What sits behind the panel still matters
Cladding fire performance is often discussed in terms of the outer surface. Reaction-to-fire classifications and panel materials tend to dominate both specification and debate. That focus is understandable, but it is only part of the picture.
Recent work by MacLeod, Butterworth and Law provides a more complete view. Their parallel panel experiments examine cladding and insulation in combination, allowing the contribution of each to overall heat release to be measured.
One finding stands out. Where combustible cladding is present, it remains the primary driver of fire size. However, the insulation behind the panel is not neutral. When combustible insulation is introduced, it contributes measurably to fire development, typically accounting for around 10–28% of the total heat release during growth.
That contribution is not dominant, but it is not negligible.
The distinction matters. It is easy to frame façade fire behaviour around the external surface, particularly where regulatory thresholds and test evidence are presented in those terms. In doing so, the internal layers can be treated as secondary. The experimental data suggests a more nuanced position.
Insulation influences the fire in two ways.
First, by participating directly as fuel when exposed. In the experiments, combustible insulation contributed to the overall heat release once the system was involved in fire, adding to the severity of the event.
Second, by influencing how the fire develops. The presence of insulation, even where non-combustible, altered peak heat release and total energy compared to a heat sink condition. This reflects the role of the assembly as a whole, rather than the behaviour of any single component.
There is a tendency in practice to rely on discrete product data as a proxy for system behaviour. That approach is efficient, but it can obscure interactions within the build-up. The façade is not a series of independent layers. It is a coupled system, with heat feedback, flame spread and material response occurring simultaneously.
The paper also reinforces a more general point. The presence of insulation has a greater effect on fire behaviour than the additional effect of making that insulation combustible. That is a subtle but important distinction. It shifts the conversation from a simple “combustible versus non-combustible” framing to one that recognises the role of configuration and interaction.
None of this undermines existing understanding. The dominant influence of combustible cladding is clear. What the study adds is resolution. It shows where the next-order effects sit, and how they combine.
For practitioners, the implication is straightforward. Product performance is only part of the story. The behaviour of the system, including what sits behind the panel, needs to be considered with equal care.
Fire safety is rarely defined by a single element. It is defined by how those elements behave together.
This commentary is offered as general professional reflection on published research. It is not intended to represent formal guidance or to assess any specific product or system.
Parallel panel experiments on cladding and insulation systems: overall behaviour and relative contributions to heat release, by Cameron E. MacLeod, Neal Butterworth and Angus Law, is part of a special issue entitled: FISJ_IAFSS 2026 published in Fire Safety Journal.