Extruded Polystyrene (XPS) Board Horizontal Burning Test: Standards, Procedures, and Evaluation Crit

The horizontal burning test for extruded polystyrene (XPS) board is a fundamental fire-performance evaluation that measures ignition resistance, flame spread distance, afterflame time, and linear burning rate when a small pilot flame is applied to a horizontally mounted specimen. Performed in accordance with recognized standards such as GB/T 2408, IEC 60695-11-10, ASTM D635, and ISO 9772, the test supports material classification (HB, HB40, HB75, and HBF ratings), flame-retardant formulation development, and compliance with building-code requirements. This article explains the test principle, sample requirements, measured parameters, procedures, calculation methods, and typical applications for XPS insulation products.

Technical Overview

Extruded polystyrene (XPS) board is a rigid, closed-cell foam insulation material produced by continuously extruding polystyrene resin together with a blowing agent and, in most commercial grades, a flame-retardant additive package. The material is valued for its low thermal conductivity, high compressive strength, and low water absorption, which make it a preferred choice for building envelopes, roofs, floors, and cold-chain facilities. Because XPS is an organic polymer foam, however, it is inherently combustible, and its burning behavior must be characterized and controlled before the product can be certified for construction use.

The horizontal burning test evaluates the response of an XPS specimen to a small ignition source applied to its free end while the specimen lies in a horizontal plane. During the test, technicians record whether the material ignites, how far the flame front propagates between gauge marks, how long afterflame and afterglow persist after removal of the pilot flame, whether flaming droplets or particles are released, and how fast the flame travels along the surface. The results are used to assign a flammability classification, verify the effectiveness of flame-retardant formulations, and provide supporting evidence for broader fire-classification schemes.

The horizontal burning test is typically performed under one or more of the following standards, depending on the target market and end use:

• GB/T 2408 — Plastics: Determination of burning characteristics by the horizontal and vertical methods, widely applied to plastic building materials in the Chinese market
• IEC 60695-11-10 — Fire hazard testing: 50 W horizontal and vertical test methods for specimens of solid electrical insulation materials
• ASTM D635 — Standard test method for the rate of burning and/or extent and time of burning of plastics in a horizontal position
• ISO 9772 — Cellular plastics, determination of horizontal burning characteristics of small specimens subjected to a small flame, which uses a wire-mesh support appropriate for foams such as XPS
• ISO 1210 — Plastics: Determination of burning behavior by the horizontal and vertical methods
• UL 94 — The horizontal burning of foamed materials for the HBF classification, relevant to North American market access

Testing Samples

Standard XPS Insulation Board Specimens

Specimens are cut from representative production boards, taking care to obtain clean, smooth edges free of crushed cells or surface defects. For tests under GB/T 2408, IEC 60695-11-10, and ASTM D635, the standard specimen is 125 mm in length and 13 mm in width, with a preferred thickness of 3 mm for comparative purposes. For tests under ISO 9772 and the UL 94 HBF procedure, larger specimens of 150 mm by 50 mm are prepared in the as-supplied thickness and supported on a wire mesh, because cellular materials with low density tend to deform, shrink, or melt away from the flame without support.

Flame-Retarded XPS Grades

Commercial XPS intended for building insulation is normally formulated with brominated or phosphorus-based flame-retardant systems. Specimens from flame-retarded grades are tested to verify that the additive package delivers the intended self-extinguishing behavior, and comparative testing between retarded and non-retarded formulations is a common part of product development.

Composite and Faced Specimens

Boards supplied with facers, skins, or laminated layers may be tested in their as-supplied configuration when the end-use condition requires it. In such cases, the report clearly identifies the tested face, the presence of facings, and any deviation from the standard bar specimen geometry.

Specimen Conditioning

All specimens are conditioned prior to testing in a controlled atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours, and commonly 48 hours, to ensure that moisture content and temperature do not bias the ignition and burning measurements.

Testing Items

The horizontal burning test on XPS board generates the following key parameters:

• Ignition behavior: whether the specimen ignites under the applied pilot flame and whether ignition occurs within the maximum application time
• Flame spread distance and damaged length: the extent of burning measured along the specimen relative to the 25 mm and 100 mm gauge marks
• Afterflame time: the duration for which the specimen continues to burn after removal of the ignition flame
• Afterglow time: the duration of glowing combustion without a visible flame after flaming ceases
• Linear burning rate: the speed of the flame front expressed in millimeters per minute, calculated over the 75 mm span between gauge marks
• Flaming drips and particles: whether burning material falls and whether falling particles ignite indicator paper placed beneath the specimen
• Classification result: assignment of HB, HB40, or HB75 under bar-specimen methods, or HBF under cellular-plastic methods

The classification logic under the bar-specimen methods can be summarized as follows:

• HB: the flame self-extinguishes before the 100 mm mark, or, if the 100 mm mark is reached, the linear burning rate does not exceed 40 mm/min for thicknesses of 3 mm or less and 75 mm/min for thicknesses greater than 3 mm
• HB40: the average linear burning rate does not exceed 40 mm/min over the 75 mm gauge span
• HB75: the average linear burning rate does not exceed 75 mm/min over the 75 mm gauge span
• HBF: assigned to foamed materials when no specimen burns beyond the 125 mm gauge mark and the burning rate over the 75 mm span does not exceed 38 mm/min

Testing Methods

Apparatus Configuration

The test is conducted inside a ventilated fume hood or enclosure free from drafts, with the ambient temperature maintained between 15 °C and 35 °C. A laboratory burner with a specified barrel diameter is inclined at 45 degrees and adjusted to produce a blue pilot flame of approximately 20 mm in height. The specimen is clamped horizontally with its long axis parallel to the burner, and reference gauge marks at 25 mm and 100 mm from the exposed end define the measurement span. For cellular-plastic methods, a wire-mesh support and a layer of indicator paper beneath the specimen complete the setup.

Test Procedure

The calibrated burner flame is applied to the free end of the horizontal specimen for the specified duration, typically up to 30 seconds under the bar-specimen methods and approximately 60 seconds under the cellular-plastic methods, or until the flame front reaches the 25 mm gauge mark. The operator records the time of flame application, observes whether the material ignites, and uses a stopwatch or optical sensor system to measure the time at which the flame front crosses the 100 mm mark. After removal or burnout of the pilot flame, afterflame and afterglow durations are timed, and any flaming drips or falling particles are noted together with their effect on the indicator paper. The damaged length is measured after the specimen has cooled.

Calculation of the Linear Burning Rate

Where the flame front travels the full 75 mm distance between the 25 mm and 100 mm gauge marks, the linear burning rate is calculated as V = 60 × 75 / t, in which V is the burning rate in millimeters per minute and t is the time in seconds for the flame to traverse the 75 mm span. If the flame self-extinguishes before the 100 mm mark, the burning rate is reported as not applicable, and the damaged length and afterflame data become the governing results.

Specimen Quantity and Result Evaluation

Standards typically require three to five conditioned specimens per test condition, with additional sets tested when borderline or inconsistent behavior is observed. The results from all specimens are evaluated together against the classification criteria, and the test report documents the standard applied, specimen dimensions and orientation, conditioning history, individual timing values, calculated burning rates, observations of dripping and afterglow, and the resulting classification.

Interpretation for XPS Foam

XPS behaves distinctly during horizontal testing because it softens, melts, and shrinks away from the ignition source, and its closed-cell structure can produce sustained flaming drips in unretarded grades. For this reason, accredited third-party laboratories pay particular attention to melt behavior, drip ignition, and the reproducibility of the extinction point, and the wire-mesh-supported cellular-plastic methods are often preferred when the as-supplied board thickness must be evaluated.

Applications

Horizontal burning test data for XPS board are used across the material value chain:

• Building envelope insulation: supporting evidence for fire-performance schemes such as GB 8624 classification of exterior wall and roof insulation products
• Flame-retardant development: screening of brominated, phosphorus-based, and synergist formulations in base polystyrene foam
• Factory quality control: routine batch verification that the flame-retardant loading and extrusion process remain within validated limits
• Cold-chain and refrigeration engineering: fire-characterization input for insulated panels in cold storage and refrigerated logistics
• Civil engineering uses: assessment of foam boards used in road and railway subgrade insulation and frost-heave mitigation where ignition sources exist
• Market access: alignment of Chinese-manufactured XPS with international expectations referencing UL 94, IEC, ISO, and ASTM flammability methods
• Insurance and design review: documentation of small-scale ignition behavior for fire-risk engineers evaluating building assemblies

FAQ

Why is the horizontal method used for XPS board?

The horizontal orientation simulates exposure conditions such as floor assemblies, soffits, and the underside of roof decks, where flame spread occurs along a horizontal surface. It also provides a sensitive, repeatable measurement of burning rate and self-extinguishing behavior that is well suited to comparing flame-retardant formulations.

What is the difference between HB ratings and HBF?

HB, HB40, and HB75 classifications apply to bar specimens of 125 mm by 13 mm tested under methods such as GB/T 2408, IEC 60695-11-10, and ASTM D635, and they are defined by maximum burning rates of 75 or 40 millimeters per minute. HBF is the foamed-material classification under UL 94 and ISO 9772, applied to larger 150 mm by 50 mm specimens supported on wire mesh, with a criterion of 38 millimeters per minute over the measured span and no burn-through beyond the 125 mm mark.

Can the horizontal burning test alone prove compliance with building fire codes?

No. The horizontal burning test is a small-scale ignitability and burning-rate method. For construction certification, results must be combined with the applicable regulatory scheme, which for the Chinese market involves test methods such as GB/T 8626 for small-flame ignition and GB/T 20284 for the single burning item test, evaluated together under the GB 8624 classification framework.

What specimen thickness should be used?

For formulation comparison and classification under the bar-specimen methods, a thickness of 3 mm is preferred to ensure comparability with published classification criteria. When the performance of the as-supplied board is required, the cellular-plastic methods with full-thickness, wire-mesh-supported specimens are more appropriate.

How do flame retardants affect the results?

Effective flame-retardant systems increase ignition resistance, shorten afterflame time, reduce the damaged length, and lower or eliminate measurable burning rates, allowing properly formulated XPS to extinguish before the flame front reaches the 100 mm gauge mark. Test data also reveal side effects such as increased smoldering or dripping behavior, which must be documented.

How long does a typical test program take?

Excluding production of the board, the process normally requires two to five working days, covering specimen cutting, a minimum of 40 hours of conditioning, burner calibration, testing of the required specimen set, and report preparation. Retesting of borderline results may extend this timeline.

Where should the test be performed?

Testing should be carried out by accredited third-party laboratories or internally validated fire laboratories operating calibrated burners, controlled conditioning facilities, and documented procedures in accordance with the selected ISO, IEC, ASTM, UL, or GB standard.