XPS Board Vertical Burning Test: Standards, Procedures, and Classification Criteria

The XPS board vertical burning test evaluates the flammability of extruded polystyrene insulation boards by exposing vertically mounted specimens to a standardized flame and measuring afterflame time, afterglow, and flaming drips. This technical guide explains the applicable standards, including UL 94, ISO 1210, GB/T 2408, and EN ISO 11925-2, along with specimen requirements, test procedures, and classification criteria used by accredited third-party laboratories and manufacturers worldwide.

Technical Overview

Extruded polystyrene (XPS) board is a rigid, closed-cell thermoplastic foam insulation material produced by extruding polystyrene resin combined with a blowing agent. Because of its low thermal conductivity, high compressive strength, and low water absorption, XPS board is widely used in wall insulation, inverted roofing, floor slabs, cold-storage panels, and below-grade applications. However, as an organic foam, XPS is inherently combustible: it softens and melts when heated, can sustain flaming combustion, and may produce flaming molten drips unless modified with flame retardants.

The vertical burning test is a small-scale laboratory method that measures how a vertically oriented specimen reacts to a controlled ignition source. The specimen is mounted with its longitudinal axis vertical, a standard gas burner flame is applied to its lower edge for a defined period, and parameters such as afterflame time, afterglow time, dripping behavior, and ignition of an indicator material are recorded. The results are used either to assign a material flammability classification, such as the V-0, V-1, or V-2 ratings defined in UL 94, ISO 1210, ASTM D3801, and GB/T 2408, or to support fire classification of building products under EN 13501-1, GB 8624, and related schemes through the single-flame-source test of EN ISO 11925-2 and GB/T 8626. These tests are routinely performed by accredited third-party laboratories as part of product certification, regulatory compliance, and quality control.

Testing Samples

Material-Level Specimens (UL 94 / ISO 1210 / ASTM D3801 / GB/T 2408)

For the classical vertical burning method, bar specimens measuring 125 mm in length and 13 mm in width are cut from the XPS board. The tested thickness is normally the thickness at which the material is used, and the full range of production thicknesses may need to be represented. Two sets of five specimens each are required for the basic test, with additional sets needed for every color, density, or thickness variant, since these parameters can significantly influence burning behavior. Specimens must be free of visible defects, skins, or edge damage caused by cutting.

Building-Product Specimens (EN ISO 11925-2 / GB/T 8626)

For the single-flame-source test used in building fire classification, specimens measuring 250 mm by 90 mm are prepared at the end-use thickness. Separate specimen sets are required for surface flame exposure and, where relevant, for edge flame exposure, and for each distinct exposed face of the product. This reflects real installation conditions in which joints and cut edges may be attacked by flame.

Conditioning Requirements

All specimens are conditioned before testing to ensure reproducible results. Typical conditioning is at least 48 hours at 23 ± 2 °C and 50 ± 5 % relative humidity. Some specifications also permit or require dry conditioning to constant mass at an elevated temperature, such as 70 °C for 168 hours followed by cooling in a desiccator, particularly when evaluating long-term thermal influences on flame-retardant performance.

Testing Items

The vertical burning test program for XPS board typically covers the following measured and derived parameters:

• Afterflame time after the first flame application (t1), recorded in seconds
• Afterflame time after the second flame application (t2), recorded in seconds
• Afterglow time after the second flame application (t3), recorded in seconds
• Total afterflame time for the specimen set
• Formation of flaming molten drips or incandescent particles
• Ignition of the cotton indicator pad placed below the specimen
• Whether the specimen burns up to the holding clamp
• Flame spread distance and time to reach the 150 mm gauge mark (EN ISO 11925-2 / GB/T 8626)
• Ignition of filter paper by flaming droplets or particles (EN ISO 11925-2 / GB/T 8626)
• Assignment of the final classification: V-0, V-1, or V-2 for material tests, or conformity assessment supporting classes under EN 13501-1 and GB 8624

Testing Methods

Apparatus and Test Environment

Testing is conducted inside a laboratory fume chamber or enclosed test cabinet that is free from drafts but provides safe extraction of combustion products. The core apparatus includes a laboratory gas burner adjusted to produce a 20 mm blue flame using methane or a specified gas of equivalent calorific value, a specimen clamp that grips the specimen at its upper end while leaving the lower edge exposed, absorbent cotton as the drip indicator, timing devices with 1-second resolution, and the necessary personal protective equipment for operators. The burner, specimen support, and indicator placement must comply with the geometric tolerances of the selected standard.

Vertical Burning Test Procedure (UL 94 / GB/T 2408 / ISO 1210)

The conditioned specimen is clamped with its longitudinal axis vertical, and the burner is positioned so that the top of the burner tube is approximately 10 mm below the free lower edge of the specimen. The flame is applied to the center of the lower edge for 10 ± 0.5 seconds and then withdrawn; the afterflame time t1 is recorded. Once flaming ceases, the flame is reapplied for another 10 ± 0.5 seconds to the same location, withdrawn again, and the second afterflame time t2 and subsequent afterglow time t3 are recorded. During both applications, the operator observes whether flaming drips fall and whether they ignite the cotton pad positioned roughly 300 mm below the specimen, and whether the flame front reaches the clamp. The procedure is repeated for all specimens in each set.

Single-Flame-Source Procedure (EN ISO 11925-2 / GB/T 8626)

The building-product specimen is mounted vertically, and a small premixed propane flame is applied at a defined angle to either the surface or the bottom edge for 15 seconds or 30 seconds, depending on the classification being sought. The operator records whether the flame tip reaches the 150 mm gauge line above the ignition point within the permitted time and whether flaming droplets or particles ignite the filter paper placed beneath the specimen. This test forms the ignitability basis for European classes B through E under EN 13501-1 and for B1 and B2 grades under GB 8624-2012 when combined with the appropriate supplementary tests.

Classification Criteria

For the material-level vertical burning test, the classification is assigned based on the combined results of the specimen set according to the following general requirements:

• V-0: each individual afterflame time does not exceed 10 seconds, the total afterflame time for the set does not exceed 50 seconds, afterflame plus afterglow after the second application does not exceed 30 seconds, no flaming drips ignite the cotton, and no specimen burns 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, afterflame plus afterglow after the second application does not exceed 60 seconds, no flaming drips ignite the cotton, and no specimen burns to the clamp
• V-2: identical timing limits to V-1, except that brief flaming drips that ignite the cotton indicator are permitted

Specimens that exceed these limits, burn to the clamp, or exhibit prolonged combustion are downgraded or reported as not classified for this method. For cellular plastics such as XPS, it should be noted that dedicated foam flammability ratings based on horizontal burning, such as HBF, HF-1, and HF-2 under ISO 9772 and ASTM D4986, also exist and are often requested in parallel, because melt shrinkage and drip behavior of foams can complicate the vertical method.

Applications

Vertical burning test data for XPS board serve multiple purposes across the construction and manufacturing value chain:

• Regulatory compliance for building insulation in markets governed by GB 8624-2012, EN 13501-1, and national building codes that restrict combustible insulation in certain assemblies
• Product certification under harmonized product standards such as EN 13164 and voluntary safety marks that require demonstrated flammability performance
• Quality control during production, verifying that flame-retardant masterbatch dosing and foaming conditions remain within qualified limits
• Research and development of flame-retarded XPS formulations, comparing afterflame and dripping behavior across candidate additives
• Import and export documentation, where buyers and authorities request test reports from accredited third-party laboratories
• Engineering selection of insulation for facades, roofing membranes, underfloor heating, cold rooms, and civil engineering works where ignition resistance of the foam layer is a design consideration

In practice, the small-scale vertical burning result is interpreted together with larger-scale reaction-to-fire tests, such as the single burning item test referenced by EN 13501-1 and surface burning characteristics tests under ASTM E84, because system performance depends on covering layers, cavities, and installation details rather than on the foam alone.

FAQ

What is the difference between V-0, V-1, and V-2? The three classes are distinguished by the permitted duration of afterflame after each flame application, the cumulative afterflame time for the specimen set, and whether flaming drips may ignite the cotton indicator. V-0 represents the strictest performance, while V-2 tolerates brief flaming drips.

Are UL 94, ISO 1210, ASTM D3801, and GB/T 2408 interchangeable? Their vertical methods are technically aligned in specimen size, flame application, and timing, and results are generally comparable. However, reports must always cite the exact standard performed, since minor differences in gas specification, conditioning, and reporting details can affect acceptance by a certification body.

Does a high vertical burning rating mean XPS is non-combustible? No. The vertical burning test is a comparative ignitability and flame-duration method for plastic materials. Non-combustibility classifications under building codes require different furnace-based tests, and XPS remains a combustible thermoplastic foam even when it achieves a good rating.

Why do untreated XPS specimens often produce flaming drips? Polystyrene is a thermoplastic that melts and flows when heated. Without effective flame retardants, melting fuel can drip while burning, which frequently results in a V-2 result or failure, since the drips ignite the cotton indicator.

How many specimens are needed and how long does testing take? The material-level test normally requires two sets of five bar specimens for the first variant, with additional sets per color, density, or thickness. Laboratory turnaround typically ranges from a few working days for routine testing to several weeks when conditioning and full certification documentation are included.

Do results apply to all thicknesses and colors of the board? No. Flammability performance can vary with thickness, density, cell structure, pigment content, and flame-retardant loading. Qualified ranges must be established by testing, and any significant formulation or process change should trigger re-evaluation at an accredited third-party laboratory.