Modern methods of construction are an essential part of the built environment today, helping us to create buildings that are resource and energy efficient, robust and visually pleasing. However, the rapid development of new and innovative products to meet demand has led to some confusion about the difference between cladding that is used purely decoratively, and systems that perform a structural as well as aesthetic role. This distinction is particularly important when it comes to understanding fire performance.
What is the difference between metal rainscreen cladding and insulated panel systems?
On the surface, metal rainscreen cladding systems and insulated panel systems can look very similar. When you look at the actual construction behind the façade they are quite different, and this is key to understanding how they might behave in a fire situation.
A rainscreen system comprises three core elements: the external façade, a supporting frame to which the façade is fixed, and an internal skin which includes an insulation layer. In the construction process, these three elements are installed separately on site. The rainscreen façade is non-loadbearing, it is simply fixed to the front of the building to provide an attractive finish, and to protect the main structure from being directly impinged on by the weather.
An important aspect of this weather protection is the presence of a cavity between the façade and the internal skin, which prevents any moisture that has passed the façade barrier to enter the main structure of the building. In high rise buildings, regular fire breaks should be installed to prevent the cavity from creating a ‘chimney effect’ in the event of a fire.
Metal faced insulated panels, sometimes called sandwich panels, are single, factory engineered components that are typically fixed directly to the structural frame to provide both insulation and weather protection. The majority of these panels feature a thermosetting Polyisocyanurate (PIR) insulation core, which is auto-adhesively bonded to the metal facings to provide a strong, durable unit with proven fire safety performance. There is no cavity in this type of cladding construction. However, insulated panels also may be used as the internal skin in a rainscreen system, simplifying the build-up process on site and reducing reliance on the cavity for weather protection.
A significant benefit of insulated panels is that they can be tested as a single component that achieves a particular standard in conjunction with the supporting frame. A built up rainscreen system, on the other hand, can vary widely in terms of composition, and each variation could affect the fire performance of the build-up. A different test would be required for each variation to get a precise indication of how it would perform as part of a construction.
Regulatory compliance, and large-scale insurer testing
Whether the cladding on a building is part of the load bearing structure or not, it still needs to comply with the relevant national fire safety regulations for external walls, especially in high rise applications. Regulatory requirements often focus primarily on life safety, and have given rise to a number of small scale ‘reaction to fire tests’, which look purely at the performance of products in isolation.
The potential issue with this approach is that it does not necessarily reflect how a real construction as a system might perform in a fire. In other words, a product that can achieve a non-combustible classification may not offer much in the way of fire resistance. The ability of a building to remain structurally intact could be better served by materials that may not be classed as non-combustible, but which perform well in both reaction to fire and fire resistance tests as part of an actual construction.
Whereas building regulations and standards are focussed on life safety, insurers have an interest in property conservation as well as life preservation, and have developed their own large-scale test standards. Of these, the most well-known and widely recognised internationally is FM Global.
The key fire test standard relevant to insulated panel systems is FM 4880 Approval requirements for fire rating of building panels. There are various levels of performance that can be tested, the key level being Class 1 with no height restriction. Achievement of Class 1 to FM 4880 with no height restriction is dependent on performance in a number of tests that can include:
- ISO 12136 Fire Propagation Apparatus
- ASTM D482 Ignition Residue tests
- ASTM E711 Oxygen Bomb tests
- UBC 26-3 Room Test
- FMRC Room Corner Test (25/50ft test)
- FM 16ft Parallel panel test
The 50ft wall test is very severe. Two walls 15.24m high with a small ceiling are lined with panels and a large fire source (345kg dry timber) is positioned in the corner. To achieve approval without any height restriction, there has to be no flame spread or fire propagation to the extremities of the panel construction as evidenced by flaming or material damage. In addition, ignition of the ceiling of the assembly must not occur.
In the UK, another stringent and well-regarded test is LPCB’s LPS 1181 test, sometimes referred to as the ‘garage test’, comprises a 10m long, 4.5m wide, 3m high enclosure clad in the materials under test. The enclosure is open at the front, with a smoke skirt at the ceiling to prevent the hot smoke layer from escaping and adding to the severity of the fire development, as in a real-life situation. The enclosure has a ventilation window at the side. A wooden crib, which generates a 1 megawatt fire load, is ignited in the corner and the fire development is monitored. Although there are a number of pass – fail criteria the key parameter is that there should be no fire propagation beyond a 1.5m zone around the crib.
Key characteristics of PIR insulated panels that have passed these tests are that they form a stable protective char, and can achieve appropriate levels of fire resistance. They demonstrate no flashover, no fire propagation and no panel collapse. Examination of the panels following the tests reveal no flame spread in the core of the panel. Importantly, to pass the test, smoke levels must also be relatively small and regarded as acceptable.
Large scale tests will clearly give a better indication of product performance in the event of a fire than small scale tests, but how well do the test results correlate to what happens in a real fire?
Over the years there have been opportunities to closely examine the behaviour of insulated panels which have been exposed to severe fires. Reports by independent experts have revealed that, in each case examined, they have performed as expected, based on their tested results.
Case study 1: Wharfedale Hospital
One argument that is often put forward against the results of large scale testing, is that the test rig is always constructed perfectly, with none of the flaws and potential exposed edges of a real building with service penetrations.
A building under construction is potentially at its most vulnerable to fire, as any detailing and safeguards will not yet be in place. On this basis, a fire at the Wharfedale hospital in Otley, West Yorkshire in the UK could have been a real disaster. The £15 million hospital was under construction in 2003 when combustible building materials stored on the ground floor were set on fire by an arsonist. The ground floor was open, with insurer approved insulated panel cladding installed at the first floor level. Much of this exterior cladding was exposed to direct flames up to 10 metres high from the ground floor fire, along with smoke and heat damage.
An independent investigation by international fire engineering consultants, Tenos, revealed that the LPCB approved insulated wall panels did not ignite, despite the fact that the building was incomplete. Indeed, the incident demonstrated that the insulated panels fire performance capabilities had helped to minimise the fire damage and prevent it from spreading.
Holes cut into the insulated panels by fire fighters to expose the steel columns for inspection revealed that the PIR core was unaffected by the fire. When the steel was peeled back from a panel that had been directly impinged on by the fire, the PIR showed only slight charring.
The Tenos report concluded that ‘the panels did not ignite, did not promote fire spread within the core or to the eaves and did not significantly contribute to the products of combustion’.
The Wharfedale hospital eventually opened on 26th January 2005.
Case Study 2: Spider Transport
The building concerned was being used as a warehouse and distribution centre, constructed of a steel frame with brickwork lower walls and insulated panels with a PIR core, approved to LPS 1181 Part 1 Grade EXT-B, installed as the upper wall cladding. A truck had been parked across the two main “up and over” doors of the building to prevent unauthorised access during the night, but arsonists set the cab on fire and this quickly spread throughout the vehicle.
The intensity of the fire and the proximity of the truck to the building meant that the panels were soon exposed to prolonged flame impingement during the 25 minutes before the fire brigade arrived. At one stage there was also an explosion and a fireball as conditions worsened.
The panels close to the fire were subjected to some intense conditions, but an investigation by Tenos found that they were essentially unaffected. Key considerations in investigation were whether the panels contributed in any way to the spread of the fire and whether they helped prevent fire entering the building.
Even though the standard construction details for the junction between the cladding and the wall and door frame had been used, which leaves a small amount of PIR core directly exposed to the fire, the report concluded that there were no signs of any spread of heat through the panel core, or that the panels contributed to the damage caused by the fire in any way. Furthermore, there was no delamination of the metal panel lining and the insulation core stayed in place, important points in maintaining system integrity and fire resistance.
Despite the ferocity of the fire, the inside of the premises was unaffected and business resumed as normal the next day.
These real-life fires provided clear evidence that the insurer approved panels caused no spread of fire within the core, no flaming droplets, had low smoke emissions, and importantly, they presented no additional risk for fire fighters.
For more information, go to www.epic.uk.com