Class B1 flame retardancy testing for extruded polystyrene (XPS) insulation boards verifies combustion performance in accordance with GB 8624-2012, the mandatory fire classification standard for building materials in China. This guide explains the B1 classification criteria, required test items such as the Single Burning Item test and limiting oxygen index, sampling requirements, and how manufacturers, contractors, and specifiers can ensure code-compliant fire performance documentation.
Technical Overview
Extruded polystyrene (XPS) board is a rigid, closed-cell foam insulation material manufactured by continuous extrusion of polystyrene resin combined with a blowing agent. It is widely used for wall, roof, and floor insulation thanks to its low thermal conductivity, high compressive strength, and resistance to moisture absorption. However, because polystyrene is a combustible thermoplastic polymer, unmodified XPS foam ignites easily, melts, and can produce flaming droplets, which is why flame retardants and mandatory fire performance testing are essential.
In China, the burning behavior of building materials and products is classified under GB 8624-2012, which assigns materials to grades A (non-combustible), B1 (flame retardant), B2 (combustible), and B3 (flammable). A B1 rating indicates that the material is difficult to ignite, limits heat release and flame spread under defined test conditions, and meets the additional smoke and droplet sub-classification requirements attached to the grade. The national fire code GB 50016 requires thermal insulation materials used in many building applications to achieve at least Class B1, and the product standard GB/T 10801.2 sets out corresponding requirements for XPS boards used in building insulation.
Class B1 flame retardancy testing is therefore a key conformity step for market access, tender qualification, construction supervision, and final acceptance of XPS insulation products. Testing should be carried out by accredited third-party laboratories with the capability to perform the full suite of GB 8624-2012 referenced test methods.
Testing Samples
Bare XPS Foam Boards
Standard samples are cut from production-line boards at the thicknesses to be placed on the market. Because burning behavior varies with thickness and density, projects and certification schemes typically require testing at both the minimum and maximum declared thicknesses. Samples must be free of surface contamination and damage, and the extruded skin should remain intact unless the product is supplied as planed or skived board.
Faced, Laminated, and Composite Products
When XPS boards are supplied with facings, foil, mortar coatings, or factory-applied composite layers, the specimens are tested in their end-use configuration, including any substrate, adhesive, or fixing system specified by the system manufacturer. Testing the assembled configuration is essential because facings and air gaps strongly influence ignition, heat release, and dripping behavior.
Sample Conditioning and Quantity
Before testing, specimens are conditioned to constant mass in a standard atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity, following the conditioning requirements referenced in the test standards. Typical specimen requirements include:
- Single Burning Item test (GB/T 20284): at least three complete sets, each consisting of a long wing (1000 mm × 1500 mm) and a short wing (500 mm × 1500 mm)
- Limiting oxygen index test (GB/T 2406.2): typically 15 to 20 small bar specimens
- Small-flame ignitability test (GB/T 8626): multiple panels of 250 mm × 90 mm for both surface and edge exposure
- Smoke density test (GB/T 8627): square panels tested in both flaming and non-flaming modes
Testing Items
The core combustion parameters used to determine the B1 classification of flat building products such as XPS boards are generated by the Single Burning Item test, supplemented by ignitability, oxygen index, and smoke tests. The key items are listed below.
- Fire growth rate index (FIGRA): the maximum ratio of heat release rate to elapsed time; for B1 the value must not exceed 120 W/s
- Total heat release (THR600s): total heat released within the first 600 seconds of exposure; for B1 the limit is 7.5 MJ
- Lateral flame spread (LFS): flame spread must not reach the edge of the specimen
- Smoke growth rate index (SMOGRA): determines the smoke sub-classification s1, s2, or s3
- Flaming droplets and particles: determines the droplet sub-classification d0, d1, or d2
- Limiting oxygen index (LOI): for B1-grade XPS, a minimum oxygen index of 30% is the widely applied industry and contractual benchmark
- Small-flame ignitability: confirms resistance to ignition by a small flame source on both surface and edge
- Smoke density and, where specified by the project or local requirements, combustion smoke toxicity evaluated under GB/T 20285
The final classification is expressed with its additional notations, for example B1 with the corresponding smoke and droplet sub-grades, giving specifiers a complete picture of fire growth, smoke production, and dripping hazard.
Testing Methods
Single Burning Item (SBI) Test — GB/T 20284
The SBI test is the principal method for classifying XPS boards under GB 8624-2012 and is technically aligned with EN 13823. Two vertical specimen wings are mounted in a corner configuration on a trolley inside a calibrated exhaust duct. A propane sandbox burner delivering approximately 30 kW is applied to the corner of the specimen, and the test records heat release rate by oxygen consumption, smoke production by an optical measuring system, lateral flame spread, and the occurrence of flaming droplets. From the recorded data, the laboratory calculates FIGRA, THR600s, and SMOGRA, which determine whether the product meets the B1 thresholds.
Limiting Oxygen Index — GB/T 2406.2 / ISO 4589-2
The LOI test measures the minimum concentration of oxygen in an oxygen–nitrogen mixture that will just support candle-like combustion of a vertically held specimen. A higher oxygen index indicates better flame retardancy. Flame-retarded XPS formulations achieving B1 performance typically reach an oxygen index of 30% or above, which is why this quick and economical method is commonly used for formulation development, factory process control, and incoming material verification.
Small-Flame Ignitability — GB/T 8626 / ISO 11925-2
In this screening test, a small diffusion flame is applied to the surface and to the bottom edge of a vertically clad specimen. Pass criteria are based on the flame tip not reaching a defined height within the specified time, and on filter paper placed beneath the specimen showing no ignition from falling flaming droplets. This test supports classification between grades and is often used together with the oxygen index test for routine quality checks.
Smoke Density — GB/T 8627
The smoke density test, based on the NBS smoke chamber principle, measures the attenuation of a light beam by smoke accumulated in a closed chamber under both flaming and non-flaming decomposition modes. Results are reported as specific optical density and a smoke density rating, which are relevant for products installed in enclosed spaces and for certain project specifications.
Combustion Smoke Toxicity — GB/T 20285
Where the purchaser, the system certificate, or local fire authorities require an assessment of toxic gas hazards, the smoke produced under controlled thermal decomposition is analyzed and the acute inhalation hazard is graded. This item is additional to the basic B1 classification but is increasingly requested for public buildings and crowded venues.
Applications
B1-rated XPS boards and corresponding test reports are required across a broad range of construction and industrial scenarios, including:
- External wall insulation and external thermal insulation composite systems (ETICS) on residential and public buildings
- Roof, inverted roof, and podium deck insulation where codes mandate flame-retardant materials
- Ground floor and underfloor heating insulation boards installed below floor screeds
- Cold storage facilities, refrigerated logistics centers, and insulated panel cores
- Insulation for HVAC ductwork, pipes, and plant rooms subject to fire safety review
- Tender qualification, factory acceptance, construction supervision, and final fire acceptance documentation for projects under GB 50016 and GB 50411
FAQ
Does a B1 rating mean XPS is non-combustible?
No. B1 means the material is flame retardant, which is difficult to ignite and limits heat release and flame spread under the specified test conditions. Only Class A materials are considered non-combustible. A B1 XPS board can still burn under sustained high heat input, which is why protective layers and fire barriers are often required in the final system design.
What is the difference between B1 and B2 for XPS boards?
Under GB 8624-2012, B2 is the combustible grade with relaxed limits, typically THR600s of no more than 15 MJ and FIGRA of no more than 250 W/s, while B1 requires FIGRA of no more than 120 W/s and THR600s of no more than 7.5 MJ. In practice, B1 XPS boards contain a higher loading of effective flame retardant and generally achieve an oxygen index of 30% or greater.
Can a European Euroclass B report replace a GB 8624 report?
Although the SBI test methods are technically harmonized, construction projects in China require a classification report issued under GB 8624-2012. Reports based on EN 13501-1 or other national schemes are useful references, but they do not substitute for a Chinese classification report when submitting for design review or fire acceptance.
How long does B1 testing take?
After sample conditioning is complete, accredited third-party laboratories commonly complete the full test suite and issue the classification report within one to three weeks, depending on the number of thicknesses and configurations being evaluated. Rush programs may be available for urgent project schedules.
Do all production batches need to be tested?
The B1 classification itself is a type test tied to a defined product formulation, thickness range, and end-use configuration. Thereafter, manufacturers typically perform routine verification, such as oxygen index or small-flame testing, on each production batch to demonstrate consistent flame retardancy, while project supervisors may request batch sampling for third-party verification.
How should samples be submitted for testing?
Samples should be taken from current production, clearly labeled with product model, thickness, density, and production date, and shipped without compression or surface damage. Because thermoplastic foams are sensitive to physical conditioning, laboratories should be consulted in advance regarding total specimen area, which is typically at least 6 to 8 square meters for a full SBI-based classification including repeats.