Whilst the surface of the char keeps on pyrolising, the rate of emission of gases lessens, and the nature of these gases changes the more pyrolised the char becomes. The net effect is that the rate of production of combustible gasses, and the intensity of the flaming produced, reduces after the formation of the initial char layer.
When the blow torch flame is taken away, the energy from combustion of these gases alone is insufficient to support further pyrolysis, and the insulation self-extinguishes.
This shows that materials classed as “combustible” will not necessarily burn or combust in all eventualities. So, “combustible” does not automatically mean “flammable”. “Flammability” is scenario specific whereas “combustibility” is an intrinsic property based solely on a material’s calorific content.
A Full System Approach
In practice, what matters is not how each individual product is labelled or classified. For example, even when the insulation and cladding materials in an external façade system are classed as Euroclass A1” (“non-combustible ) or Euroclass A2 (of “limited combustibility” ), there will still be a surprising amount of combustible material within the overall construction, such as thermal breaks, sealants, vapour barriers and tapes. It is the interaction between the different components, their spatial arrangement, and how they are installed that will ultimately determine how a façade system behaves when exposed to fire.
Therefore, whilst the individual product tests used within the Euroclass system and the National Classes to determine reaction to fire performance can provide a useful baseline measure, they have severe limitations when assessing the performance of a complete system, such as a rainscreen façade. These limitations have been highlighted by recent testing which has shown that, even when both the cladding and the insulation are of ‘limited combustibility’, they can fail a large-scale test as part of a system
This is why it is so important to test the whole wall assembly to ensure that it will achieve the desired performance.
Put to the Test
Kingspan Insulation recently commissioned testing of constructions which matched those for the Government Building Safety programme. The tests were carried out to ISO 13785: 1
- an intermediate scale reaction to fire test for façade systems. As with BS 8414
(the large-scale test used within the Government programme), it is designed to simulate a fire beginning in a building, breaking out through the window and impinging on the façade.
Image 2 - ISO 13785: 1 test rig
The ISO 13785: 1 test comprises a 2.4 m tall wall with a corner. It is, essentially, one third of the size of the BS 8414 test with a lower fire load to compensate for the smaller scale. During the test, a gas burner at the base of the wall is ignited and allowed to burn at 100 kW for a period of 30 minutes or until the top of the specimen is extensively flaming.