Fire classes

Reaction-to-fire class describes a product's reaction to fire — how much it itself contributes to a fire, how much smoke it produces, and whether it sheds flaming droplets. The classification system EN 13501-1 is the European harmonised standard and applies in all EU countries. In Denmark it is implemented as DS/EN 13501-1.

Before the EU's classification system, every country had its own. Germany used DIN 4102 with classes A1, A2 (non-combustible), B1 (hard to ignite), B2 (normally ignitable) and B3 (easily ignitable). The United Kingdom used BS 476 with Class 0, Class 1, etc. Denmark had its own "Class A", "Class B", "Class 1", "BS", "BD" and "F". This meant the same product had to be tested several times, translated between systems, and clients across borders could not compare offers.

With CE marking, one common language was introduced. The AMU compendium on DS 428 describes the introduction: "In connection with CE marking, new European terminology, test methods and a classification system in the fire field have been introduced. Appendix 4, 1 March 2002, to the building regulations BR 95 introduces new European reaction-to-fire classes to replace the national ones." [Passiv brandbeskyttelse af stålkonstruktioner, Flemming Rasmussen 2023, p. 8].

The principle is: one test, one language, one stamp. A German product with class "B-s1,d0" may be used exactly where a Danish architect requires "B-s1,d0" — without translation, without extra testing.

For you as an insulator, the classes are daily practice: you must know which class a product has, which class the client requires, and whether the two match. It is not up for discussion — it is either legal or illegal.

The classification has seven main classes, from the most fire-safe to the completely untested:

A1 — completely non-combustible

A1 is the strictest class. An A1 product contributes in no way to a fire, no matter how far the fire has progressed — not even in a fully developed fire. This means:

• No flame development
• No heat development (PCS ≤ 2.0 MJ/kg, total)
• No significant smoke production
• No flaming droplets

A1 cannot be combined with smoke or droplet classes — it makes no sense, since an A1 product by definition gives off neither smoke nor droplets [AMU compendium p. 5].

Test: A1 requires two tests:

• ISO 1182 (non-combustibility test) — the specimen is lowered into a furnace heated to 750 °C. Temperature rise in the furnace, duration of flaming and mass loss are recorded. Criteria: temperature rise max 50 °C in the furnace, duration of flaming max 20 seconds, mass loss max 50 % [Passiv brandbeskyttelse af stålkonstruktioner p. 3]
• ISO 1716 (heat of combustion, PCS) — determines the gross heat of combustion in a bomb calorimeter

Typical A1 products: Stone wool and glass wool without binder (or with < 1 % organic content), cellular glass (Foamglas), concrete, brick, plasterboard (plain), steel, aluminium, glass. Cellular glass is the most interesting insulation material in A1, because it is completely inorganic (soda-lime glass foamed with carbon — the carbon burns off during production).

Where should A1 be used? Escape routes, penetrations in fire compartments, external walls on high-rises > 22 m, fire-exposed duct constructions, and everywhere BR18 or a fire strategy explicitly requires it.

A2 — very limited contribution

A2 lies very close to A1 — the product contributes only marginally to a fire. The difference from A1 is that A2 products may contain a little organic material (typically binder in mineral wool or paper backing on plasterboard).

A2 must always be combined with a smoke class (s1, s2, s3) and a droplet class (d0, d1, d2). The Danish standard class is A2-s1,d0 — which you will find again and again in datasheets and requirement texts.

Test: A2 requires two of the three major tests:

• ISO 1716 (PCS) — total heat development ≤ 3.0 MJ/kg or equivalent criteria
• EN 13823 — SBI (Single Burning Item) — the central test for classes A2-D. Described in detail below.

In the SBI test, A2 must meet: FIGRA ≤ 120 W/s, LFS < edge of specimen, THR₆₀₀ₛ ≤ 7.5 MJ [UNCERTAIN: exact limit values may vary between revisions — check the latest EN 13501-1]

Typical A2 products: Mineral wool (stone or glass wool) with binder — the most common insulation material in Denmark. Plasterboard with paper backing. Calcium silicate boards (Promatect-L, Promaduct-S). Aluminium-foil-faced mineral wool (when the foil thickness and adhesive quantity are kept below threshold values).

Where should A2 be used? Almost everywhere that A1 is not directly required. A2-s1,d0 is the minimum for fire-insulation products for DS 428 ventilation ducts [AMU compendium p. 7].

B — very limited contribution

B is still a low contribution to a fire, but the product may have a more noticeable organic component. The class is typically required for flame-retardant foam-plastic products (PIR with flame retardant, certain phenolic foams) and for system solutions where a combustible surface treatment (e.g. PVC foil) has been tested together with a non-combustible insulation.

Test: B requires two tests:

• EN 13823 — SBI — as for A2, but with less strict requirements. FIGRA ≤ 120 W/s, LFS < edge of specimen, THR₆₀₀ₛ ≤ 7.5 MJ. Classes A2 and B have the same FIGRA/THR requirements in the SBI test; the difference is that A2 must also meet ISO 1716 (low total heat development) [UNCERTAIN: confirm against the current EN 13501-1 version]
• EN ISO 11925-2 — ignitability with small flame, 30 seconds of exposure

Typical B products: Flame-retardant PIR (polyisocyanurate) in certain makes achieves B-s1,d0 or B-s2,d0. Flame-retardant phenolic foam (Kingspan Kooltherm). A flame-retardant PVC-foil system on mineral wool can typically be classified as B-s2,d0 or B-s3,d0 depending on the test build-up [example: Isotop PVC system, classification report referenced in local material].

Where may B be used? In most contexts within technical installations — but not in escape routes without a specific assessment, and not as a surface material in high-risk rooms. The class is fully acceptable for pipe-insulation surfaces in technical rooms, roof spaces and shafts where a specific fire strategy does not prescribe A2.

C — limited contribution

C is the first level where the product noticeably contributes to a fire. The class is often found for plastic-based insulation products with standard flame retardant.

Test: As B — SBI + EN ISO 11925-2 — but with looser requirements on FIGRA and THR. The limit values rise gradually through the scale C → D → E.

Typical C products: Certain polyurethane foams (PUR) with flame retardant, certain elastomeric foam products (Armaflex AF flame-retardant), paper+canvas on pipes (system-tested).

D — acceptable contribution

D is the lowest level that BR18 generally recognises as "acceptable contribution to fire" for construction products. The class covers many common materials, including wood.

Test: SBI + EN ISO 11925-2, with considerably looser requirements.

Typical D products: Structural timber (spruce, pine), many standard PUR foams with flame retardant, polyester felt (acoustics), certain PE foams with flame retardant.

E — ignitable under standard flame

E covers materials that ignite under a standardised small flame. The material burns, but stays above a minimal limit.

Test: Only EN ISO 11925-2 — ignitability with small flame, 15 seconds of exposure. If the flame does not reach the top of the specimen (150 mm) within 20 seconds, the class is E.

Typical E products: Standard PUR foam without flame retardant, standard PVC (not flame-retardant), polystyrene (EPS, XPS) without flame retardant in some forms, ordinary polyethylene foam.

F — not classified / not tested

F simply means that the material has not been tested to EN 13501-1 — or that it has been tested and failed all tests including E. It is NOT the same as "highly flammable" — it is lack of documentation. An F product could in principle be non-combustible, but without testing you do not know.

Class F cannot be combined with smoke or droplet classes [AMU compendium p. 5].

In practice: An F product may only be used where BR18 sets no fire requirement, or where it is encapsulated in a building element that itself has sufficient fire resistance. An F insulation product between two plasterboard walls in an EI 60 construction is, for example, OK, because the plasterboard walls deliver the fire class — the insulation is only thermal insulation.