Extruded polystyrene (XPS) foam boards are among the most widely used thermal insulation materials, but as combustible foamed plastics they must pass strict fire rating testing before they can be used in buildings. This article explains how the fire performance of XPS boards is evaluated under the GB 8624, EN 13501-1 and ASTM E84 classification systems, covering sample requirements, key testing items, test methods, typical rating criteria and frequently asked questions.
Technical Overview
Extruded polystyrene (XPS) boards are rigid, closed-cell foamed plastics produced by continuously extruding polystyrene resin together with a blowing agent and fire-retardant additives. Thanks to their low thermal conductivity, high compressive strength and excellent resistance to water absorption, XPS boards are extensively used in roof insulation, wall systems, floor underlayment, cold-storage facilities and civil engineering projects.
However, polystyrene is an organic polymer and therefore inherently combustible. When exposed to fire, XPS softens, melts, shrinks away from the flame and can produce flaming molten droplets. Building codes in virtually every market therefore require XPS products to be tested and classified for their reaction-to-fire performance before use. In China, the governing classification standard is GB 8624; in Europe it is EN 13501-1; in North America the predominant method is the ASTM E84 (or UL 723) tunnel test. Accredited third-party laboratories perform these tests and issue classification reports that form the basis of code compliance, CE marking, tender submissions and export documentation.
Testing Samples
Standard Unfaced XPS Boards
Samples are taken directly from normal production and must be representative of the commercial product in terms of density, thickness, cell structure and fire-retardant loading. Each test method requires its own specimen geometry: the single burning item (SBI) test requires a corner assembly consisting of a long wing approximately 1500 mm high by 500 mm wide and a short wing approximately 1500 mm by 230 mm; the small-flame ignitability test uses strips of about 250 mm × 90 mm; oxygen index tests use small strips roughly 150 mm × 10 mm; and cone calorimeter tests use 100 mm × 100 mm plaques.
Faced, Laminated and Composite Samples
Boards supplied with factory facings, foil skins, mineral coatings or bonded to other layers must be tested in their end-use configuration, because the facing can significantly alter heat release, dripping and smoke behavior. Complete external thermal insulation composite systems (ETICS) built with XPS are assessed as full kits rather than as bare boards, so the system classification may differ from that of the foam alone.
Conditioning and Ageing
All specimens are conditioned to constant mass at 23 ± 2 °C and 50 ± 5 % relative humidity prior to testing, in accordance with standard atmosphere requirements such as ISO 291 or GB/T 2918. Because the fire performance of flame-retardant-treated foams can decline over time, some certification schemes additionally require artificial ageing (heat and humidity cycling) followed by retesting, so that both initial and aged fire behavior are documented. Sample grouping rules based on thickness, density and formulation determine how many product variants a single test series can cover.
Testing Items
A complete fire performance assessment of XPS boards typically includes the following items:
- Overall reaction-to-fire classification (for example B1 or B2 under GB 8624-2012, or B–E Euroclasses under EN 13501-1)
- Fire growth rate index (FIGRA) and total heat release within 600 seconds (THR600s) from the SBI test
- Lateral flame spread (LFS) over the specimen surface
- Smoke production rate index (SMOGRA) and total smoke production (TSP600s)
- Production of flaming droplets and particles
- Ignitability under small-flame impingement
- Limiting oxygen index (LOI)
- Smoke density rating (SDR) from smoke chamber testing
- Gross heat of combustion (PCS), where the classification scheme requires it
Testing Methods
Classification Under GB 8624 (China)
GB 8624-2012 classifies building materials into levels A(A1, A2), B1(B, C), B2(D, E) and B3(F). Combustible boards such as XPS are assessed using the single burning item test (GB/T 20284, equivalent to EN 13823) and the small-flame test (GB/T 8626, equivalent to EN ISO 11925-2), with supplementary grading for smoke (s1–s3) and flaming droplets (d0–d2). Typical acceptance criteria include:
- Level B: FIGRA ≤ 120 W/s, THR600s ≤ 15 MJ, no lateral flame spread reaching the specimen edge, and passing the 30 s small-flame test
- Level C: FIGRA ≤ 250 W/s, THR600s ≤ 15 MJ, flame spread not reaching the edge, and passing the 30 s small-flame test
- Level D: FIGRA ≤ 750 W/s and passing the 30 s small-flame test
- Level E: passing the 15 s small-flame test with the flame tip not reaching 150 mm within 20 s and no droplets igniting filter paper
The limiting oxygen index (GB/T 2406.2, aligned with ISO 4589-2) and smoke density (GB/T 8627) are frequently specified as supporting quality-control items. Many legacy specifications still reference oxygen index thresholds of at least 32 % for flame-retardant grade and 26 % for general grade, which originate from the superseded 1997 edition of the standard and remain common in procurement documents.
Classification Under EN 13501-1 (Europe)
For CE marking under EN 13164, XPS boards must achieve at least Euroclass E, while flame-retardant formulations commonly reach class E or class B/C depending on the system. The classification combines the SBI test (EN 13823) and the small-flame test (EN ISO 11925-2), with additional requirements such as gross heat of combustion (EN ISO 1716) applied to higher classes. The final designation carries suffixes for smoke production (s1, s2, s3) and flaming droplets (d0, d1, d2); for example, a typical designation for a flame-retardant board might be B-s2, d0.
ASTM E84 / UL 723 Tunnel Test (North America)
The 25-foot Steiner tunnel test measures flame spread index (FSI) and smoke developed index (SDI) over a 10-minute exposure. Materials are classified as Class I/A (FSI ≤ 25), Class II/B (FSI 26–75) or Class III/C (FSI 76–200), with smoke developed typically limited to 450. Building codes impose additional conditions on foam plastics, generally requiring testing at maximum thickness and installation behind a thermal barrier or approved protective assembly in most occupied spaces.
Supporting and Research Tests
- Oxygen index (ISO 4589-2 / GB/T 2406.2): a rapid screening method widely used for incoming-material control and formulation comparison
- Cone calorimeter (ISO 5660-1): provides heat release rate, total heat release, smoke yield and mass loss data for R&D and fire-hazard modeling
- Smoke density chamber (ASTM D2843 / GB/T 8627): quantifies specific optical density and smoke density rating for quality control
Applications
Fire rating test reports for XPS boards are used across the construction supply chain. Architects and fire engineers rely on the classification to select insulation that satisfies the combustibility requirements of building and fire codes for external walls, roof systems and public buildings. Contractors and developers submit the reports for project approval, tender qualification and site inspection. Manufacturers use them for product certification, CE marking, export documentation and ongoing factory production control. Typical end uses of classified XPS include inverted and conventional roof insulation, external wall and core insulation, ground floor and underfloor heating panels, cold-store panels, plaza and roadway insulation, and foundation or railway subgrade insulation, each of which may impose a different minimum fire class depending on the code and the building occupancy.
FAQ
What fire rating can XPS boards achieve?
With effective flame-retardant treatment, XPS boards commonly achieve B1 under GB 8624-2012 and Class B or E with limited smoke and droplets under EN 13501-1. Untreated or poorly compounded boards typically fall to B2/B3 or Euroclass E/F, and their use is restricted or prohibited in many applications.
Does an oxygen index of 26 % automatically mean the board is flame retardant?
Not by itself. Under the current GB 8624-2012 system, B1 and B2 classification is based on the SBI and small-flame tests, not on oxygen index alone. Oxygen index values of 26 % and 32 % are legacy thresholds from the 1997 edition that survive in many specifications as supplementary quality indicators, so both the LOI value and the current classification report should be checked.
How many samples are needed and how long does testing take?
SBI testing normally requires three complete corner assemblies plus spares, while the other methods need only small numbers of strips or plaques. After conditioning, a full GB 8624 or EN 13501-1 series typically takes two to four weeks at an accredited third-party laboratory, longer if artificial ageing is included.
Does one report cover all thicknesses and colors of a product range?
Only if the variants fall within the grouping rules for thickness, density and facing defined by the relevant standard and certification scheme. Thicker sections, different facings or changed formulations generally require separate testing or an extended assessment.
Why do some boards lose their fire performance over time?
Fire retardants can migrate or degrade under heat, humidity and UV exposure. Testing boards both in the initial state and after defined ageing protocols demonstrates whether the classification remains valid throughout the declared service life, which is increasingly required by certification bodies and large procurement projects.