The performance of cladding materials under fire conditions is crucial and should be considered in detail in order to draw conclusions about the fire safety of any facade system. However, before addressing this issue, we need to answer two obvious questions: Why do we NEED to consider the performance of external wall systems in terms of fire performance and; Why can it be a major concern for high rise buildings?
Given the nature of current building contents, if a fire occurs in a compartment within a high-rise building, there is a good chance that the fire will develop to flashover. At this stage, the fire could break out of the compartment via the windows for example and manifest itself on the external surface of the construction. When this happens the cladding suffers a massive heat shock. However, the fire should remain local and not spread throughout the building to give fire brigades the chance to get to site and deal with the fire. However, due to limitations in the equipment, they can only deal with fires at a limited height and they need to deal with such fire using different methods e.g. from the inside of the building.
If such a fire cannot be controlled, there is a chance that the fire will spread in an uncontrolled manner and lead to a tragic incident. Unfortunately, there are some well-publicised fires which were tragic in their impact such as the Windsor building in Madrid, Spain; the First Interstate Bank in Los Angeles, USA; Grenfell Tower in London, UK; the Mandarin Hotel in Beijing, China; the Gronzy-City Towers in Chechnya, Russia; and the Torch Tower in Dubai, UAE. Figure 1 shows some of those examples.
A review of the nature of the cladding for above examples, shows that the cladding used was quite different. Indeed, it can be said that cladding for construction does not only change from one construction to another but also the same building could be clad with various types of material sitting side by side. Examples of the most common type of systems are external thermal insulation composite systems (ETICS), sandwich panels, high pressure laminates structural insulation panel systems (SIPS), rainscreen cladding (RSC), external timber panels and many more. All these systems will perform differently when exposed to fire. So, the next question would be how can we make sure that the design system is safe in the event of a fire?
To answer this question, we need to know the mechanism of spread of the flame:
- What is the acceptable level spread of the flame?
- How do the products proposed for the façade system perform should they get exposed to fire?
- How does the cladding as a system perform in a fire, given that materials are used as part of a system and not individually?
In terms of the mechanism of flame spread, a cladding could be exposed to fire from an external fire located close to the cladding, for example if a refuse bag located close to the cladding is ignited. The other scenario would be when a fire occurs inside a compartment and the fire develops to flashover and the fire projects from the window. In this case, even if the fire does not interact with any of the products installed on the cladding, the flame height can be as much as two metres high or more. In this case, the flames could impinge directly on the floor above and may re-enter the building, causing a secondary fire in another compartment. Alternatively, the fire could interact with cladding products and spread along the surface or spread through the cavity and exposes many storeys at the same time.
While all the mentioned mechanisms are possible, which of them would be most acceptable? Based on current regulation, rapid spread of flame should be prevented. This means the flame should not expose and spread more than a maximum of two storeys at any time. This is because such a spread of the flame mechanism would provide the chance for fire brigades to deal with the fire. There are instances where the fire can find the chance to spread on the surface for more than two storeys either due to flame spread occurring due to the interaction between the products used on the cladding or spread through concealed areas where a chimney effect can occur leading to flame heights of between five to 10 times higher than its original length on the outer surface. Managing such fire presents considerable difficulties and this increases the risk of uncontrolled flame spread. Therefore, such a scenario is not acceptable.
But how can we make sure that a façade system is within that acceptable range should it be exposed to fire?
To be able to answer to this question we need to look into the issue from two angles:
- The material’s performance under fire conditions. This would allow us to have an initial evaluation of the system and in this way the reaction to fire classification (for example to EN 13501-1) would be quite helpful.
- Performance of the entire system under fire conditions representing initial flashover
However, the tests used for classifying materials use fire conditions which are considerably less stringent than an actual fire. As a result, the performance of the materials or systems under such conditions would not be a true representation of the real situation. Moreover, even if they have a good reaction to fire performance, these products would be installed as a system and as a system they may not perform in an acceptable way. This presents the need for a large scale test in which the overall performance of the cladding system can be checked.
Currently, we do not have a European harmonised standard under which a façade system could be tested. However, a European Commission review has studied the various national test standards to suggest a large-scale fire test which can be used across Europe. Based on this review they have narrowed down their consideration to BS8414 test series [2,3] as the large-scale test and DIN 4102-20  as a medium size test.
With regard to the BS8414 test, it is a large scale test published in 2015 by BSI to consider performance of the façade systems under fire conditions [2,3]. The test rig in this test is an L-shaped arrangement where the main wall is 2.6m and the wing wall is 1.5m. The height of the system is a minimum 8m high. The fire load in the system is 400kg of timber crib that could create a heat flux of 45 to 90 KW/m2, with a constant heat flux of 75KW/m2 for a period of 20 minutes. See Figure 2
This test lasts for 60 minutes. During the first 30 minutes the timber crib burns and at the end of this period, which is about the time where timber crib is largely consumed, the timber crib would be extinguished and spread of the flame is monitored over the remaining 30 minutes. Flame spread to the top of the test specimen is noted [2,3].
In terms of instrumentation, the specimen is instrumented at two levels (2.5m and 5m) above the chamber. The thermocouples at level 1 are mainly used as an indication to see when the timber is fully ignited and when the thermocouples at this level reach a temperature of 200oC that is counted as the start time. The temperature recording at level 2 is used for analysis of the fire performance of the system. That is why at this level, thermocouples are not only located on the external surface but also at the middle of cavity, the insulation and any layers more than 12mm thick.
Figure 3 Effect of external cladding on spread of the flame [7-13].
Currently, BS8414 test series [2,3] does not have any failure criteria other than for safety reasons and spread of the flame to the top of the rig. As a result, in the UK a norm has been defined by which the results of a test could be seen as a pass or fail . Based on those criteria:
- A temperature of over 600oC above ambient temperature for at least 30 seconds at level 2 within the first 15 minutes from the starting time (i.e. after temperature of 200oC recorded at level 1);
- A temperature of over 600oC above ambient temperature for at least 30 second at any internal part of level 2 within first 15 minutes from the starting time;
- A sustained flame of at least 60 seconds duration on unexposed surface at a height of 0.5m (or higher) above the chamber on internal layer within 15 minutes since the starting time (integrity failure);
The results of that test would be interpreted as a failure.
Knowing the test method and its failure criteria, it would be interesting to know what the parameters are that could have an impact on the performance of the cladding system when tested to BS8414.
Figure 4 Effect of detailing on spread of the flame.
One of the parameters is the products used as a part of cladding system. An example of this can be found in tests commissioned by MHCLG (DCLG at the time), on similar systems with different external claddings [7-13]. The test was performed on three systems identical in every aspect except for the external cladding where they varied from ACM PE core to ACM FR PE core and an ACM A2 core. The results of these tests indicated that the system with ACM PE core failed after about six minutes due to spread of the flame to the top, ACM FR PE failed due to record the temperature over 600oC while it demonstrated a considerably improved performance compared to ACM PE core and finally ACM A2 core which passed the test. The results of these tests can be seen as an indication of the importance of external cladding material in terms of spread of flame when exposed to fire, see Figure 3.
The other key parameter which could have a considerable impact on the performance of the cladding is the detailing of the system. As an example of this scenario, we can refer to a test which was built to be the same as the arrangement with ACM FR PE core. However, in this test the gap around the individual cladding panels was reduced from 20 mm to 10mm and cavity barriers were moved to be further away from the gaps between the panels. Comparing the recorded data, this showed that the modified test arrangement managed to pass the set criteria and spread of flame was limited and in the acceptable range, Figure 4.
The final factor which can have a considerable impact on spread of the flame on a cladding is the cavity width. Based on our experience, we observed various systems which had similar arrangements except their cavity size. In those cases, due to the air flow inside the cavity, the flame found the chance to spread through the cavity so that the temperature criteria were exceeded, leading to the failure of the test.
These are some of the main parameters which influence the performance of cladding as a part of a BS8414, however there are other factors which could change the dynamic of the fire in a façade system e.g. penetrations, installation details etc. However, based on our experience the above mentioned parameters had the main impact on the test results.
In conclusion, it can be said that the façade performance is a major issue which should be considered with the utmost attention to avoid further tragic incidents. To do so, responsible persons should consider not just the fire performance of individual products used as a part of a cladding system but also the detailing of the construction of the cladding and the effect this may have on the overall performance of the system that they are proposing to use. The best way to be sure of performance is to test the entire system (materials, fixings, cavity size, cavity barriers etc.) to an acceptable standard, which in this case is BS8414.
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- BSI. (2002) “BS 8414-1:2002 Fire performance of external cladding systems. Test methods for non-loadbearing external cladding systems applied to the face of a building”. UK.: British Standards Institute; 2002.
- BSI. (2015) “BS 8414-1:2015+A1:2017 Fire performance of external cladding systems. Test methods for non-loadbearing external cladding systems applied to the face of a building”. UK.: British Standards Institute; 2015.
- BSI. (2015) “BS 8414-2:2015+A1:2017 Fire performance of external cladding systems Test method for non-loadbearing external cladding systems fixed to and supported by a structural steel frame”. UK.: British Standards Institute; 2015.
- DIN 4102-20:2017-10 “Fire performance of construction materials and building elements; part 20: Special determination of the fire performance of external wall cladding systems”,2017.
- BS EN 13501-1:2018 “Classification of building products; part 1: classification of building products with the results of tests to their reaction to fire”,2019.
- Colwell S. and Baker T. “BR135”: Fire performance of external Thermal Insulation for Walls of Multi-storey Buildings 3 (IHS/BRE), BRE report, 2013.
- BRE 2017 Fire test report DCLG BS 8414 test no.1. 7 August.
- BRE 2017 Fire test report DCLG BS 8414 test no.2. 3 August.
- BRE 2017 Fire test report DCLG BS 8414 test no.3. 8 August.
- BRE 2017 Fire test report DCLG BS 8414 test no.4. 11 August
- BRE 2017 Fire test report DCLG BS 8414 test no.5. 14 August.
- BRE 2017 Fire test report DCLG BS 8414 test no.6. 25 August.
- BRE 2017 Fire test report DCLG BS 8414 test no.7. 21 August.
Dr. Mostafa Jafarian
Dr. Janet Murrell