Fire Performance Classes: Differences Between A and B, A1 and A2, B1 and B2

The fire performance classification for building materials is a highly specialized and critical safety regulation. China's current classification is primarily based on the national standard ​​GB 8624-2012 "Classification for the burning behaviour of building materials and products"​​, developed with reference to the European standard EN 13501-1. The classifications are clearly defined. Below are the key differences between Classes A and B, and their sub-classes A1, A2, B1, and B2.

​​Core Distinction: Fire Performance Classes​

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1. ​​Class A (Non-combustible Materials):​​

   • ​​Definition:​​ Materials that do not burn or contribute significantly to a fire. They will not ignite or sustain combustion when exposed to flames.
   • ​​Fire Behaviour:​​ Do not produce sustained flaming combustion, even when directly exposed to a fire source.
   • ​​Representative Materials:​​ Concrete, masonry, metal, gypsum board (types without significant organic content), glass, rockwool, glasswool, specific inorganic foam insulation boards.
   • ​​Highest Safety Level:​​ The primary requirement for areas demanding the highest fire resistance, such as load-bearing walls, firewalls, escape stairwells, and elevator shaft walls.
   • ​​Sub-classes:​​ Class A is further divided into the higher standards ​​A1​​ and ​​A2​​ (see below).

2. ​​Class B (Flame-retardant Materials):​​

   • ​​Definition:​​ Materials possessing good flame-retardant properties. They can ignite when exposed to an open flame or high temperatures, but exhibit difficulty in sustaining combustion independently after the ignition source is removed.
   • ​​Fire Behaviour:​​ Surface flame spread is limited after ignition. Combustion produces smoke and heat. Materials classified as B1 and above typically exhibit "self-extinguishing" characteristics.
   • ​​Representative Materials:​​ Flame-retardant-treated wood, specifically formulated fire-retardant coatings (applied to potentially combustible substrates), some fire door cores, high-performance organic insulation boards (rigorously flame-retardant treated), specific flame-retardant plastic products, fire-rated gypsum board (containing minimal organic additives).
   • ​​High Safety Level (Secondary to A):​​ Widely used in internal partitions, ceilings, floors, furniture, and decorative materials where fire safety is required but not critical (must comply with specific design regulations).
   • ​​Sub-classes:​​ Class B is further divided into ​​B1 (Flame-retardant)​​ and ​​B2 (Combustible)​​ (see below).

​​Sub-class Differentiation within Class A: A1 vs. A2​​

• ​​A1 (Highest Level of Non-combustibility):​​

  • ​​Fire Behaviour:​​ Do not burn at all. Exhibit no combustion even in a furnace at high temperatures, and do not release sufficient heat to sustain combustion (extremely low total heat release).
  • ​​Strictness:​​ Very demanding test conditions, including stringent limits on furnace temperature rise (average and maximum peak) and mass loss rate.
  • ​​Application:​​ Used where the highest fire resistance is mandated, e.g., elevator shafts, structural cores, firewalls (compartment walls), core sections of escape routes in large public buildings.
  • ​​Representative Materials:​​ Pure inorganic materials like cement, quartz, most rocks, steel, glass. High-quality rockwool and mineral wool boards often achieve A1.
  • ​​Key Distinction:​​ Under no standard test methods should A1 materials exhibit any signs of combustion.

• ​​A2 (Non-combustible Materials, Slightly Less Stringent than A1):​​

  • ​​Fire Behaviour:​​ Essentially non-combustible. Under specific severe test conditions (e.g., direct burner impingement), they may exhibit very brief flaming or smoke emission but extinguish quickly. The material itself does not sustain combustion. The total heat release remains very low.
  • ​​Strictness:​​ High testing requirements remain, but allow slightly higher tolerance for furnace temperature rise and mass loss compared to A1 (while staying within the non-combustible category).
  • ​​Application:​​ Also permissible for applications requiring Class A materials. Materials like gypsum boards with minimal organic binders or high-quality glasswool may be classified as A2.
  • ​​Key Distinction:​​ A2 materials may exhibit minimal signs of combustion (very brief, faint) in certain tests, whereas A1 materials must not exhibit combustion at all in any standard test. The net calorific value of A2 materials is slightly higher than A1 but remains very low (typically capped at ≤ 3 MJ/kg).

​​Sub-class Differentiation within Class B: B1 vs. B2​​

• ​​B1 (Flame-retardant):​​

  • ​​Fire Behaviour:​​ Can ignite upon exposure to an open flame. However, after removal of the ignition source, flames extinguish relatively quickly ("self-extinguishing" under standard test conditions). Burning is very slow. In flame spread tests (e.g., room corner tests), fire growth and lateral spread are effectively limited. Smoke density and toxicity production are controlled according to standard requirements.
  • ​​Core Characteristic:​​ Demonstrated "self-extinguishing" property under standardized test conditions. The material can burn but has strong inherent self-inhibition.
  • ​​Application:​​ Widely used within buildings for non-load-bearing/non-fire compartment walls, ceilings, insulation layers (where height regulations permit), furniture upholstery (flame-retardant fabrics), some interior decoration materials (complying with codes), flame-retardant cable sheathing, and plastic components in high-risk areas.
  • ​​Representative Materials:​​ High-quality flame-retardant-treated timber (e.g., fire-retardant plywood), phenolic foam insulation boards (high-grade), specific fire-retardant PIR/PUR insulation boards (must pass testing), effectively flame-retarded plastics, certified fire-retardant wallpapers/carpets (installed per regulations).

• ​​B2 (Combustible):​​

  • ​​Fire Behaviour:​​ Will ignite upon exposure to an open flame and may continue to burn for a short period after the ignition source is removed. Burning speed is relatively slow compared to untreated materials. Provides some flame-retardant effect (delays combustion) but is fundamentally combustible.
  • ​​Core Characteristic:​​ Capable of continued burning for a limited time after ignition source removal, but does not lead to extremely rapid, uncontrolled fire spread. Flame-retardant properties are lower than B1.
  • ​​Application:​​ Suitable for locations with lower fire safety requirements, or as cladding/decorative layers over B1-rated materials (subject to the overall fire performance testing of the assembly). Minimum requirements for wall/floor finishes in public areas often mandate B1, but some areas may allow B2 (consult specific codes carefully). More common in residential applications.
  • ​​Representative Materials:​​ Untreated timber, standard plastics (non-flame-retardant PVC pipes/trunking), standard polyethylene (PE) foam boards (Note: eps foam insulation faces heavy restrictions in buildings; PU foam typically needs B1). Standard textiles and paper.
  • ​​Key Distinction:​​ B2 materials can sustain limited burning after the ignition source is removed, whereas B1 materials self-extinguish quickly. B2 offers less fire resistance than B1.

​​Summary Table:​​

​​Important Notes:​​

1. ​​Performance Hierarchy:​​ A1 > A2 > B1 > B2. Fire safety demands decrease down the scale.
2. ​​Application Standards:​​ The usage of different classes must comply with China's mandatory standards like the ​​Code for Fire Protection Design of Buildings (GB 50016, etc.)​​, dictated by the specific building element and location. For example, firewalls must be Class A; ceiling finishes in high-rise building escape corridors must be Class A or B1.
3. ​​Certification:​​ Material selection requires verified test reports conforming to GB 8624-2012, explicitly stating the fire performance class (e.g., A1, A2, B1, B2). Terms like "fireproof board" or "flame-retardant material" are insufficient without the official classification.
4. ​​Assembly Performance:​​ The fire resistance of building elements (e.g., walls, doors, floors) must be verified through tests on the complete assembly (e.g., fire resistance rating tests per GB/T 9978). While the individual material's reaction to fire class is fundamental, the element's design and installation are equally critical.
5. ​​Smoke & Toxicity:​​ Modern classification standards (like GB 8624-2012) include additional sub-classes for smoke generation (s1/s2/s3) and flaming droplets/particles (d0/d1/d2). Material selection should consider these alongside the primary A/B class.

Understanding these fire performance classes is crucial for building safety in a fire event and forms the foundation for architectural design and material specification. It is imperative to select the appropriate material class strictly according to the specific regulations and design codes applicable to the project location.