Beyond the Label
The question every technical audience should ask about combustible insulation claims.
A Technical Fire Safety Review — Modern Building Alliance
—
Polymer-based insulation materials have been an accepted insulation choice in building facades and flat roofs for many years.
Most people assume a material’s combustibility tells the whole story. It doesn’t. Fire performance is an emergent property of the complete building assembly (insulation, protective layers, fixings, fire barriers, and installation acting together) not a property of any single component considered on its own. The ultimate issue is the fire performance of the complete end-use system, not materials in isolation. That distinction is the one that most often goes unexamined.
This review examines seven of the most widely circulated claims about combustible insulation and separates the accurate core of each statement from the broader conclusion typically attached to it.
“Reaction to fire class” of a material is one input into fire safety — not the verdict on it.
—
Myth 1: Combustible materials result in unsafe buildings
Does the combustibility class of a single material determine whether a building is fire-safe?
Fact: Combustibility classes alone do not determine fire safety.
Under EN 13501-1, construction materials are classed from A1 to F, with each successive class permitting greater combustion and heat release.
Contrary to normal assumptions, A1 materials may not be non-combustible in a physical sense — they are allowed to sustain combustion, but that combustion has to stay below certain low thresholds. In real-world terms, A1/2 rock mineral fibre insulation can smoulder, which can, in certain circumstances, spread and produce very high temperatures (up to 500°C or more).
The fire performance of a wall, roof, or floor is not determined by any single material. It is a **property of the complete assembly**: the insulation, the protective layers around it, the fixings, the fire-stopping details, and the installation quality all interact. That is precisely why full-scale, system-level testing is required. A combustible insulation material within a tested, certified, well-detailed assembly is a different proposition from the same material left exposed.
Myth 2: Plastic foam burns
Does the combustibility of plastic foam insulation make its use inherently hazardous?
Fact: Plastic foam can burn — but the assembly in which it is installed may not.
It is correct and uncontested that plastic-based foam insulation will ignite and sustain combustion when directly exposed to a powerful enough heat source for a sufficient duration. However, insulation is rarely exposed in buildings. In external thermal insulation systems (ETICS), insulated roofing, and insulated cladding, insulating materials are typically enclosed by a protective coating, board, membrane, cladding layer, or fire-stopping that restricts fire propagation. Full-scale fire testing of insulated systems confirms this.
Research does not support a blanket conclusion that plastic insulation automatically causes uncontrolled fire propagation. It supports careful design: tested assemblies, robust surface protection, fire breaks and compartments, ignition-source control, and maintenance.
Myth 3: Non-combustible materials inherently confer fire resistance
Does a non-combustible insulation material inherently confer a fire-resistance rating on the element it’s installed in?
Fact: Non-combustible products may offer comparatively little fire resistance.
This conflates two separately regulated properties. **Reaction to fire** (EN 13501-1) categorises how a material behaves when directly exposed to heat and fire, using heat-release-based and other parameters. **Fire resistance** (EN 13501-2, tested under EN 1364, EN 1365, or ISO 834) is a time-based rating that applies only to a complete, tested **assembly** — never to a raw material in isolation. It measures how quickly a fire will burn through a complete construction. Because fire must burn through each successive layer of a construction, the overall fire resistance of an assembly is largely the sum of its parts.
It is possible for some A1/2 materials to offer very little to the fire resistance of an assembly. Conversely, it is also possible for some materials with Euroclass ratings of B–F to offer quite a lot.
Myth 4: Grenfell proves combustible insulation alone causes catastrophe
Did the Grenfell Tower fire establish that combustible insulation alone causes catastrophic fire to spread?
Fact: The predominant cause of the speed and extent of flame spread in the Grenfell Tower fire was the presence of PE-cored ACM cladding panels.
It is accurate that Grenfell Tower’s refurbished wall system comprised PE-cored ACM cladding panels outside of combustible insulation. However, the Inquiry described the predominant cause of the speed and extent of flame spread as the combustion of the PE core of the ACM panels — not the combustion of the insulation.
The UK’s Ministry of Housing, Communities and Local Government (MHCLG) testing and other research have shown that, had the insulation been non-combustible, the result would have been the same — the combustibility of the insulation was irrelevant. This is supported by the fire incident that occurred at a high-rise building in Valencia, Spain, on 22 February 2024.
Myth 5: Combustible insulation produces toxic smoke
Does non-combustible insulation produce less toxic smoke, making those constructions inherently safer?
Fact: Combustion toxicity is a legitimate concern; however, all fires produce toxic gases and particulates in the form of smoke.
Toxic gases are produced by all fires. The quantity and nature of the gases produced depend on several factors: the quantity of different materials involved, their configuration, the proximity of other combustibles, and the ventilation conditions. The concentration of gases also depends on the exposure time and the volume of the compartments in which combustion products accumulate.
In most cases, the bulk of any early-stage fire hazard comes from the combustion of room contents and decoration, not from the building fabric or any insulation contained within it.
Myth 6: Flame retardants make insulation toxic by default
Does the need for flame retardants make combustible plastic-based insulation inherently problematic?
Fact: Risk depends on the specific chemistry, not the mere presence of a flame retardant.
Most plastic-based insulation materials include flame retardants to meet specific fire classification requirements. Historically, this often meant using halogenated compounds. Some, such as HBCDD, have since been substantially restricted under the Stockholm Convention on Persistent Organic Pollutants and largely superseded by other chemistries, including polymeric and non-halogenated formulations.
The presence of a flame retardant does not, by itself, establish that a product is toxicologically problematic. The impact depends on the specific compound, its persistence, its bioaccumulation potential, and its current regulatory status. Tarring all flame retardants with the same brush ignores important differences — and the progress made in replacing legacy substances that are now largely restricted or discontinued.
Myth 7: Installation of PV panels on roofs results in a higher fire risk
Does combining photovoltaic panels with combustible insulation constitute a major, insulation-driven fire risk?
Fact: The additional risk is inherently due to the PV system, and the insulation can be irrelevant.
The addition of PV systems to flat roofs introduces new and complex fire safety risks. Incident data show most PV-related fires originate from electrical faults, DC-side connector failures, DC arc faults, and installation defects such as poor cabling, faulty mounting, or defective inverters — rather than from the insulation or substrate beneath the panels.
PV panels installed on flat roofs create a cavity between the panel and the roof, which supports heat feedback and causes heat build-up underneath. The fire performance of any roof depends on the type of roof build-up and the presence of a mitigation layer.
The available evidence suggests that flat roofs insulated with combustible insulation materials, in some cases protected by appropriate layers, can perform as well as, or better than, flat roofs insulated with some A1 insulation materials.
—
The takeaway
Across all seven claims, the pattern is the same: a true, narrow fact about a material gets stretched into a broader conclusion the evidence doesn’t support. Fire safety is decided at the level of the tested, certified, well-installed assembly — not by the Euroclass rating of any one component read in isolation.
Disclaimer: This content is provided for general informational purposes only and does not constitute legal, regulatory, or engineering advice. Readers should verify all claims against the applicable standards, project conditions, and competent professional judgment.*