The vertical burning test is a fundamental fire-safety evaluation used to determine how thermal insulation materials—particularly polymer foams—respond to direct flame exposure, measuring afterflame time, dripping behavior, and self-extinguishing capability. This article provides a comprehensive technical guide to the vertical burning test for insulation materials, covering applicable standards such as UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408, along with sample requirements, test procedures, classification criteria, and typical industrial applications.
Technical Overview
The vertical burning test is a small-scale laboratory flammability test in which a test specimen of thermal insulation material is mounted in a vertical orientation and exposed twice to a standardized low-energy flame, typically from a 50 W premixed gas burner. The test evaluates whether the material ignites, how long it continues to burn after the flame source is removed (afterflame time), whether it glows after flaming ceases (afterglow time), and whether flaming droplets or particles fall and ignite an indicator medium placed below the specimen.
For thermal insulation products such as rigid polyurethane (PU), polyisocyanurate (PIR), expanded and extruded polystyrene (EPS/XPS), phenolic foam, and elastomeric rubber-plastic foams, the vertical burning test serves as a key screening method for comparing flame-retardant formulations and verifying compliance with product specifications. Because these materials are widely installed in walls, roofs, ducts, and pipe networks, their behavior under direct flame impingement is directly relevant to fire risk in buildings and industrial equipment.
The most widely referenced standards for this test include:
- UL 94 – Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, vertical burning tests V-0, V-1, and V-2.
- IEC 60695-11-10 – Fire hazard testing: 50 W horizontal and vertical flame test methods, technically aligned with UL 94 V methods.
- ASTM D3801 – Standard Test Method for Measuring the Comparative Burning Characteristics of Solid Plastics in a Vertical Position.
- GB/T 2408 – Chinese national standard for burning characteristics of plastics, horizontal and vertical methods, harmonized with IEC 60695-11-10.
- ISO 9773 – Determination of burning behavior of thin flexible vertical specimens in contact with a small-flame ignition source.
- ASTM D6413 – Flame resistance of textiles (vertical test), applicable to insulation facings, jackets, and membranes.
In building-product conformity assessment, the vertical burning result is typically used alongside larger-scale reaction-to-fire tests, such as those referenced in EN 13501-1 and GB 8624, to support the overall fire classification of insulation systems.
Testing Samples
Rigid Polymer Foam Boards
Rigid foam boards including PU, PIR, phenolic foam, EPS, and XPS are cut into rectangular specimens, typically 125 mm in length and 13 mm in width, at the product's actual supplied thickness. If the material is produced in a range of thicknesses, testing is generally performed at both the minimum and maximum thickness, because thin specimens often burn more readily than thick ones. Specimens must be free of surface skins, facings, or defects unless these are integral to the finished product and are to be evaluated as supplied.
Elastomeric Foam Sheets and Tubes
Flexible rubber-plastic (NBR/PVC) insulation materials in sheet, roll, or tubular form are sampled so that the tested configuration represents the finished product. Flat specimens are die-cut from sheets; tubular products may be slit and flattened or tested in representative geometries where the standard permits. For materials that distort, shrink, or warp when free-standing, thin specimens may be supported on a specified mandrel or fixture in accordance with the applicable standard.
Facings, Jackets, and Composite Constructions
Aluminum foil facings, polymer films, glass-fabric jackets, and coated composites used with insulation products may be evaluated by vertical burning methods such as ASTM D6413, particularly when the facing contributes to the fire performance of the assembly. Specimens are conditioned to remove moisture and tested in the orientation corresponding to actual installation.
Specimen Conditioning and Quantity
Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours prior to testing. A standard vertical burning evaluation requires two sets of five specimens each (ten specimens total), allowing the laboratory to assess variability and to complete the full classification protocol described in UL 94 and IEC 60695-11-10.
Testing Items
The vertical burning test records the following measurable parameters for each specimen and each of the two flame applications:
- Afterflame time (t1, t2) – the duration, in seconds, that the specimen continues to flame after the first and second removal of the burner flame.
- Afterglow time (t3) – the duration of glowing combustion after flaming ceases following the second application.
- Flaming drips and particles – whether flaming or non-flaming material falls from the specimen during or after flame application.
- Cotton indicator ignition – whether flaming drips ignite a layer of surgical cotton placed approximately 300 mm below the specimen.
- Specimen damage and burning to the clamp – whether flame or glowing reaches the holding clamp, indicating excessive upward flame propagation.
- Flammability classification – assignment of a rating, typically V-0, V-1, or V-2, or no rating (fail), based on the combined criteria.
Testing Methods
Apparatus and Test Setup
The test is conducted in a ventilated fume chamber or enclosure that provides a draft-free environment during the burn period. A laboratory burner of the specified type is adjusted to produce a blue flame of defined height, typically 20 mm for the 50 W flame used in UL 94 V, IEC 60695-11-10, and GB/T 2408 tests. The specimen is clamped vertically with its long axis uppermost, and a cotton pad is positioned on a support 300 mm below the lower edge of the specimen.
Test Procedure
The general procedure, common to UL 94 and IEC 60695-11-10, proceeds as follows:
- Mount the conditioned specimen vertically in the clamp and confirm the burner flame height and gas supply.
- Apply the burner flame to the lower edge of the specimen for 10 ± 0.5 seconds, with the burner tube inclined at approximately 20 degrees from vertical or positioned as prescribed by the standard.
- Withdraw the burner and record the afterflame time t1, noting any dripping and cotton ignition.
- Apply the flame a second time for 10 ± 0.5 seconds after the specimen has ceased burning, and record the afterflame time t2 and afterglow time t3.
- Repeat the sequence for all five specimens in the set, then conduct the second set to complete the evaluation.
Classification Criteria
Based on the recorded results, the material is assigned one of the following classifications:
- V-0 – Afterflame time for each specimen does not exceed 10 seconds after each application; the total afterflame time for both applications on all specimens does not exceed 50 seconds; no flaming drips ignite the cotton; and afterglow does not persist beyond 30 seconds.
- V-1 – Afterflame time for each specimen does not exceed 30 seconds; total afterflame time does not exceed 250 seconds; no flaming drips ignite the cotton; and afterglow does not persist beyond 60 seconds.
- V-2 – Timing criteria are the same as V-1, but flaming drips are permitted provided the cotton is ignited only briefly, as defined by the standard.
- No rating (fail) – The material does not meet any of the above criteria; supplementary requirements, such as additional testing at higher thickness or glow-wire testing, may then be considered.
Reporting
The test report should identify the material and supplier designation, specimen dimensions and thickness, density where relevant, conditioning history, burner gas and flame height, individual afterflame and afterglow times, observations of dripping and cotton ignition, and the resulting classification. Reports issued by accredited third-party laboratories provide traceable evidence for regulatory submission, tender documentation, and quality-control records.
Applications
Vertical burning test data for thermal insulation materials support fire-safety decisions across multiple sectors:
- Building envelope insulation – Evaluating flame-retarded PU, PIR, phenolic, and polystyrene boards used in external wall, roof, and floor systems, in support of national fire classification schemes such as GB 8624 and EN 13501-1.
- HVAC and air-duct systems – Qualifying duct wrap, duct liner, and elastomeric foam insulation where proximity to air movement and ignition sources demands verified self-extinguishing behavior.
- Pipe and vessel insulation – Screening flexible foam tubes and rigid sections used on chilled-water, heating, and process piping in commercial and industrial plants.
- Refrigeration and appliances – Confirming the flammability performance of insulation foams used in cold rooms, display cabinets, and household appliance cabinets.
- Transportation – Supporting material qualification for thermal and acoustic insulation in rail, marine, and automotive applications, often in combination with cabin-interior flammability standards.
- Electrical and electronic equipment – Verifying that foam components installed within enclosures meet the UL 94 V requirements called up by product safety standards based on the IEC 60950 or IEC 62368 series.
FAQ
What is the vertical burning test for thermal insulation materials?
It is a standardized flammability test in which a vertically mounted insulation specimen is exposed twice to a small gas flame. The laboratory records afterflame time, afterglow time, dripping behavior, and cotton ignition, then assigns a classification such as UL 94 V-0, V-1, or V-2 that describes the material's self-extinguishing performance.
Which standards govern the test?
The core methods are defined in UL 94, IEC 60695-11-10, ASTM D3801, and GB/T 2408, which are closely harmonized for the 50 W vertical flame procedure. Flexible and textile-based products may be tested to ASTM D6413 or ISO 9773. For building products, the vertical burning result complements the reaction-to-fire classifications of GB 8624 and EN 13501-1.
What is the difference between V-0, V-1, and V-2?
The ratings differ mainly in permitted afterflame time and dripping behavior. V-0 is the most stringent, allowing no more than 10 seconds of afterflame per application and no flaming drips that ignite cotton. V-1 allows up to 30 seconds of afterflame with no cotton ignition, while V-2 permits flaming drips that ignite the cotton indicator, subject to timing limits.
How does the vertical test differ from the horizontal burning test?
The vertical orientation exposes the specimen to upward flame spread and preheating of unburned material, making it a more severe test than the horizontal configuration. Materials that pass a horizontal test may fail a vertical test, so the vertical method is specified where higher fire performance is required, such as around electrical components and in occupied building spaces.
Why can test results differ between laboratories?
Small-scale flammability results are sensitive to specimen thickness, density, cell structure, aging, and conditioning. Consistent specimen preparation, strict control of flame height and timing, and testing at the minimum product thickness all improve reproducibility. Selecting an accredited third-party laboratory with experience in foam materials reduces the risk of divergent results.
Does a V-0 rating mean the insulation is non-combustible?
No. A UL 94 V rating describes behavior under a small ignition source only. Polymer foam insulation remains a combustible material, and full-scale fire classification for building use must be established through the reaction-to-fire test suites referenced by GB 8624 or EN 13501-1, which assess heat release, flame spread, smoke production, and flaming droplets under larger-scale exposure.