XPS Board Vertical Burning Test: Standards, Sample Requirements, Procedure, and Classification

The XPS board vertical burning test is a standardized laboratory method used to evaluate the flammability of extruded polystyrene foam insulation by applying a small flame to vertically oriented specimens and measuring afterflame time, afterglow time, and flaming drips. Based on recognized standards such as UL 94, GB/T 2408, IEC 60695-11-10, and ASTM D3801, the test assigns V-0, V-1, or V-2 classifications that are widely used in product specifications, quality control, and international trade. This article explains the test principles, sample preparation, procedure, classification criteria, and typical applications for XPS foam boards.

Technical Overview

XPS (extruded polystyrene) board is a rigid, closed-cell thermoplastic foam widely used for thermal insulation in walls, roofs, floors, cold-storage facilities, and civil engineering applications. Because polystyrene foam is inherently combustible, flame-retardant grades are commonly produced, and their burning behavior must be verified through standardized testing. The vertical burning test is one of the most widely used screening methods for this purpose.

In the vertical burning test, a small methane or natural-gas burner flame is applied to the lower end of a rectangular specimen held in a vertical position. The test evaluates whether the material self-extinguishes after removal of the ignition source, how long flames and glowing persist, and whether flaming droplets or particles fall and ignite an indicator material placed beneath the specimen. The results are used to assign a flammability classification that indicates the relative flame retardancy of the material.

The vertical burning test for XPS boards is typically performed in accordance with one or more of the following standards:

• UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances — the vertical burning test with a 20 mm flame (V-0, V-1, V-2 classifications)
• GB/T 2408, Plastics — Determination of burning characteristics — Horizontal and vertical burning, the Chinese national standard technically aligned with IEC 60695-11-10
• IEC 60695-11-10, Fire hazard testing — Test flames — 50 W horizontal and vertical flame test methods
• ISO 1210, Plastics — Determination of flammability characteristics by small-scale flame exposure, with vertical and horizontal configurations
• ASTM D3801, Standard Test Method for Measuring the Comparative Extinguishing Characteristics of Solid Plastics in a Vertical Position

These tests are small-scale, laboratory-based screening methods. For building-code compliance of XPS insulation, the vertical burning result is usually combined with or supplemented by reaction-to-fire classification systems such as GB 8624 in China or EN 13501-1 in Europe, which use additional test methods such as the single flame source test and the single burning item (SBI) test.

Testing Samples

Specimen Dimensions and Preparation

Standard vertical burning specimens are bars measuring 125 mm in length and 13 mm in width. The test is normally conducted at the thickness relevant to the end use, because flammability results depend strongly on specimen thickness — thinner specimens generally produce more severe results. For XPS boards, which are commonly supplied in thicknesses from approximately 10 mm to 100 mm, testing is usually performed on specimens cut from the thinnest marketed thickness, and, where required by the specification, on additional thicknesses to establish the thickness dependence of the classification.

Specimens should be cut cleanly so that edges are smooth and free of dust, skin damage, or crushed cells. Any molded surface skin is generally retained as supplied, since it represents the actual product condition. Reference-condition specimens of defined thickness (for example, approximately 3 mm) may also be prepared when a baseline classification for the base polymer formulation is required.

Conditioning Requirements

Before testing, specimens are conditioned as follows:

• Ambient conditioning: at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity for one set of specimens
• Thermal conditioning: a second set of specimens is conditioned for 168 ± 2 hours at 70 ± 1 °C in a ventilated oven, then cooled to room temperature in a desiccator for at least 4 hours before testing, in order to evaluate the stability of the flame-retardant effect after aging

This dual conditioning regime is specified in UL 94, GB/T 2408, and IEC 60695-11-10 and reflects the fact that some flame-retardant systems can lose effectiveness over time or at elevated temperature.

Sample Quantity

A complete test normally requires two sets of five specimens each — ten bars in total — one set for each conditioning state. Additional backup specimens are recommended, because borderline results may require testing a supplementary set of five specimens under the decision rules of the standards.

Testing Items

The vertical burning test on XPS board specimens measures and records the following items:

• Individual afterflame time after the first flame application (t1), in seconds
• Individual afterflame time after the second flame application (t2), in seconds
• Afterglow time following the second application, measured after flaming ceases (t3), in seconds
• Total afterflame time for the set of five specimens
• Whether flaming drips or particles fall from the specimen and ignite the cotton indicator pad placed beneath it
• Whether afterflame or afterglow extends to the clamped upper end of the specimen (burn to the clamp)
• Final classification of the material: V-0, V-1, V-2, or not classified

For XPS foam, the dripping behavior is a particularly critical observation, because thermoplastic foams tend to melt and shrink away from the flame, and flaming melt droplets can propagate fire downward if the flame-retardant system is inadequate.

Testing Methods

Apparatus and Test Conditions

The test is conducted inside an essentially draft-free enclosure or fume chamber. A specimen clamp holds the bar with its long axis vertical, with the lower edge of the specimen positioned 10 mm above the top of the burner barrel. The burner is a Bunsen or Tirril type operated on methane or natural gas of specified calorific value, adjusted to produce a blue flame 20 ± 1 mm in height. A pad of dry, absorbent surgical cotton approximately 50 mm × 50 mm × 6 mm is placed 300 mm below the bottom edge of the specimen to detect flaming drips.

Test Procedure

The general procedure is as follows:

• Mount the conditioned specimen vertically in the clamp and confirm the burner flame height is correctly adjusted.
• Apply the flame to the center of the lower edge of the specimen for 10 ± 0.5 seconds so that the blue cone just touches the specimen surface, then withdraw the burner and record the afterflame time t1.
• As soon as flaming stops, immediately reapply the flame for a second 10 ± 0.5 second period, then withdraw and record the afterflame time t2 and the subsequent afterglow time t3.
• Observe and record whether flaming drips or particles fall and ignite the cotton pad, and whether afterflame or afterglow reaches the clamp.
• Repeat the sequence for all five specimens in the set, then test the oven-conditioned set under identical conditions.

Because XPS foam shrinks, melts, and may retreat from the burner, the operator moves the burner as necessary during the application period to keep the flame in contact with the specimen, in accordance with the provisions of the applicable standard.

Classification Criteria

The classification is determined by comparing the recorded results with the following criteria:

• V-0: each individual afterflame time does not exceed 10 seconds; the total afterflame time for the set of five specimens does not exceed 50 seconds; no flaming drips ignite the cotton; and no specimen burns or glows up to the clamp.
• V-1: each individual afterflame time does not exceed 30 seconds; the total afterflame time for the set does not exceed 250 seconds; no flaming drips ignite the cotton; and no specimen burns or glows up to the clamp.
• V-2: identical time limits as V-1, but flaming drips are permitted provided the remaining criteria are met — that is, flaming particles that ignite the cotton pad are acceptable.
• Not classified: if the specimen fails all of the above criteria, the material is unclassified in the 20 mm vertical test, and testing at more severe levels (for example, the 125 mm flame 5VA/5VB tests) or other reaction-to-fire methods may be considered.

High-quality flame-retarded XPS board is typically specified to achieve at least a V-1 or V-0 classification at the relevant thickness, although the exact requirement is defined by the purchaser, the product standard, or the applicable regulatory framework.

Applications

The XPS board vertical burning test supports a wide range of engineering, commercial, and regulatory activities:

• Building insulation: verification of the flame-retarded grade of XPS used for external wall insulation, roof insulation, and floor underlayment, in support of requirements such as GB 8624 B1 classification, which also involves GB/T 8626 and GB/T 20284
• Product specifications: demonstration of compliance with the flammability clauses of XPS product standards and buyer specifications for domestic and export markets
• Quality control: routine batch release testing and incoming material inspection at manufacturing plants to confirm consistent flame-retardant dosing and mixing
• Research and development: comparative evaluation of flame-retardant formulations, including brominated, phosphorus-based, and mineral-based additive systems, during formulation optimization
• Cold-chain and prefabricated construction: assessment of foam boards used in cold-storage panels, insulated doors, and composite sandwich panels
• Trade and certification: generation of test reports from accredited third-party laboratories for customs clearance, tender documents, and supplier qualification

It should be emphasized that the 20 mm vertical burning test is a small-scale screening method. For the formal reaction-to-fire classification of construction products, the results must be combined with the full suite of tests required by GB 8624, EN 13501-1, or the corresponding regional building codes.

FAQ

What is the difference between V-0, V-1, and V-2 classifications?

All three classifications indicate that the specimen self-extinguishes after removal of the flame. V-0 is the most demanding rating, requiring individual afterflame times of no more than 10 seconds and a set total of no more than 50 seconds without flaming drips. V-1 allows up to 30 seconds per specimen and 250 seconds total without flaming drips. V-2 has the same time limits as V-1 but permits flaming drips that ignite the cotton indicator.

Why does specimen thickness matter so much for XPS foam?

Flammability results are thickness-dependent. Thin sections heat up faster, lose flame retardant through migration or volatilization more readily, and are more prone to sustained burning. A classification obtained at one thickness cannot automatically be extended to other thicknesses, which is why standards require testing at the minimum relevant thickness.

How does the vertical burning test differ from the horizontal burning test?

The vertical configuration is more severe because heat and flames rise and preheat the material above the flame front, promoting upward flame propagation. Materials that pass the horizontal test may still fail the vertical test, so the vertical method is preferred for evaluating flame-retarded XPS grades.

Is a vertical burning test alone sufficient to prove fire safety of XPS insulation?

No. The vertical burning test is a small-scale screening method. Regulatory approval of building insulation generally requires a complete reaction-to-fire classification under systems such as GB 8624 or EN 13501-1, which involve additional tests including the single burning item test. The vertical burning result is best used for material comparison, specification control, and as supporting evidence within a broader fire-safety assessment.

What fuel and flame conditions are used?

The test uses a Bunsen-type burner fueled by methane or natural gas of specified calorific value, adjusted to a blue flame 20 ± 1 mm high. The flame is applied to the bottom edge of the vertical specimen for two successive 10-second periods.

How many specimens are required and what is the typical testing sequence?

Ten specimens are normally required: five conditioned at 23 °C and 50 % relative humidity, and five oven-aged at 70 °C for one week. Each specimen receives two 10-second flame applications, and the recorded afterflame times, afterglow times, and dripping behavior determine the final classification.