Insulation & Cladding

Two families of material sit at the centre of the combustible-cladding problem: the cladding panel and the insulation behind it. Understand these and you understand most of the danger.

Many cladding panels are composites - the common type is an aluminium composite material (ACM, or ACP): two thin aluminium skins bonded to a core, making a light, flat, easily-formed panel. The aluminium faces are not the issue; the core is everything. A polyethylene (PE) core is, in effect, a layer of solid hydrocarbon - a plastic very close to candle wax in its chemistry - sandwiched in the panel. Heat it and it melts, flows, and burns fiercely, releasing a great deal of energy. Cores described as fire-retardant (`FR`) contain less polymer and more mineral filler and burn less readily; mineral-filled or limited-combustibility (`A2`) cores are designed to contribute little. The same-looking silver panel can therefore range from essentially fuel to essentially non-combustible depending on a core you cannot see from outside.

The insulation behind the panel tells a parallel story. Plastic foam insulations - polyisocyanurate (`PIR`), polyurethane (`PUR`) and expanded/extruded polystyrene (`EPS`/`XPS`) - are excellent insulators but are organic and combustible, and some release large volumes of dense, toxic smoke when they burn. Mineral wool (stone or glass wool) is non-combustible and is the conservative choice where fire performance governs. The gravest situation combines a combustible panel, combustible insulation and a ventilated cavity: fuel on the outside, fuel on the inside, and a chimney between them. It is precisely this combination, in various forms, that has produced rapid, catastrophic facade fires. The lesson of the materials is blunt: on the external wall, what a product is made of - the hidden core, the foam behind - matters far more than how it looks.

It is worth stating what happened at Grenfell Tower soberly, because the facts carry the lesson better than any rhetoric. In the early hours of 14 June 2017, a fire started in a kitchen on a lower floor of the 24-storey residential tower in west London. On its own, that was an ordinary flat fire of a kind the building's design could have contained. But the fire broke out of the kitchen window and reached the building's recently-installed external cladding system - aluminium composite panels with a polyethylene core, over combustible insulation, with a ventilated cavity. The cladding ignited, and fire then spread up the outside of the tower with extraordinary speed, and subsequently around and back down other faces, involving the full height of the building in a matter of minutes to hours rather than being held to its floor of origin. Seventy-two people died.

The public inquiry that followed examined the causes in depth, and several lessons are now settled enough to teach as principles. The combustible cladding system was the principal reason the fire spread as it did - the external envelope became the fuel and the route. The building operated a 'stay-put' strategy - residents were advised, as is common for compartmented apartment buildings, to remain in their flats unless directly affected, on the assumption that compartmentation would keep fire from spreading. That assumption was overwhelmed, because the fire was not travelling through the compartments at all; it was travelling outside them, on the facade, re-entering flat after flat through the windows. Means of escape, smoke control and the limits of firefighting a facade fire all came under scrutiny too. (The stay-put question itself is taken up in Module 8.)

The single most important thing for a designer to carry from this is a principle, not a rule: a combustible external envelope can defeat an otherwise reasonable internal fire strategy. All the internal compartmentation in the world does not help if the fire goes up the outside. That is why the materials on the external wall are a life-safety decision of the first order.

A hard and uncomfortable lesson of the cladding crisis is that being widely used, marketed, or even permitted is not proof that a material is safe. The combustible panels and insulations involved in facade fires around the world were not obscure or black-market; they were mainstream products, sold with test certificates, specified by professionals, and signed off in their thousands. Some were tested in ways that did not reflect how they would behave in a real facade fire, or had individual-component data that said nothing about the whole system, or were covered by 'desktop study' assessments that extended a test result to a build-up that had never actually been tested. The paperwork existed; the safety did not.

The technically sound way to judge a cladding system is to assess it as a whole system, not component by component. That means either demonstrating that the complete build-up passes a large-scale facade fire test (such as BS 8414, judged against defined performance criteria), or using construction that is non-combustible (or of limited combustibility) throughout so the question of how the system burns largely does not arise. A stack of certificates for individual products - the panel here, the insulation there, the barrier somewhere else - does not add up to a safe system, because fire finds the interactions between them.

Regulators worldwide responded to the disasters by tightening toward non-combustible external walls on tall and high-risk buildings, restricting or banning combustible materials in those walls and closing the loopholes that let unsuitable products through. The exact thresholds - which building heights, which uses, which materials - vary by jurisdiction and are changing, so they must be checked against the current code and authority for your project. But the professional stance is clear and does not depend on the thresholds: approach cladding and insulation with scepticism, demand system-level evidence rather than component claims, and where life-safety margins are thin, choose non-combustible and be glad of it.

What, concretely, does a responsible designer do with all this? The principles are now widely shared, even as the exact code figures differ. On tall and high-risk buildings, favour non-combustible or limited-combustibility external wall construction - this is the single most robust protection, because a skin that cannot burn cannot become the fuel that Grenfell showed us. Limit or avoid combustible insulation in the external wall, preferring mineral wool where fire performance governs. Ensure cavity barriers are specified and installed correctly, as the previous lesson set out, so that any cavity cannot act as a hidden flue. And hold on to the idea that the envelope has a containment role - it is part of the building's fire strategy, not a decorative afterthought bolted to the structure.

But this lesson must end where honesty requires. External fire spread and cladding safety exceed simple architectural judgement. The architect leads the strategy - sets the intent to use a safe, ideally non-combustible envelope, coordinates the many trades, and insists on system-level evidence - but the *verification* that a specific build-up, at a specific height, with specific materials and junctions, will perform is the work of a facade engineer and a fire engineer, tested or demonstrably non-combustible, and approved by the authority having jurisdiction. Knowing to bring that expertise in early, and to treat cladding as a matter to get right rather than to value-engineer down, is the competence that matters.

Finally, a word on existing buildings. The cladding crisis is not only about what we design new; it is a vast, ongoing remediation problem, as thousands of existing buildings were found to carry suspect cladding after Grenfell. If you work on an existing building with cladding of uncertain performance, that is not a DIY assessment: it calls for survey, system-level investigation and assessment by specialists, and remediation planned with the fire engineer and the authority. The duty is to find out, honestly, and to act - not to assume that because a building has stood, its skin is safe.