XPS Board Horizontal Combustion Testing: Standards, Specimens, Methods and Classification

XPS board horizontal combustion testing measures the ignitability, flame spread rate, and burning behavior of extruded polystyrene foam insulation when a small flame is applied to a horizontally oriented specimen. Performed by accredited third-party laboratories in accordance with GB/T 2408, GB/T 8626, ASTM D635, UL 94, and related standards, the test underpins the fire performance classifications required for building code approval of insulation products. This article explains the sample requirements, measured parameters, test procedures, classification criteria, applications, and frequently asked questions.

Technical Overview

Extruded polystyrene (XPS) board is a closed-cell rigid foam insulation material widely used in walls, roofs, floors, and civil engineering applications due to its low thermal conductivity, high compressive strength, and moisture resistance. As an organic polymer foam, however, XPS is inherently combustible, and untreated grades can ignite easily, propagate flame rapidly, and produce flaming melt drips. Fire performance testing is therefore a mandatory step in product development, certification, and construction approval.

Horizontal combustion testing is one of the most fundamental small-scale flammability evaluations for plastic and foam materials. A standardized small flame is applied to the leading edge or surface of a horizontally mounted specimen, and parameters such as ignition behavior, afterflame time, extent of burning, linear burning rate, and flaming drips are recorded. The principal standards frameworks include GB/T 2408 (horizontal and vertical burning of plastics, technically aligned with IEC 60695-11-10), GB/T 8626 (single-flame-source test for building products, aligned with ISO 11925-2), ASTM D635, UL 94 (HB class), and ISO 9772 for cellular plastics. Results feed directly into classification systems such as GB 8624 in China, EN 13501-1 in Europe, and UL 94 material ratings.

Testing Samples

Bar Specimens for Material-Level Screening

For horizontal burning tests under GB/T 2408, UL 94 HB, and ASTM D635, small rectangular bar specimens are cut from the XPS board. Typical requirements include:

• Dimensions of approximately 125 mm in length and 13 mm (or 12.7 mm) in width, with reference marks at 25 mm and 100 mm from the exposed edge.
• Thickness tested as supplied or trimmed to the range specified by the standard; where the product exceeds the permitted thickness, the standard's rules for reduced or full-thickness testing apply.
• A minimum of three to five specimens per condition, with additional specimens retained for retest in case of borderline results.

Panel Specimens for Building Product Classification

When the objective is fire classification under GB 8624, larger panel specimens are tested according to GB/T 8626 or ISO 11925-2:

• Panels of 250 mm × 90 mm, tested at the actual product thickness, typically up to the maximum thickness stated in the classification standard.
• Separate specimen sets for edge exposure and surface exposure flame attack, since XPS boards behave differently when flame contacts the cut edge versus the facer or skin surface.
• Optional mounting on a defined substrate or in the end-use configuration when the classification scheme requires it.

Specimen Conditioning

All specimens are conditioned prior to testing, typically at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 88 hours (or as specified in the governing standard). Specimens must be free of dust, machining burrs, facer damage, and residual solvents that could alter burning behavior.

Testing Items

The horizontal combustion test program typically evaluates the following parameters:

• Ignition behavior: whether and how quickly the specimen ignites under the applied pilot flame.
• Afterflame time: the duration, in seconds, for which flaming combustion persists after removal of the ignition source.
• Afterglow time: the duration of glowing combustion without flame after flaming ceases, where applicable.
• Extent of damage: the burned or charred length measured in millimeters, and whether the flame front passes the 25 mm and 100 mm reference marks.
• Linear burning rate: expressed in mm/min and calculated over the span between the reference marks.
• Flaming drips and particles: whether molten flaming material falls and, in filter-paper protocols, whether it ignites the indicator paper.
• Classification result: for example HB, HB40, or HB75 under GB/T 2408 / UL 94 HB logic, HF classes under ISO 9772, or pass/fail and flame spread criteria under GB/T 8626 supporting GB 8624 grades.

Testing Methods

Specimen Mounting and Preparation

The specimen is clamped horizontally at its far end, with the long axis parallel to the airflow of a draft-free enclosure or fume hood. Reference marks are verified at 25 mm and 100 mm from the free edge, and a layer of dry surgical cotton or filter paper is placed below the specimen where the standard requires drip detection. The gas supply (propane, butane, or methane as specified) is regulated to produce the calibrated blue flame.

Flame Application

For bar specimens under GB/T 2408, UL 94 HB, and ASTM D635, the burner is inclined at approximately 45 degrees and the blue flame, typically 20 to 25 mm in height, is applied to the leading edge for about 30 seconds or until the flame front reaches the 25 mm mark, whichever occurs first. For building product panels under GB/T 8626 and ISO 11925-2, the burner tube is applied to the edge or surface for a fixed duration of 15 or 30 seconds as defined by the classification route, with the gauge line at 150 mm monitored.

Observation and Data Recording

During and after flame application, the operator records the time at which the flame front crosses each reference mark, the total afterflame and afterglow durations, the occurrence of flaming drips, and the final damaged length. Any burning that extinguishes before the 100 mm mark is noted, because early self-extinction is a key criterion for achieving higher ratings. Tests are repeated on all specimens and the complete data set is used for classification.

Classification Criteria

Under the HB logic of GB/T 2408 and UL 94, a specimen achieves the basic HB class if flaming does not reach the 100 mm mark, or if the linear burning rate does not exceed the limit defined for the specimen thickness, with flaming drips restricted as specified. Higher flame spread rates fall into the HB40 or HB75 classes, while products that cannot meet these thresholds are considered unclassified for this method. Under the GB 8624 route, results from GB/T 8626 are combined with data from other fire tests, such as the single burning item test and oxygen index, to assign grades such as B1 (flame retardant) or B2 (ordinary combustible) for organic insulation boards.

Applications

Horizontal combustion test reports are used throughout the life cycle of XPS insulation products:

• Regulatory compliance: demonstrating conformity with national building codes and fire protection design standards for exterior insulation, roofing, and underfloor systems.
• Product certification: supporting fire classification marks under GB 8624 and Euroclass schemes under EN 13501-1, which rely on the horizontal small-flame test as the entry-level criterion.
• Research and development: comparing flame-retardant formulations, additive loadings, facer designs, and polymer modifications during new grade development.
• Quality control: verifying batch consistency of flame-retardant XPS in factory laboratories and during third-party surveillance testing.
• Project documentation: providing test evidence for tender submissions, engineering acceptance, supervision review, and market access files.

FAQ

What is the difference between a horizontal and a vertical burning test?

In the horizontal test the specimen lies flat and the flame attacks its edge or lower surface, which generally produces slower flame propagation and makes the method suitable for screening and classification of materials such as XPS foam. In the vertical test the specimen stands upright, so the flame attacks the bottom edge and buoyancy accelerates burning, resulting in more severe criteria. The two orientations are complementary and are governed by the same families of standards.

Which standard should be selected for XPS boards used in construction?

For construction approval in China, the classification route through GB 8624 is normally required, with GB/T 8626 as the small-flame ignition test and, for certain grades, the oxygen index test under GB/T 2406.2 as a supplementary item. GB/T 2408, ASTM D635, and UL 94 HB are more commonly used for material-level characterization, supplier specifications, and export documentation.

What fire classification must XPS insulation typically achieve?

Requirements depend on the jurisdiction and application. In Chinese building practice, exterior insulation boards are commonly specified as B1 grade with an oxygen index of about 30% or higher, while lower-risk positions may accept B2 grade. Material-level HB ratings are used for non-construction or component applications. Project specifications and local codes always take precedence.

Does the flame retardancy of XPS change over time?

Yes. Flame retardants can migrate or degrade during long-term thermal and UV exposure, so some specifications require testing in both the as-manufactured state and after defined aging treatments. Comparing initial and aged horizontal burning results helps predict long-term fire performance.

How many specimens are needed and how long does testing take?

A typical program uses three to five bar specimens per condition for material screening, or multiple panel sets for edge and surface exposure under GB/T 8626. Including conditioning, testing, and report issuance, accredited third-party laboratories generally complete the work within five to ten working days.