Why fire performance is about the system, not the material

There is a persistent assumption by some in construction that a construction product’s individual fire classification will determine how safe a building will be. It does not, and new independent testing on flat roofs beneath rooftop PV systems shows exactly why, writes Simon Storer, Chief Executive of Insulation Manufacturers Association.

Testing the system

IMA commissioned the Fire Protection Association to carry out two comparative fire tests on flat roof assemblies fitted with identical reinforced bituminous membranes and mounted PV arrays. One assembly used PIR insulation, classified as combustible under EN 13501-1. The other used stone wool, classified as non-combustible. Everything else in the build-up, including the roof deck, membrane and PV configuration, was kept identical, so any difference in fire behaviour could be attributed to the insulation layer.

A propane burner was used to simulate a localised fire starting beneath one PV panel. The fire was allowed to develop freely for almost 22 minutes before extinguishing.

What the results actually showed

The results were clear. In both tests, fire performance across the roof was broadly similar. Neither insulation allowed fire to penetrate through its full depth, and the air and vapour control layer stayed intact underneath both. If anything, the damaged surface area was smaller on the PIR assembly than on the stone wool one.

That result runs contrary to the instinct that a non-combustible product must always outperform a combustible one. It is a difficult message to land, because it sounds counterintuitive to argue that a combustible material can be just as fire safe as one that is not. But that instinct misunderstands what fire classification actually measures. A product's performance in isolation tells you very little about how it behaves once fitted within a complete system alongside a membrane, a deck and everything above it. We accept this readily enough with timber, which burns, yet remains a trusted structural material because of how it is designed and detailed within a building. The same logic applies to insulation beneath PV.

This is not an argument for complacency but empirical evidence must outweigh simple opinion. Fire safety in any roof build-up depends on getting the whole system right, from the deck up to the waterproofing and everything mounted above it. But it is an argument against decisions that fixate on one material's classification while ignoring how that material actually performs once installed.

Policy built on assumption

This matters because policy is increasingly being shaped by exactly the assumption this testing challenges and can lead to poorer performing buildings overall. Proposals such as those within the Draft London Plan¹, which sets out a presumption against combustible materials in certain buildings, respond to public concern that is understandable but not always well founded. A building has to perform in many different ways, of which fire safety is only one category, and system-level testing is a more reliable guide than material classification alone.

Rooftop PV is only going to become more common on flat roofs as decarbonisation targets bite. That makes evidence like this more valuable, not less. Architects, specifiers, insurers and regulators all need a system-level understanding of fire risk, rather than relying on assumptions about individual materials.

To download a summary of the testing, which includes links to the full report, click here

For more information about IMA visit www.insulationmanufacturers.org.uk

¹https://www.london.gov.uk/programmes-strategies/planning/london-plan/next-london-plan/draft-london-plan-consultation