Since fire barriers are a part of ventilated façade systems, it is very important to know what kind of testing method is being used to ensure that the building can be treated as safe in the event of fire [
8]. Fire test methods and the generated data must be assessed and evaluated to determine whether the specific test conditions are applicable for the specific fire scenario or the final use of the tested sample [
9]. For this paper, literature on the analysis of fire propagation and influence of barriers for three methods was reviewed: numerical analysis, medium-scale, and large-scale testing.
1.1.2 Medium-Scale Testing
Medium-scale testing results may be used for the development of products and indication of material behaviour with the advantage of not being as costly as large-scale tests. However, they are often incomparable with the large-scale testing, due to lower fire exposure and less embedded construction details for the use in the elements such as joints, barriers, etc. [
1].
The test conducted according to the standard ISO 13785-1 [
12] is the example of a medium-scale test. The effects of cavity barriers, aluminium composite panels as claddings, and several insulation materials were observed on nine samples. In the research by Guillaume et al., it is shown that cavity barriers were largely ineffectual in the three tests with the combustible ACM-PE cladding, but they showed good results when being combined with the non-combustible ACM-A2 cladding. They also state that mineral wool and phenolic foam insulation have shown similar behaviour when the fire retardant ACM-FR and the non-combustible ACM-A2 cladding is used. Their results for the ACM-PE based cladding compositions also confirmed the results obtained during the BS 8414-1 DCLG tests [
13‐
16].
However, authors believe that such comparison, in general, should be done carefully since many other parameters should be considered when testing the fire performance of facades. Although correlation is not evident, all scales can give important details. For the good rules of extension of application, there is a need to develop a decision scheme based on both scales to be able to accommodate all variations in facade systems. Performing intermediate-scale tests can be a good way to complete assessments performed at large-scale but it is not only about combining the individual test results into system that is more complex, it is about the reliability and the validation of the tests methods being applied in the first place. It is important to develop the skills to be able to properly assess the results.
1.1.3 Large-Scale Testing
Large-scale testing is often seen as the most representative way of showing the full performance of a building during fire as long as it is well designed and constructed. Numerous large-scale test methods exist in Europe and North America but we are still missing an harmonized test method for Europe [
17]. In the work reported by RISE, together with a group of laboratories [
18], authors explained how the repeatability and reproducibility of test methods, as well as this harmonisation, can be maintained. They suggest that a possible solution could be to define a heat exposure curve, as for fire resistance testing. They propose the usage of gas burners that can be regulated instead of using a defined amount of free burning fuel and advise to use plate thermometers to measure the heat exposure on the façade instead of conventional thermocouples. Advices proposed by RISE is very promising but still under evaluation.
As early as in 1986, Jeffs et al. conducted large-scale testing using the French standard from 1964 [
19]. Even then, the significance of fire barriers was noticed. Researchers stated that in tall buildings, it is necessary to prevent the fire spread by using adequately designed construction details. They also noticed that fire propagation along the facade, as well as its penetration into the neighbouring fire compartments (flats), could be prevented by using barriers in the shape of horizontal fasteners for ventilated facade cladding. Another research conducted by Kolaitis et al. [
20] investigated the main aerodynamic and thermal phenomena which influence the flow of hot gasses and flames through the cavities of ventilated facade systems. Fire barriers were not installed, which is the “worst-case scenario” for the system without flammable materials. The researchers also concluded that regulation was not good enough in defining the use and positioning of barriers essential for ventilated facades (because the flame moves unnoticed through the cavity). The real fire exposure brings in question the open-state barriers used in façade systems today, and this fire scenario is not adequately simulated by any of the large-scale tests. Currently the barriers can be examined using the North American method at a bench scale ASTM E2912-1.
Tamás Bánky and Hideki Yoshioka, authors from Hungary and Japan performed their tests using identical “aluminum composite panel” specimens provided by the same supplier and tested it with two national standards. Almost identical specimen with aluminum composite panel façade, ventilated layer and rock wool insulation installed, has passed the criteria of MSZ 14800-6 (large-scale Hungary standard) but failed the criteria of JIS A 1310: 2019 (intermediate-scale Japan standard).
Reviewing the available literature, it can be concluded that Europe should set up an adequate fire barrier test so that one test can be applied for both composites such as ETICS and ventilated façades (or elsewhere - CLT-based façades, curtain façades, etc.). Fire spread along the wall exterior surface is not part of the European rating system and remains subjected to national fire protection regulations. Numerous test methods covered by this scenario exist in Europe and North America at national level and they are compared in [
17]. The European Commission contracted a consortium led by RISE to develop a harmonised European test for façades with varied requirements based not only on fire spread, but also on falling parts, burning debris, smouldering fire, smoke and detailing (windows) [
21]. Two methods have been proposed, one simply keeping two existing tests (BS8414 and DIN 4102-20) and an alternative method proposing several changes that can address some of the weak points of the two methods. The outcome of the report was an tender for the EU Commission to continue the work on the alternative method and so is therefore relevant to discuss the weak points BS 8414. The setup and technical specifications of new European test standard together with the fire exposure in a new test method through numerical simulation was discussed during the 3rd International Symposium on Fire Safety of Facades—FSF2019 [
22]. However, one of the most important questions which arose were, what kind of safety should be obtained and is the one standard test and classification enough? It is not feasible to test all combinations of different products or system characteristics for their reaction to fire or fire resistance performances, but it is important to know that such characteristics can substantially influence a test result. The potential of façade being safe is highly dependent on which risk is to be assessed and one should be aware that tests in laboratory are made under perfect and controlled conditions where ‘’standardized’’ fire scenario is used. Detailing is difficult to evaluate, and more complex geometries are difficult to be assessed by using only one method.
Following the Grenfell fire public investigation [
2,
23], Lane [
24] noticed that the BS 8414 Parts 1 and 2 ought to include window openings and other relevant fixtures and fittings. Lane also states that a more robust testing framework, reflecting the real building design and construction detailing, would also assist in establishing whether materials of “limited combustibility” are suitable. According to the report from Torero [
23], such tests provide a single scenario consistent with an external fire, has very limited number of measurements and a simple failure criterion, which is not adapted to the complexity of a real fire scenario.
Based on the research by Lane and the tests carried out for this research, we assess that fire barriers can radically alter test performance, but the number and location are unspecified in the standard BS 8414. However, the use of fire barriers is implied by the requirement to report any barrier failure during the test. It is important to mention that it is not enough to describe its behaviour after testing because the failure of cavity barriers will appear in the manufacturer’s test report, but this information may not be conveyed to the fire risk assessor or end user. Another important assumption is that cavity barriers must be identical to that in the façade design used in real buildings and thus façade tests should also include openings such as windows or doors. Otherwise, composite design for the test is insufficiently detailed. Because of that, standard BS 8414 should include a failure criterion, such as “any fire penetration through cavity barriers incorporated within the cladding system, or around it through breaches in the external cladding panels.”
The aim of this research is to examine the preferred number and position of barriers in the design of ventilated façades and to assess the impact of combining the fire barriers with the combustible and non-combustible insulation on the fire spread. The tests were performed outdoors, according to the environmental conditions specified in the BS 8414-1 [
26], and the duration of the test was 60 min. Some of the samples consist of the same elements (insulation and cladding) as in the DCLG tests ordered by the UK government [
13‐
16], but DCLG tests were examined indoors and extinguished as soon as the BR135 criterion failed.
It must be noted that for this research, we focused on a large number of different tests and each system was tested only once due to the limited financial resources. The large-scale sample behaviour in the standard testing always differs from the real fire scenario, but it allows for the comparability of the repeated tests. Therefore, the results can serve as a basis for comparison for other researchers, which ultimately guides the update of the existing standards and thus the possibilities of implementation in the recommendations for the design and execution of such ventilated facade systems.