A1 vs A2 vs B Fire Rating: Which Euroclass Should You Specify?

Why Compare A1, A2 and B (Not Just “A1 Is Best”)

A1, A2 and B sit next to one another on the Euroclass scale, yet they answer different specification questions. This guide is a practical A1 vs A2 vs B fire rating comparison for insulation specifiers. A1 represents the non-combustible family. A2 covers products with limited combustibility. B is a combustible class with limited fire contribution. Treating A1 A2 B as a single quality ladder — and assuming the highest class is always the right answer — leads to over-specified, expensive systems in some projects and under-specified systems in others.

The real decision is not which letter wins. It is whether the material, the complete system and the building regulation route can all clear the same fire-safety bar. This guide compares A1, A2 and B at that decision level: what each class means, how the classification routes differ, where each is normally required, what it costs in thickness and thermal design, and how to choose without confusing product reaction with system approval.

Use this guidance for initial selection only. Reaction-to-fire classification, national regulations and system approvals change. This article does not replace the current UK Building Regulations, Approved Documents, applicable EN/CEN standards, EU national rules, fire-engineered design, or advice from building control and a competent fire-safety professional.

The Test Stack Behind Each Class

Euroclass A1, A2 and B are assigned through EN 13501-1, which organises reaction-to-fire tests rather than measuring one single property. The most useful way to compare the three classes is by the classification route — what a product must demonstrate — which is why the A1 A2 B comparison below is framed as a decision table rather than a list of fixed thresholds. For the full Euroclass system including the A1–F scale and the s1–s3/d0–d2 subscripts, see the Euroclass fire classification guide.

For A1, classification normally relies on non-combustibility and gross heat of combustion. EN ISO 1182 assesses non-combustibility, while EN ISO 1716 measures gross calorific value (PCS). SBI, from EN 13823, is not the defining A1 route.

For A2, the route is often described as a combination of near-non-combustible or calorific-value evidence together with SBI performance. Public technical summaries commonly associate A2 with a PCS boundary around 3.0 MJ/kg and SBI values such as FIGRA ≤120 W/s, but these figures must be confirmed against the version-specific classification rules and the product’s Declaration of Performance (DoP). The same caution applies to A1’s 2.0 MJ/kg boundary: it is a useful industry reference, not a substitute for the applicable standard and product construction.

Class B typically follows the SBI route and includes EN ISO 11925-2 small-flame considerations. B is therefore not defined by having a uniquely different FIGRA value from A2. In many published classification structures, A2 and B can occupy the same SBI performance band. Their distinction lies in whether the product meets the near-non-combustible or limited-combustibility path — not in a simple “A2 has SBI, B does not” rule.

Do not write a laboratory report when the project needs a specification decision. Precise thresholds depend on standard version, homogeneous versus non-homogeneous construction, whole-product versus minor-component limits, and the declared product form. If a number matters to a compliance claim, take it from the DoP and the applicable standard, not from a generic comparison table.

A1 vs A2: The Combustion-Heat Line

The practical boundary between A1 and A2 is often discussed as a combustion-heat line: A1-like performance at very low calorific value, with A2 permitting a more limited combustible contribution. This is a reasonable shorthand, but it should never be presented as a single universal number that automatically decides every product’s class.

A1’s strength is regulatory certainty. Inorganic fibers, mineral-based boards, ceramic-type materials and calcium silicate products can sit in the A1 candidate family. That does not mean every inorganic material is automatically A1. Binders, facings, coatings, composite cores, thickness, orientation and test construction can all affect classification. The non-combustible insulation systems guide covers the A1 route in more detail.

A class A2 fire rating is therefore not a weaker A1; it is a separate classification route. A2 exists because real building products are rarely perfectly homogeneous and entirely free of organic components. Thin composite panels, faced mineral wool systems, boards with higher binder content and certain clad constructions may achieve a limited-combustibility position where a pure A1 route is harder to demonstrate. A2 should be described as near-non-combustible or limited-combustibility performance with additional classification conditions — not as “A1 on a budget.”

The correct writing position is conditional:

Where the applicable EN 13501-1 classification rules require gross heat of combustion limits, the relevant PCS values must be taken from the declared product construction and the standard version stated on the DoP.

That phrasing protects both accuracy and the reader. It also prevents a common error: converting an A1 vs A2 comparison into a marketing absolute about “≤2.0 MJ/kg for A1, ≤3.0 MJ/kg for A2” when the actual rules can distinguish between the whole product and internal or external minor components.

A2 vs B: Combustible, but How Limited?

A2 and B are sometimes presented as if the only difference is a more relaxed SBI score. The research does not support that simplification. Both classes may use SBI-based evidence, and both can appear with the same FIGRA-style performance under typical classification summaries. The more useful distinction is how far the product moves away from the non-combustible or limited-combustibility boundary.

• A2 is a limited-combustibility class. It is attractive where the specification requires a strong reaction-to-fire position but the product cannot, or need not, be classified as fully non-combustible.
• B is a combustible class with limited fire contribution. Class B insulation — often organic foam insulation — targets B or lower classes because it can deliver strong thermal efficiency, but its use is application- and specification-dependent.
• Similar SBI does not mean A2 and B are interchangeable. Jurisdiction, building element, complete-system approval and the precise Euroclass (including s and d subscripts) can make one suitable and the other unacceptable.

A2 does not automatically solve a high-rise façade problem. In the UK, external wall safety is assessed at system level: insulation core, cladding or render, cavities, fire barriers, fixings, junctions, window reveals and the relevant large-scale test or engineered assessment. A material labelled A2-s1,d0 may be essential to a compliant design, but the label alone does not “approve” the wall.

B is not automatically unsafe either. B-s1,d0 products may be appropriate for many applications where the complete specification and local rules permit them. The error is to select B for a high-risk, exposed or regulated element without checking whether the jurisdiction requires A1, A2 or a tested complete system.

Smoke and Droplet Classes in the A1/A2/B Context

A Euroclass is more than its first letter. The full notation can include s for smoke production and d for flaming droplets or particles:

• s1 indicates the best smoke-performance category in the standard’s scale; higher numbers indicate more smoke.
• d0 indicates no, or very limited, flaming droplets; d1 and d2 indicate progressively greater droplet behaviour.
• A full class might read B-s1,d0, not simply “Class B.”

A1 products are non-combustible in the classification context, so smoke and droplet sub-levels are not the same decision as they are for combustible classes. For A2 and class B insulation, however, ignoring the subscripts is a material-selection mistake. Material-level fire behaviour for a common insulation family is covered separately in is glass wool fireproof. A2-s1,d0 and B-s2,d1 are not equivalent just because their main letters differ by one step; the smoke and droplet behaviour may materially change acceptability in internal linings, escape routes and sensitive enclosures.

Rule of thumb: choose the complete Euroclass first, then check whether the system, location and regulation require a particular s/d pair. A project that accepts “A2” in theory may still reject a product with the wrong smoke or droplet rating in practice.

When Each Class Is Required

The following table is a decision framework, not a promise that a class letter automatically grants permission. Height thresholds, building types and regulation versions vary by jurisdiction and project date. In England, for example, relevant amendments to the building regulations introduced an 18-metre reference for combustible materials in certain external walls of relevant buildings; the exact obligation still depends on the building category, construction or major-refurbishment timing, and the current regulatory wording.

Height needs a warning label. The well-known 18-metre threshold in England is specific to certain buildings and regulatory conditions. An 11-metre figure appears in some remediation, funding and risk-assessment discussions, but should not be treated as a direct replacement for the 18-metre construction rule. A 30-metre threshold should not be stated as a general UK legal rule unless original legislation or local building control provides project-specific support.

For every high-rise or external-wall application, ask:

1. Which regulation and version applies to this project?
2. Does the rule apply to the whole system or the insulation core?
3. What does the current DoP declare for this exact product and thickness?
4. Is there large-scale system evidence or a recognised assessment route?
5. Has a competent fire-safety professional reviewed the complete construction?

If the answer to any of these is unknown, stop short of declaring the product “high-rise approved.”

The Trade-off Reality: Cost, Thickness, Thermal and Design Freedom

If A1 is the most conservative fire answer, why not specify it everywhere? Because insulation selection is a multi-objective decision: fire risk × regulation × thermal performance × thickness × weight × installation × budget × design intent.

A1 mineral wool insulation may be thicker and heavier than class B insulation using an organic foam core for a comparable target U-value. That can increase structural considerations, fixings, labour and overall build depth. A2 composite or faced systems can reduce some of those constraints while retaining a limited-combustibility position. B-class organic foam can look highly efficient on paper because of its thermal properties, but its fire restrictions can eliminate it from exactly the façade or escape-route applications where thickness matters most.

The selector below turns this into an actionable sequence: establish the fire constraint first, then optimise thermal and cost within the allowed classes.

A1/A2/B Across Standards

No cross-standard mapping should be presented as an automatic legal equivalence. EN 13501-1, UK building regulations, BS 476, ASTM E84 and China’s GB 8624 address different test concepts, product forms and jurisdictions. Use the table below only as a starting orientation.

In the UK, Class 0 is a well-known performance description derived from BS 476-6 and BS 476-7, commonly associated with wall and ceiling linings. It is not a direct substitute for A1, nor does a Euroclass certificate automatically become Class 0. A valid comparison needs the test report, standard version and the applicable route for the intended building element.

In the US, ASTM E84 measures surface burning characteristics, commonly reported through a flame spread index and smoke-developed index. It is not a mechanism for translating FIGRA, PCS or an EN 13501-1 letter class into a US legal class without a tested, jurisdiction-specific comparison.

China’s GB 8624 classification is included here only for orientation. It should be described as a separate national system with its own version-specific mapping — not as a line-by-line equivalent of EN 13501-1.

Worked Decision Selector

Use this selector in order. Move to the next question only after the previous constraint is satisfied.

Example 1 — tall residential façade: A1 is usually the starting point because of regulatory and fire-strategy requirements. Even then, the full cladding system, cavity barriers and fixings must be assessed. A2 cannot be assumed simply because its letter is close to A1.

Example 2 — mid-rise ETICS or rainscreen: A2 may provide the optimum balance between fire position and design flexibility. Confirm the exact product, thickness, system route and building control requirements before specification.

Example 3 — low-rise internal element: B-s1,d0 may be compliant and thermally efficient. Check the lining rules, smoke/droplet class, occupancy and escape-route implications rather than assuming B is banned or universally acceptable.

The selector is intentionally conservative. Its purpose is to prevent a familiar specification error: choosing a product class before choosing the system and regulatory route.

Common Mistakes When Choosing the Class

“A1 is always mandatory.” No. A1 is mandatory where the jurisdiction, building type or fire strategy says so. Many buildings and internal applications can be designed with A2 or B where the rules allow. The mistake is not using A1 everywhere; it is failing to identify where A1 is actually required.

“Class 0 equals A1.” No. They arise from different test cultures and product contexts. Class 0 may represent strong UK lining performance, but it is not a one-to-one Euroclass A1 equivalent.

“A2 is the same as A1 on a façade.” No. The classes are not interchangeable, and neither class alone proves complete-wall compliance. The system matters as much as the insulation.

“B is unsafe.” No. B-s1,d0 is used in compliant applications every day. The correct question is whether B is permitted for the specific element, location, smoke/droplet condition and jurisdiction — not whether the letter “B” sounds acceptable in isolation.

“A1 insulation makes the building fireproof.” No. A1 describes reaction to fire under a defined classification. Fire resistance, fire spread, compartmentation, cavity design and installation quality remain separate considerations.

“A2 and B differ because B has a much higher FIGRA limit.” Unsupported. Classification summaries often place A2 and B in the same SBI band. Their difference is better understood through the limited-combustibility route and overall classification conditions.

Our Material Scope

Our insulation portfolio focuses on inorganic, high-temperature and mineral-based material families. The following are candidate routes for A1 or A2 performance, depending on the exact product, thickness, facing, binder system, test construction and DoP:

• Rock wool / mineral wool insulation — commonly used where non-combustible or limited-combustibility performance is required.
• Glass wool insulation — suitable for selected thermal and acoustic applications, with classification confirmed by product data.
• Ceramic fiber insulation — used in high-temperature industrial and thermal-processing contexts.
• Calcium silicate insulation — an inorganic board and structural-insulation family with strong fire-performance potential.

Some faced boards, composites and systems within these families may be classified A2 rather than A1. The correct class must always be taken from the current DoP for the exact product and construction, not inferred from the generic material name.

Honest boundary: organic-foam insulation in the B-class range, such as PIR- or phenolic-core products, is not part of our current insulation portfolio. Where a specification requires an A1 or A2 route, our mineral-fiber, ceramic-fiber and inorganic-board families can be assessed. Where a B-class organic-foam solution is preferred for thermal efficiency, the choice must be justified against the complete system and the applicable fire-safety design — not selected on thermal performance alone.

Explore our rock wool insulation, glass wool insulation, ceramic fiber insulation and calcium silicate insulation families to find product data and declarations matched to your specification.

Related Reading

• Euroclass Fire Classification for Insulation — the full A1–F system, smoke/droplet subscripts and risk context.
• Non-Combustible Insulation Systems: The A1 Fire Rating — what A1 means, how it is demonstrated and where it is required.
• Is Glass Wool Fireproof? — material-level behaviour and the limits of single-material fire claims.