Extruded polystyrene (XPS) insulation boards are the dominant thermal insulation material for cold storage construction, yet their organic polymer matrix makes combustion performance testing essential for fire safety compliance. This article explains the sample requirements, test items, methods, and classification criteria for XPS combustion performance testing under GB 8624-2012 and related international standards, helping manufacturers, cold chain project owners, and fire design engineers select and verify compliant flame-retardant insulation. All tests described can be performed by accredited third-party laboratories.
Technical Overview
Extruded polystyrene (XPS) foam board is a rigid, closed-cell insulation material produced by continuous extrusion of polystyrene resin blended with blowing agents and flame retardants. With a thermal conductivity of approximately 0.028–0.034 W/(m·K), low water vapor permeability, minimal long-term water absorption, and compressive strengths ranging from 150 kPa to over 500 kPa, XPS is widely specified for cold store floors, perimeter walls, roofs, and freezer slabs, where it must withstand heavy racking loads and chronic moisture exposure.
However, polystyrene is an organic hydrocarbon polymer. When exposed to an ignition source, unmodified XPS softens, melts, shrinks away from flame, produces flaming droplets, and releases dense black smoke containing carbon monoxide and styrene derivatives. Cold storage facilities present compounded fire risk: large uninterrupted areas of exposed or concealed insulation, high combustible loading from stored goods and plastic packaging, electrical defrost and refrigeration equipment, and restricted evacuation paths. Consequently, Chinese fire design codes, including GB 50016 (Code for Fire Protection Design of Buildings) and GB 50072 (Code for Design of Cold Store), require thermal insulation materials used in cold storage to achieve a combustion performance of Grade B1 or higher, verified through testing under GB 8624-2012.
Combustion performance testing of XPS boards systematically evaluates ignitability, flame spread, heat release rate, smoke production, and flaming droplet behavior. The resulting classification supports product certification, factory quality control, engineering acceptance, and export compliance.
Testing Samples
Scope of Sample Types
Samples typically cover the following categories submitted to accredited third-party laboratories:
- Flame-retardant XPS boards of declared B1 or B2 grade, with or without factory-applied facings such as polymer films, mineral coatings, or metal skins
- Boards across the representative thickness range (commonly 20–120 mm) and density range (commonly 25–45 kg/m³) for the product family
- New formulations after changes to resin grade, blowing agent, or halogenated or halogen-free flame retardant packages, requiring requalification
- Benchmark samples for comparative evaluation or failure analysis following site incidents
Specimen Preparation
Specimens are cut from boards representative of normal production, free of crushed edges and surface defects. For the single burning item test, thick or multilayer products are assembled to reproduce the end-use installation configuration, including any joints or facings. Mechanical fastening or freestanding mounting is used as appropriate, and each specimen is uniquely labeled with orientation, batch, and thickness records.
Conditioning Requirements
Prior to testing, all specimens are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity until mass equilibrium is reached, following the conditioning requirements of the applicable test standards. Conditioning periods commonly range from 48 to 96 hours depending on the method. Accurate conditioning is critical because moisture content measurably influences ignitability and smoke results for foamed plastics.
Typical Specimen Dimensions and Quantities
- Single burning item test (GB/T 20284): long wing 1000 mm × 1500 mm and short wing 495 mm × 1500 mm, normally three complete specimen sets
- Small-flame ignitability (GB/T 8626): 250 mm × 90 mm panels at end-use thickness up to 60 mm, tested in both surface and edge exposure orientations
- Oxygen index (GB/T 2406.2): specimens of 80–150 mm × 10 mm × 4 mm or at product thickness as permitted, typically 15 or more pieces to bracket the limiting concentration
- Cone calorimeter (ISO 5660-1): 100 mm × 100 mm specimens, at least three to six replicates
- Smoke density (GB/T 8627): 25 mm × 25 mm specimens cut to the required test thickness
Testing Items
A complete combustion performance evaluation of XPS boards for cold storage generally includes the following items:
- Limiting oxygen index (LOI), the minimum oxygen concentration required to sustain candle-like burning
- Small-flame ignitability under surface and edge exposure, including flaming droplet and particle behavior
- Single burning item parameters: fire growth rate index (FIGRA), total heat release within 600 seconds (THR600s), lateral flame spread (LFS), and smoke growth rate index (SMOGRA)
- Smoke density rating measured by light attenuation in a closed chamber
- Gross heat of combustion (PCS), where applicable for higher classification levels
- Smoke production toxicity grading under GB/T 20285, when specified for particular occupancy categories
- Final combustion performance classification under GB 8624-2012, reported with smoke (s) and droplet (d) subclasses, for example B1 (B-s1, d0)
Testing Methods
Oxygen Index Test (GB/T 2406.2, ISO 4589-2, ASTM D2863)
A small vertically mounted specimen is ignited at its top and burns downward in a glass chimney through which a controlled mixture of oxygen and nitrogen flows. The oxygen concentration is adjusted stepwise to determine the minimum level that just supports burning over a defined length or time. Flame-retardant XPS intended for Grade B1 application is commonly specified by manufacturers with an LOI of 30% or higher for internal quality control, although LOI alone does not constitute a GB 8624 classification.
Small-Flame Ignitability Test (GB/T 8626, EN ISO 11925-2)
A 15 mm or 20 mm flame from a small burner is applied to the specimen surface or bottom edge for 15 seconds (for Grade E evaluation) or 30 seconds (as required for Grades B and C). Filter paper beneath the specimen detects flaming droplets. For Grade E, the flame tip must not reach the 150 mm gauge mark within 20 seconds and no filter paper ignition is permitted; for Grades B and C, the corresponding criterion applies within 60 seconds. This test directly assesses the ignitability behavior most relevant to incipient fire scenarios in equipment rooms and wall penetrations.
Single Burning Item Test (GB/T 20284, EN 13823)
The specimen wings are mounted vertically in a corner configuration and exposed to a 30.7 kW propane gas burner simulating a burning wastepaper basket placed in a room corner. Heat release rate is calculated continuously by oxygen consumption calorimetry, and smoke production is measured by laser photometry. The test yields FIGRA, THR600s, LFS, and SMOGRA, which are the governing parameters for Grades B, C, and D classification. This is the core intermediate-scale test for B1 verification of XPS boards.
Smoke Density Test (GB/T 8627)
Specimens are exposed to a radiant heat source inside a closed chamber, and the attenuation of a horizontal light beam by the accumulating smoke is recorded. The resulting smoke density rating characterizes the smoke hazard of the material, which is significant for XPS given its aromatic polymer backbone.
Cone Calorimeter Test (ISO 5660-1, GB/T 16172)
A 100 mm × 100 mm specimen is irradiated at 25 to 50 kW/m² beneath a cone heater while heat release rate, total heat release, total smoke release, effective heat of combustion, and mass loss are recorded. Cone calorimetry is widely used for formulation development, comparative screening of flame retardant systems, and fire hazard modeling, and provides supporting evidence for classification under EN 13501-1 in some approval routes.
Supplementary Methods for International Projects
For export projects, XPS boards may additionally be evaluated under UL 94 (HF-1, HF-2, or HBF classifications for foamed materials), ASTM E84 or UL 723 surface burning characteristics (flame spread index and smoke developed index), and EN 13501-1 Euroclass testing, which shares the single burning item and small-flame methodology with the GB system.
Classification Criteria under GB 8624-2012
GB 8624-2012 aligns its grading with EN 13501-1, where Grades B, C, and D correspond to the traditional B1 category, Grade E corresponds to B2, and Grade F corresponds to B3. Key criteria are summarized below:
- Grade B: FIGRA0.2MJ not greater than 120 W/s, LFS not reaching the specimen edge, THR600s not greater than 7.5 MJ, plus passing small-flame test with 30 s exposure
- Grade C: FIGRA0.2MJ not greater than 250 W/s, LFS not reaching the specimen edge, THR600s not greater than 15 MJ, plus passing small-flame test
- Grade D: FIGRA0.2MJ not greater than 750 W/s and THR600s not greater than 15 MJ
- Grade E: passing GB/T 8626 with 15 s flame exposure, no flame tip beyond 150 mm within 20 s, no ignition of filter paper
- Smoke subclasses: s1 requires SMOGRA not greater than 30 m²/s², s2 not greater than 180 m²/s², otherwise s3
- Droplet subclasses: d0 indicates no flaming droplets or particles within 600 s, d1 indicates brief flaming droplets, otherwise d2
The test report issued by an accredited laboratory must state the standards applied, specimen description and conditioning, full measurement data, and the resulting classification with subclasses, and it must be accompanied by laboratory accreditation scope confirmation.
Applications
Combustion performance test reports for cold storage XPS boards are used across the entire project and product lifecycle:
- Design compliance for new cold stores, refrigerated logistics centers, and food processing plants, demonstrating that insulation meets the B1 requirement of GB 50072 and GB 50016
- Type testing and product certification for XPS manufacturers establishing or renewing declared classification for a product family
- Factory production control verification, including periodic confirmation testing and requalification after raw material or formulation changes
- Construction acceptance and supervision, where supervising engineers verify that delivered boards match the certified combustion grade
- Fire risk assessment and performance-based fire engineering for high-bay automated cold warehouses and insulated sandwich panel assemblies
- International project acceptance requiring EN 13501-1, ASTM E84, or UL 94 documentation in addition to GB classification
- Failure investigation and insurance loss control following fire incidents involving insulated structures
FAQ
What combustion grade must XPS boards achieve for cold storage?
Under GB 50072 and GB 50016, insulation materials used in cold storage construction must generally achieve Grade B1, meaning Grade B, C, or D under GB 8624-2012, with Grade B or C and favorable smoke and droplet subclasses (for example B-s1, d0) preferred for exposed or minimally protected applications. Where boards are fully protected by non-combustible layers such as concrete screeds or gypsum boards, design codes may permit specific relaxations, subject to fire design review.
Is an oxygen index of 30% sufficient to declare B1 grade?
No. LOI is a sensitive and repeatable screening indicator widely used for factory quality control, but Grade B1 classification under GB 8624-2012 must be based on the single burning item test results for FIGRA, THR600s, LFS, and SMOGRA together with the small-flame ignitability test. LOI data alone cannot support a formal classification claim.
How many samples are needed and how long does testing take?
A full B1 evaluation typically requires three complete single burning item specimen sets, one full production board for cutting small-flame specimens, and additional material for oxygen index and smoke density testing. After conditioning, the testing cycle at an accredited third-party laboratory commonly takes ten to fifteen working days, allowing for replicate verification if borderline results occur.
Do facings and metal skins affect the test result?
Yes. GB 8624-2012 evaluates products in their end-use configuration. A film facing, mortar skin, or metal facing changes ignitability, melt drip behavior, and heat release, so XPS boards must be tested exactly as installed. Metal-faced insulated sandwich panels are classified as composite assemblies rather than bare boards.
What is the difference between B1 and B2 XPS?
B1 corresponds to Grades B, C, or D, which must pass the intermediate-scale single burning item test controlling fire growth rate and total heat release. B2 corresponds to Grade E, which only requires passing the small-flame ignitability test. B2 material can still contribute significantly to a developing fire, which is why cold storage codes mandate B1.
How often should combustion performance be retested?
Type classification applies to a defined product family covering resin system, flame retardant formulation, thickness range, and facing configuration. Retesting is required when the formulation, thickness beyond the qualified range, facing, or manufacturing process changes, and periodic confirmation testing on an annual or agreed interval is recommended as part of factory production control.