Burning Duration Test for Thermal Insulation Materials: Principles, Methods, and Standards

The burning duration test for thermal insulation materials is a critical fire safety evaluation that measures how long a specimen continues to flame after removal of an ignition source. This article provides a comprehensive technical overview of the test principles, applicable standards such as ISO 11925-2, GB/T 8626, and EN 13501-1, sample preparation requirements, testing procedures, and practical applications in construction and product certification.

Technical Overview

The burning duration test, also referred to as the sustained flaming or afterflame duration test, is a fundamental fire performance evaluation for thermal insulation materials. During the test, a standardized specimen is exposed to a small ignition flame for a defined period, after which the ignition source is removed and the specimen is observed to determine whether flaming combustion continues and for how long. The recorded burning duration, together with flame spread distance and the production of flaming droplets or particles, directly influences the fire classification of the material.

Thermal insulation products are widely used in walls, roofs, floors, and piping systems, where they occupy concealed cavities and large surface areas. Because insulation materials can significantly influence fire growth within a building, regulatory frameworks in most jurisdictions require demonstrated control of ignitability and sustained combustion. Burning duration data provide regulators, designers, and manufacturers with quantifiable evidence of how a material behaves once ignition occurs, which is essential for classification under fire safety standards.

Key parameters evaluated during the test typically include:

• Ignition occurrence and time to sustained flaming
• Afterflame time (burning duration after removal of the ignition source)
• Flame spread distance relative to the gauge line
• Generation of flaming droplets or particles that ignite filter paper
• Specimen damage length and burn-through behavior

Accredited third-party laboratories perform these tests under controlled environmental conditions to ensure reproducible results suitable for certification, regulatory compliance, and product development purposes.

Testing Samples

Organic Foam Insulation Materials

Expandable polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PU), polyisocyanurate (PIR), and phenolic foam boards are among the most commonly tested insulation products. These combustible thermoset and thermoplastic foams are evaluated for burning duration because their low density and high surface area can promote rapid flame propagation and sustained combustion. Specimens are typically cut to standard dimensions, for example 250 mm × 90 mm strips for small-flame ignition testing, with thickness representative of the product as placed on the market.

Inorganic Fiber Insulation Materials

Mineral wool (rock wool and slag wool) and glass wool products are also subject to burning duration evaluation. Although these materials are inherently non-combustible or limited-combustible, they may contain organic binders and facings that can sustain brief flaming. Testing verifies that any binder combustion self-extinguishes quickly and that the product qualifies for the highest fire classification grades under relevant standards.

Composite and Faced Insulation Products

Metal-faced sandwich panels, foil-faced foam boards, paper- or glass-fleece-faced products, and reflective insulation assemblies are tested in their end-use configuration where required by the governing standard. The interaction between facings, adhesives, and core materials can significantly affect burning duration, so specimens must include all functional layers unless the standard permits testing of the core alone.

Pipe and Duct Insulation

Preformed pipe sections, flexible elastomeric foam tubes, and duct wrap materials are tested using specimen geometries adapted to their cylindrical or wrapped application. Burning duration results for these products support fire performance requirements in building services installations.

Testing Items

The burning duration test program generally encompasses the following measured and observed items:

• Burning duration (afterflame time) following removal of the pilot flame, recorded in seconds
• Flame spread distance, including whether flaming reaches the 150 mm gauge line within the specified exposure time
• Occurrence of flaming droplets or particles and whether they ignite the filter paper placed beneath the specimen
• Time to ignition, measured from initial flame application to the onset of sustained flaming
• Post-test char length, damaged length, and burn-through assessment
• Observed melting, shrinkage, deformation, or delamination behavior
• Additional supporting parameters such as gross heat of combustion (per ISO 1716) or heat release rate (per ISO 5660 or GB/T 20284) when full classification is required

Testing Methods

Burning duration is most commonly determined using the small-flame ignitability method described in ISO 11925-2 and its Chinese counterpart GB/T 8626, which forms part of the classification systems defined in EN 13501-1 and GB 8624 respectively. The general procedure is as follows:

• Specimen conditioning: samples are conditioned to equilibrium at a temperature of 23 ± 2 °C and relative humidity of 50 ± 5% for a minimum specified period, typically at least 24 hours or until mass stabilization.
• Specimen preparation: test pieces are cut to the dimensions prescribed by the standard, wrapped in aluminum foil on the rear face where required, and mounted vertically on the specimen holder behind a U-shaped clamp.
• Flame application: a standardized propane burner flame of 20 mm height, verified by calibration, is applied to the specimen surface or bottom edge at a 45° angle for an exposure duration of 15 or 30 seconds depending on the classification route.
• Observation phase: after removal of the ignition flame, the specimen is observed to record whether flaming persists, the total burning duration, the maximum flame spread, and whether flaming droplets or particles fall and ignite the filter paper.
• Documentation: all observations, timing measurements, and post-test specimen conditions are recorded, and the test is repeated on the required number of specimens to establish compliance.

Acceptance criteria vary with the target classification. For example, under GB 8624 and EN 13501-1, products classified as grade B, C, or D must satisfy the ignitability test requirements, which include flame spread not reaching the gauge line within the specified time, no ignition of filter paper by flaming droplets, and no sustained flaming exceeding the permitted duration. For products seeking grade A classification, additional non-combustibility testing per GB/T 5464 and ISO 1182, and gross heat of combustion per GB/T 5454 or ISO 1716, are required.

Complementary methods may be applied depending on the material and market. UL 94 vertical and horizontal burning tests and ASTM D6413 are widely used for polymeric and textile-based insulation components, particularly in electrical and industrial applications under IEC and UL frameworks. Japanese market requirements may reference JIS-based flame tests. When combined with the single burning item test per GB/T 20284 or EN 13823, burning duration data contribute to a complete fire performance profile.

Applications

Burning duration test results for thermal insulation materials serve multiple technical and regulatory purposes:

• Building code compliance: demonstrating conformity with mandatory fire performance classifications for exterior wall insulation, roofing systems, and interior linings under GB 8624, EN 13501-1, and equivalent national regulations.
• Product certification and CE or CCC marking support: providing the test evidence required for conformity assessment of insulation products placed on regulated markets.
• Quality control: verifying that production batches maintain the fire performance of the originally certified formulation, particularly after raw material or process changes.
• Research and development: comparing flame retardant systems, binder chemistries, and facing materials to optimize self-extinguishing behavior.
• Fire risk assessment and engineering design: supplying input data for fire safety engineers evaluating combustible contents within concealed cavities and façade systems.
• Tender and specification documentation: demonstrating to architects, contractors, and building owners that specified insulation products meet the required fire classes.

FAQ

What is burning duration in a fire test?

Burning duration, often called afterflame time, is the length of time a test specimen continues to flame after the ignition source has been removed. It is a key indicator of a material's ability to self-extinguish and directly affects its fire classification.

Which standards govern the burning duration test for insulation materials?

The primary methods are ISO 11925-2 and GB/T 8626 for small-flame ignitability, supported by classification standards EN 13501-1 and GB 8624. For polymeric components, UL 94 and ASTM D6413 are also commonly applied, and supplementary tests such as ISO 1182, ISO 1716, and EN 13823 may be required for full classification.

How are specimens prepared for the test?

Specimens are cut to standard dimensions representative of the product in its end-use state, conditioned at 23 ± 2 °C and 50 ± 5% relative humidity, mounted vertically with any required foil backing, and fitted with filter paper below to capture flaming droplets.

What results cause a specimen to fail?

Typical failure criteria include flame spread reaching the 150 mm gauge line within the prescribed time, flaming continuing beyond the permitted duration after removal of the pilot flame, and flaming droplets or particles igniting the filter paper.

Can fire-retardant-treated foams pass the test?

Yes. Flame-retardant modifications to EPS, XPS, PU, and PIR foams can substantially reduce burning duration and prevent sustained flaming, enabling qualifying products to achieve B-grade or higher classifications when supported by test evidence from accredited third-party laboratories.

How long does the test take?

The flame exposure itself lasts only 15 or 30 seconds, with an observation period of a few minutes per specimen. Including conditioning, mounting, and multiple replicate specimens, a complete test series is typically completed within one working day in the laboratory.