Flame retardancy testing of thermal insulation materials determines how polymer foams, mineral wools, and elastomeric products resist ignition, flame spread, heat release, and smoke generation under standardized fire conditions. Test results are essential for demonstrating compliance with classification standards such as GB 8624, EN 13501-1, UL 94, and ISO 4589-2 across building, industrial, and transportation applications. This article explains the sample types, key test items, evaluation methods, and application requirements typically handled by accredited third-party laboratories.
Technical Overview
Thermal insulation materials occupy a broad spectrum of combustion behavior, ranging from non-combustible mineral fibers to highly combustible polymer foams such as expanded polystyrene (EPS), extruded polystyrene (XPS), and polyurethane (PU). Because insulation products are installed in walls, roofs, ducts, pipes, vehicles, and electrical enclosures, their fire performance directly affects overall fire safety. Flame retardancy determination therefore evaluates multiple parameters simultaneously: ease of ignition, flame propagation, heat release rate, smoke production, flaming droplets, and combustion gas toxicity.
Most regulatory frameworks express results as a classification grade rather than a single number. In China, GB 8624-2012 grades building materials from A1 and A2 (non-combustible to limited combustibility) through B and C (B1, flame retardant), D and E (B2, combustible), down to F (B3, highly combustible). The European system EN 13501-1 defines Euroclasses A1 to F with additional classifications for smoke production (s1 to s3) and flaming droplets (d0 to d2). For polymeric components, UL 94 ratings (HB, V-2, V-1, V-0, 5VB, 5VA) are the dominant benchmark in electrical and electronic applications, while ASTM E84/UL 723 reports a flame spread index and smoke developed index for interior finishes in North America. Accredited third-party laboratories select the appropriate method set based on the end use of the insulation product.
Testing Samples
Rigid Polymer Foam Insulation
Typical samples include PU and polyisocyanurate (PIR) board, phenolic foam, EPS, and XPS. Specimens are cut from finished boards at production thickness or at the thickness specified by the product standard, with cut surfaces flat and free of dust. Because foam density, cell structure, and flame retardant loading strongly influence results, samples must represent the actual commercial formulation.
Elastomeric Foam Insulation
Flexible rubber-plastic (NBR/PVC) insulation supplied as sheets or preformed tubes is tested in its installed configuration. Specimens may include both sheet material and tube sections, since curved geometry and closed-cell structure affect ignition behavior and after-flame time.
Fibrous and Mineral Insulation
Glass wool, rock (mineral) wool, ceramic fiber blanket, and calcium silicate board are sampled as representative boards, blankets, or pipe sections. These materials are primarily evaluated for non-combustibility, heat of combustion, and melting behavior rather than sustained flaming.
Specialty and Composite Insulation
Aerogel blankets, melamine foam, polyethylene foam, and composite boards with aluminum foil or glass fabric facings require testing of the complete composite assembly, because facings, adhesives, and stitching can alter flame spread, dripping, and smoke results compared with the core material alone.
Prior to testing, specimens are normally conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 48 hours (or to constant mass for hygroscopic fibrous products) to ensure reproducible moisture content, unless the governing standard specifies otherwise.
Testing Items
Depending on the target classification and application, a flame retardancy test program for insulation materials typically includes the following items:
- Limiting oxygen index (LOI) — the minimum oxygen concentration required to support candle-like combustion.
- Burning behavior classification per GB 8624 (A1, A2, B1, B2, B3) or Euroclass per EN 13501-1.
- Small-flame ignitability — surface and edge exposure, after-flame time, and ignition of filter paper by drips.
- Single Burning Item (SBI) parameters — FIGRA (fire growth rate index), THR600s, LFS (lateral flame spread), SMOGRA, and total smoke production.
- Heat release rate, peak heat release rate, and total heat release from cone calorimeter testing.
- Smoke density and specific optical density under flaming and non-flaming conditions.
- Smoke toxicity grading of combustion gases (CO, CO₂, HCN, HCl, and other species).
- Gross heat of combustion (calorific potential).
- Non-combustibility furnace test results — mass loss, sustained flaming, and furnace temperature rise.
- UL 94 vertical and horizontal burning ratings, including after-flame time, afterglow, and cotton ignition.
- Hot metal nut and glow-wire performance (GWFI, GWIT) for electrically adjacent insulation.
Testing Methods
Oxygen Index Method (ISO 4589-2 / ASTM D2863 / GB/T 2406, JIS K 7201)
A small specimen is vertically supported in a glass chimney and ignited at its top edge in a flowing nitrogen–oxygen mixture. The oxygen concentration is reduced stepwise until flaming combustion can no longer be sustained for the specified time or length. Rigid specimens are commonly 80–150 mm long by 10 mm wide at the relevant thickness, and at least 15 specimens are tested to bracket the LOI value. Because ambient air contains about 21% oxygen, LOI values above roughly 26–28% indicate self-extinguishing tendency, and many Chinese product specifications for B1-grade polymer insulation require LOI ≥ 32%.
Burning Classification per GB 8624-2012
The Chinese building-material classification combines several component tests. Ignitability is assessed with GB/T 8626 (a small propane flame applied to 250 mm × 90 mm specimens, both edge and surface exposed, with 30 s or 15 s application times depending on grade). The Single Burning Item test per GB/T 20284 exposes a specimen assembly consisting of a 1500 mm × 1000 mm short wing and a 1500 mm × 500 mm long wing to a 30 kW burner, yielding FIGRA, THR600s, LFS, and SMOGRA. For higher grades, ISO 1182 (750 °C furnace non-combustibility on 45 mm × 50 mm cylinders), ISO 1716 (oxygen bomb calorimetry for calorific value), and GB/T 20285 (smoke toxicity grading) are added, while flooring products follow the radiant panel method of GB/T 11785. The corresponding European route to EN 13501-1 uses EN ISO 11925-2, EN 13823 (SBI), EN ISO 1182, and EN ISO 1716.
Vertical and Horizontal Burning (UL 94 / IEC 60695-11-10 / GB/T 2408)
Specimens of 125 mm × 13 mm are clamped vertically or horizontally and exposed twice to a standardized 20 mm (or 50 mm for 5VA/5VB plaque tests on 150 mm × 150 mm plaques) flame. The classification depends on after-flame time after each application, total after-flame time across the specimen set, afterglow duration, whether flaming particles ignite surgical cotton placed below the specimen, and whether the flame reaches the clamp or holding fixture. The V-0 rating represents the most stringent vertical performance, followed by V-1 and V-2; HB denotes horizontal burning.
Cone Calorimetry (ISO 5660-1 / GB/T 16172)
A 100 mm × 100 mm specimen is irradiated at 25–75 kW/m² beneath a conical heater while exhaust gases are analyzed for oxygen consumption. The method provides time to ignition, heat release rate curve, peak and total heat release, effective heat of combustion, mass loss rate, and smoke production rate. It is widely used for product development and fire modeling, and for demonstrating performance in rail and marine applications.
Smoke Density and Toxicity (ASTM E662 / GB/T 8627 / GB/T 20285)
Specific optical density is measured in a closed chamber with 75 mm × 75 mm specimens exposed to a radiant heater with and without piloted ignition. Toxicity testing collects combustion effluent from an imposed decomposition condition and quantifies asphyxiant and irritant gases, which is particularly important for nitrogen- and chlorine-containing foams.
Tunnel and Radiant Panel Tests (ASTM E84 / UL 723 / ISO 9239-1)
The Steiner tunnel test burns a 0.61 m × 7.32 m specimen in a 25 ft furnace to determine a flame spread index and smoke developed index, with Class A corresponding to flame spread of 0–25. Horizontal radiant panel testing per ISO 9239-1 measures critical heat flux for extinguishment of flooring and roof insulation assemblies.
Applications
Flame retardancy determination directly supports regulatory approval and safe deployment of insulation in the following sectors:
- Building construction — external thermal insulation composite systems (ETICS), curtain wall infill, roof and floor insulation requiring GB 8624 or EN 13501-1 grades.
- HVAC and plumbing — duct wrap and pipe insulation requiring B1 or B2 grades, LOI data, and smoke limits.
- Cold chain and refrigeration — panel cores and pipe insulation for cold storage facilities.
- Rail transit — components tested against EN 45545-2 and TB/T 3138 using cone calorimeter and smoke toxicity data.
- Marine and offshore — insulation and linings qualified under the IMO FTP Code.
- Electrical equipment — foams and elastomers near live parts verified with UL 94 and IEC 60695-2 glow-wire methods.
- Industrial and energy facilities — fireproofing wraps, equipment insulation, and fire barriers for petrochemical plants.
FAQ
What oxygen index value qualifies insulation as flame retardant?
A material with an LOI below 21% burns readily in normal air. Values between 22% and 27% indicate slow-burning to self-extinguishing behavior, while values of 28% and above are generally regarded as flame retardant. Many B1-grade polymer insulation specifications in China require LOI of 32% or higher, so the acceptance threshold should always be checked against the applicable product standard.
How do GB 8624 grades relate to Euroclasses?
GB 8624-2012 is technically aligned with EN 13501-1: A1 and A2 correspond to Euroclasses A1 and A2; B and C together constitute B1; D and E constitute B2; and F corresponds to B3. The underlying test methods (furnace, calorimetry, SBI, and small-flame ignition) are also harmonized, so one test campaign can often support both certifications.
Which insulation materials can achieve A1 non-combustibility?
Inorganic products such as rock wool, glass wool, ceramic fiber, and calcium silicate typically pass the ISO 1182 furnace test and ISO 1716 calorific value limits for A1. Organic polymer foams, even when heavily flame retarded, are limited to A2 or, more commonly, B1 grades because of their inherent calorific content.
What sample quantities are needed for a full GB 8624 classification?
Small-flame ignition typically requires 10 to 20 specimens, the SBI test requires at least three complete wing assemblies, and furnace and calorimetry tests require several 45 mm cylinders and approximately 50 g of granulated material respectively. Accredited third-party laboratories usually request full-size production boards to cut all specimens from one homogeneous batch.
How long does flame retardancy testing take?
Individual apparatus tests such as LOI or UL 94 can be completed within one to two days, while a full classification involving SBI, furnace tests, calorimetry, and smoke toxicity, including conditioning and report review, typically requires two to four weeks. Retests triggered by borderline FIGRA or ignition results should be factored into project schedules.