Extruded polystyrene (XPS) boards are the most common thermal insulation layer installed beneath underfloor heating pipes and cables, and because the base polymer is inherently combustible, their flame retardant performance must be verified through standardized testing. This article explains the sample requirements, test items, and test methods—including oxygen index, small flame ignitability, and the single burning item test—used to determine the fire performance of floor heating XPS boards in accordance with GB, ISO, EN, ASTM, and UL standards.
Technical Overview
Extruded polystyrene (XPS) foam board is a closed-cell rigid insulation material produced by extruding polystyrene resin together with a blowing agent. In underfloor heating systems, it is placed below the heating pipes or electric heating cables to reduce downward heat loss, distribute load, and act as a moisture-resistant layer. Because polystyrene is a thermoplastic hydrocarbon polymer, unmodified XPS softens, ignites, and produces flaming melt drips when exposed to fire. Flame retardants—today typically polymeric brominated systems, following the global phase-out of HBCD under the Stockholm Convention—are therefore compounded into the resin during extrusion.
Flame retardant performance determination verifies that the finished board achieves the fire classification declared by the manufacturer and required by building regulations. In the Chinese market, classification is performed under GB 8624-2012, which assigns the grades A, B1, B2, and B3 for burning behavior of building materials and products. Parallel frameworks include EN 13501-1 (Euroclass system) in Europe, ASTM E84 and UL 723 in North America, and UL 94 for small-scale specimen ratings. The tests themselves are conducted by manufacturers' laboratories or accredited third-party laboratories using the specific methods referenced in each classification standard.
Testing Samples
Finished Production Boards
Samples are taken from normal production lots and must be representative of the commercial product in density (typically 30–45 kg/m³ for floor heating grades), thickness (commonly 20–50 mm), cell structure, and flame retardant loading. Batch records, formulation identification, and production date should accompany the samples so results can be traced to the manufacturing process.
Cut Laboratory Specimens
For small-scale methods such as the oxygen index and horizontal/vertical burning tests, specimens are cut from the board with clean, smooth edges, free of dust and melted deposits. Foam specimens are typically cut to 100–150 mm in length and 4–10 mm in thickness, with at least 15 pieces prepared for oxygen index testing because the bracketing procedure consumes multiple specimens.
Full-Size Panels for the SBI Test
The single burning item test requires two vertical wing panels forming a corner: a long wing of approximately 1500 mm × 1000 mm and a short wing of approximately 1500 mm × 495 mm, both at the full product thickness with the intended exposed surface facing the burner. One spare set is recommended in case a retest is needed.
Conditioning and Aged Samples
All specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours, or longer where the test standard requires constant mass. Because underfloor heating exposes boards to elevated temperatures for decades (surface temperatures up to about 29 °C and continuous material service temperatures of roughly 60–70 °C), certification schemes may also require thermal aging or humid aging before retesting to confirm the long-term durability of the flame retardant system.
Testing Items
Depending on the classification route and the target market, the determination typically includes the following items:
- Limiting oxygen index (LOI) of the foam material
- Horizontal and vertical burning behavior and rating (e.g., UL 94 HB)
- Single flame source ignitability with 15 s and 30 s flame application, including surface and edge exposure
- Heat release and flame spread parameters from the single burning item test: FIGRA, THR600s, LFS
- Smoke production parameters: SMOGRA and smoke density rating
- Flaming droplets and particles behavior
- Overall fire classification: GB 8624-2012 grade or EN 13501-1 Euroclass
- Flame spread index and smoke developed index for North American listings (ASTM E84 / UL 723)
Testing Methods
Oxygen Index Test
The test is performed to GB/T 2406.2, ISO 4589-2, or ASTM D2863. A specimen held vertically in a glass chimney is ignited at its top edge in a flowing mixture of oxygen and nitrogen. The oxygen concentration is adjusted stepwise to find the minimum level that just supports burning for 3 minutes or consumes 50 mm of the specimen. Polystyrene foam is combustible in air (21% oxygen), so a higher LOI indicates greater flame retardancy; procurement specifications for B1-grade floor heating XPS commonly require an LOI of 30% or above. LOI is a fast screening and quality control method rather than a classification test by itself.
Horizontal and Vertical Burning Test
Small-scale burning behavior is assessed to GB/T 2408, IEC 60695-11-10, or UL 94 using bar specimens of approximately 125 mm × 13 mm. For the horizontal (HB) configuration, a specimen passes if burning extinguishes before the 100 mm gauge mark or the burning rate does not exceed 40 mm/min for thicknesses of 3–13 mm (75 mm/min below 3 mm). Vertical ratings additionally evaluate afterflame time and flaming drips. These ratings are useful for comparing formulations but do not replace building material classification.
Single Flame Source Ignitability Test
The test follows GB/T 8626 or EN ISO 11925-2 using plate specimens of approximately 250 mm × 90 mm. A small propane burner flame is applied to the surface and, separately, to the bottom edge for 15 s or 30 s. The specimen passes if flame spread does not reach the 150 mm gauge line within the specified observation time and if flaming droplets or particles do not ignite filter paper placed beneath the specimen. Passing this test is a prerequisite for classification under both GB 8624-2012 and EN 13501-1.
Single Burning Item (SBI) Test
The SBI test to GB/T 20284 or EN 13823 exposes the corner specimen to a propane burner of approximately 30 kW for 20 minutes while oxygen depletion, smoke opacity, and flame spread are recorded continuously. The derived parameters are FIGRA0.2MJ (fire growth rate), THR600s (total heat release), SMOGRA (smoke growth rate), and LFS (lateral flame spread). Under GB 8624-2012, the principal criteria for B1 grade are FIGRA0.2MJ ≤ 120 W/s and THR600s ≤ 7.5 MJ, and for B2 grade FIGRA0.2MJ ≤ 250 W/s and THR600s ≤ 15 MJ, in each case combined with a pass in the small flame test. Additional smoke and droplet sub-classifications (s1–s3, d0–d2) are assigned from SMOGRA and flaming droplet behavior, giving a full designation such as B1(B-s1, d0) or, under EN 13501-1, B-s1, d0.
Smoke Density Test
Smoke production may be additionally characterized to GB/T 8627 using a smoke density chamber, which yields a smoke density rating (SDR). Low smoke generation is an important safety property for interior applications such as underfloor heating, where escape routes may be affected by smoke from burning insulation materials.
Tunnel Flame Spread Test
For the North American market, boards are tested to ASTM E84 or UL 723 in a 25-foot tunnel furnace, producing a flame spread index (FSI) and a smoke developed index (SDI). Interior finish classes are defined by FSI limits (for example, Class A ≤ 25, Class B 26–75, Class C 76–200), and foam plastics are generally required to be covered by a thermal barrier or qualified as part of a tested assembly.
Classification and Reporting
The laboratory combines the individual results into a classification statement under GB 8624-2012 or EN 13501-1. The report should identify the sample and batch, list all standards and deviations, tabulate the measured parameters with their criteria, state the resulting grade with sub-classifications, and note that the classification applies to the board as tested. Under the European product standard EN 13164, a declared reaction-to-fire class under EN 13501-1 is part of the conformity documentation for XPS products.
Applications
Flame retardant performance determination of floor heating XPS boards supports the following activities:
- Factory quality control and batch release of flame-retarded XPS production
- Type testing and product certification for fire classification marks and labels
- Construction project documentation, fire design review, and site acceptance
- Verification of formulation changes, new flame retardant grades, and supplier changes
- Export compliance, including EN 13501-1 classification under EN 13164 and ASTM E84 listings
- Research and development benchmarking of polymeric flame retardant systems replacing legacy HBCD formulations
FAQ
Question: Which fire grade is required for XPS boards used under floor heating? Answer: Requirements depend on the building code and jurisdiction. Under GB 8624-2012, combustible insulation used in buildings must normally achieve at least B2 grade, and B1 grade is commonly specified for interior flooring applications in China. In Europe, the declared Euroclass under EN 13501-1 must satisfy the national fire safety regulations applicable to the floor assembly.
Question: Is a high oxygen index alone sufficient to claim B1 grade? Answer: No. Oxygen index is a material-level screening indicator. Formal classification under GB 8624-2012 or EN 13501-1 requires the defined combination of small flame ignitability and, for B1 and B2 plate products, the single burning item test results.
Question: How does the melting behavior of polystyrene affect the tests? Answer: As a thermoplastic, XPS shrinks and melts away from the flame, and melt drips can propagate fire. Test methods therefore monitor flaming droplets explicitly—filter paper in the small flame test and the d0/d1/d2 sub-classification in the SBI test—and specimens are mounted on supports in the defined orientation.
Question: Does the flame retardant performance change over the service life of the floor heating system? Answer: Modern polymeric brominated flame retardants are designed to resist migration and thermal degradation. However, verification after thermal or humid aging is a recognized way to demonstrate that the fire performance is maintained under the long-term elevated temperatures typical of underfloor heating.
Question: How much sample should be submitted for testing? Answer: A typical program needs at least 15 oxygen index specimens, several burning-test bars, a set of small flame plates, and one to two full SBI wing sets of approximately 2.3 m² each, plus retain samples. Submitting complete production boards allows the laboratory to cut all specimens from representative material.
Question: Can a UL 94 or ASTM E84 result be converted to a GB 8624 grade? Answer: No. Each classification system uses its own test methods and criteria, and results are not interchangeable. Products for the Chinese market must be tested under the GB framework, while UL 94 and ASTM E84 serve component-level and North American interior finish purposes respectively.
