate concerns that Approved Document B (ADB) guidance has not always been properly followed on construction sites. This is despite evidence that certain multi layered wall assemblies currently being constructed, whilst in some instances conforming to ADB guidance, do not perform well with respect to their resistance to spread of fire when tested.
Most concerns stem from the vastly different performances witnessed from the different brands of aluminium composite material (ACM) available that claim to meet the same EN 13501-1/2.
Types of ACM
ACM is a type of cladding material commonly used on public buildings such as schools and hospitals, as well as social housing. There are generally three types of ACM available in the industry, each with different levels of fire performance influenced by the nature of the coated aluminium outer layers, the core material, and the way the core is bonded or fused to the coated aluminium. We now know that the type of core material – and how it is fused to the aluminium outer layers – is the primary determinant of the complete ACM’s fire performance.
It is not just the calorific value of the core that needs to be considered. There are three main grades of ACM: those with pure polyethylene (PE) cores, those with fire retarded PE (FR) cores and those with solid mineral (Euroclass A2 or limited combustibility) cores. These are generally called PE, FR and A2 solid cored ACMs, respectively. Some manufacturers refer to FR solid cored ACMs as ‘Plus’ grades. FR solid cored ACMs have core materials which contain a mixture of PE and fire retardant minerals that limit the spread of flames and development of smoke.
The core of PE solid cored ACMs generally has a calorific value over 35 MJ/kg, while generally the core of FR solid cored ACMs has a calorific value greater than 3 MJ/kg and less than 35 MJ/kg. Both types are combustible products, as defined in ADB. To put that into perspective, petrol has a calorific value of 44.8 MJ/kg.
In my view, PE solid cored ACMs, some of which have cores with a higher calorific value than petrol, have no place in modern architecture or the built environment, regardless of the height of the building (and they are not supplied by Booth Muirie, an architectural cladding specialist, which is a division of Euroclad Limited).
Booth Muirie and Euroclad have carried out, and are continuing to carry out, unprecedented numbers of ventilated rainscreen system tests to BS 8414-1 and BS 8414-2. The details of all systems that have met the requirements of BR 135 are publicly available online at www.boothmuirie.co.uk/technical/fire-performance/. This testing has helped an evidence based understanding of the effects that changes in critical design variables have on a wall assembly’s fire performance.
To date, the evidence gathered suggests that the main determinants of system 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 system performance can be delineated by the calorific value of the core and the manner in which this core is bonded to the aluminium surface layers of the composite. Insulation type, thickness and cavity size also influence performance, but not to the same extent.
As an industry, it is our responsibility as individuals to act on the evidence from testing and restrict cladding systems for new builds, refurbishments and re-clads to multi layered systems that have been proven to perform, and make a change now, rather than waiting for regulations to catch up. Noone wants to witness another Grenfell.
Incredibly, it seems that there is still a bias in the industry to use the quickest, cheapest routes to compliance for current regulations, which poses an extra obstacle to change.
Routes to compliance
Four routes to compliance exist: the linear, performance based, desktop studies and holistic engineering routes.
The simplest is the linear route, but unfortunately this does not necessarily ensure delivery of the safest multi layered wall solutions. It allows the use of materials that are of ‘limited combustibility’ (in England and Wales) or better, without consideration to their performance as component parts of a multi layered wall assembly. It does not consider how other elements of that system will interact.
This is a more reliable route, leaving less to chance. It uses data from large scale fire tests comprising the entire external cladding system, built exactly as the proposed system is to be supplied and constructed for any particular element of a project. It is however an expensive route, as 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.
A third route is to obtain a desktop study report (DTS) from a suitably qualified independent UKAS accredited testing body. This should state whether, in its opinion, BR 135 criteria would be met by the proposed system.
A DTS is a comparative assessment between a system or systems that have been tested and the system that is fixed or proposed, showing that there is no adverse impact on the fixed or proposed system based on the differences in construction and/or materials.
Such an assessment will be required if combustible materials are included as any major element of the multi layered wall and/or if there is any variance between the system proposed and any system tested. These variances could include, but are not limited to: panel material type and brand; panel modules; panel system; panel joints; cavity size; cavity barrier type and brand; cavity barrier frequency; position of cavity barriers relative to panel joints; insulation type; insulation thickness; support rail and bracket centres; and nature of the backing wall.
DTSs are very much project specific and not transferable from one project to another. Although the full scale tests do not take into account any form of suppression system that may be installed in the building or any other active life safety measures, such aspects may provide further support to any DTS case that may be presented.
Some commentators have expressed doubts about the rigour of DTSs. We believe that guidance governing the use of DTS reports needs to be tightened. For instance, a published register of all approved DTSs, setting mandatory qualifications for those performing them and better prescription of what test data can and cannot be considered in the production of a DTS would all improve reliability.
If none of the above options is suitable, compliance with the functional fire safety standards may be demonstrated by alternative means, such as the adoption of a fire safety engineering approach. Based upon scientific principles from an integrated or a ‘whole building’ perspective, fire safety engineering not only considers the performance of structures, systems, products and materials when they are exposed to fire, but also includes human behavioural aspects, fire prevention, and active and passive fire protection measures. For example, effective means of egress and adequate measures for alarm, detection, control and extinguishment.
Furthermore, this approach can enable innovation in building design without compromising fire safety, particularly in some large and complex buildings, as well as in multi purpose buildings where it may be the only practical way to achieve a satisfactory level of fire safety. If taking this advanced route to compliance, the guidance given in a number of supporting published documents can be followed. However, Approved Document B – Volume 2: Buildings other than dwellinghouses (both England and Wales) refers directly to BS 7974: 2001: Application of fire safety engineering principles to the design
of buildings. Code of practice, while Technical Handbooks Section 2 refers directly to International Fire Engineering Guidelines 2005.
In summary, the linear approach to compliance may not provide a reliable indication of how an overall wall assembly might react in the case of fire. Whether using systems that have been successfully tested to BS 8414/BR 135 or DTSs based on compliant tests, given the amount of test data that is now available, there is a clear and proven route to compliance which the industry can follow and have confidence in.
Nick Jenkins is executive director at Booth Muirie. For more information, view page 5