Oxygen Index Testing of Thermal Insulation Materials: Principles, Standards, and Procedures

The oxygen index test, also known as the limiting oxygen index (LOI) test, is a standardized laboratory method for evaluating the flammability of thermal insulation materials such as PIR/PUR foam, phenolic foam, polystyrene boards, and elastomeric rubber foam. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test determines the minimum oxygen concentration required to sustain candle-like combustion of a vertically mounted specimen. This article explains the test principle, sample requirements, testing procedure, result interpretation, and typical applications of oxygen index testing for insulation product development, certification, and quality control.

Technical Overview

The oxygen index (OI), or limiting oxygen index (LOI), is defined as the minimum concentration of oxygen, expressed as a percentage by volume, in a flowing mixture of oxygen and nitrogen that will just support flaming combustion of a material. Because ambient air contains approximately 20.9% oxygen, the LOI value provides an immediate indication of how readily a material will burn under normal atmospheric conditions: materials with an LOI below 21% can ignite and continue burning in air, while materials with higher LOI values are progressively more difficult to ignite and more likely to self-extinguish.

For thermal insulation materials, which are frequently installed in walls, roofs, HVAC ducting, and industrial equipment, the oxygen index test is one of the most widely used small-scale screening methods for flame-retardant performance. The principal test standards include:

• ISO 4589-2 (Determination of burning behaviour by oxygen index, ambient temperature test)
• ASTM D2863 (Minimum oxygen concentration to support candle-like combustion of plastics)
• GB/T 2406.2 (Chinese national standard, technically aligned with ISO 4589-2)
• JIS K 7201-2 (Japanese method for polymers based on the same principle)
• ISO 4589-3 (oxygen index testing at elevated temperature, used to derive a temperature index)

As a general guideline for interpreting results on organic foam insulation:

• LOI below 21%: readily combustible; the material sustains burning in ordinary air (for example, non-flame-retardant polystyrene or polyethylene foam)
• LOI of 22% to 25%: slow-burning with a tendency to self-extinguish
• LOI of 26% to 30%: generally regarded as flame-retardant; typical of flame-retardant EPS/XPS and PIR products
• LOI above 30%: high flame retardancy; typical requirements for B1-class elastomeric foam insulation and phenolic foam, which often exceeds 38%

Testing Samples

Rigid Foam Insulation Boards

Rigid foam products such as polyisocyanurate (PIR), polyurethane (PUR), phenolic foam, and extruded or expanded polystyrene (XPS/EPS) boards are the most common subjects of oxygen index testing. Specimens are cut from representative samples of the finished board with smooth, clean edges and no skin defects or crushed cells. Typical specimen dimensions are 80 mm to 150 mm in length with a 10 mm by 10 mm cross-section for cellular materials, in accordance with the specimen types specified in ISO 4589-2 and ASTM D2863.

Flexible and Elastomeric Insulation

Flexible elastomeric foam (NBR/PVC rubber foam), flexible polyurethane foam, and polyethylene foam used for pipe insulation and duct lagging are tested in their as-supplied form where possible. Because soft, low-density foams may deform in the rising gas stream, a support frame or specimen holder may be used, provided the holder does not interfere with gas flow or flame spread along the specimen surface.

Film, Granular, and Other Product Forms

Facings, membranes, and thin film components of insulation systems are tested using a film support (a small frame or wire holder) specified in the standards. Loose-fill or granular insulation can be evaluated by loading the material into a fine mesh cylinder, although alternative reaction-to-fire tests are often more appropriate for such products. Mineral wool products, being essentially non-combustible, are normally characterized by non-combustibility tests rather than the oxygen index method.

Testing Items

A standard oxygen index test campaign for insulation materials typically includes the following deliverables:

• Limiting oxygen index at ambient temperature (23 °C), reported to one decimal place as a volume percentage
• Observation of burning behavior, including afterflame time, dripping, flaming debris, and whether combustion reaches the 50 mm measurement mark
• Identification of the ignition method applied (top-surface ignition, Method A, or propagation ignition, Method B)
• Optional temperature-index testing per ISO 4589-3, in which the oxygen index is measured at elevated temperatures or the temperature at which the LOI falls to a specified value is determined
• Comparison of measured values against specification limits, such as the minimum LOI requirements stated in product standards or purchase specifications

Testing Methods

Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney (typically 450 mm or more in height), a vertical specimen holder mounted centrally in the chimney, a gas mixing and metering system supplied by oxygen and nitrogen cylinders of at least 98% purity, calibrated flow controllers or an oxygen analyzer, a small propane or butane flame igniter, and timing and measuring devices. The total mixed gas flow is regulated to achieve a linear velocity of approximately 40 mm/s through the chimney as required by the standards.

Specimen Conditioning

Specimens are conditioned to equilibrium at 23 ± 2 °C and 50 ± 10% relative humidity prior to testing, normally for a minimum of 88 hours per ISO 291 or an equivalent conditioning period per ASTM D618. At least 15 specimens are usually prepared for each material, since the bracketing procedure requires multiple concentration steps and confirmation runs.

Test Procedure

• Mount the specimen vertically in the holder with the top of the specimen a defined distance below the top of the chimney, then establish the initial oxygen concentration (21% for unknown materials or an estimated value based on prior data).
• Purge the chimney with the gas mixture for at least 30 seconds to ensure a stable atmosphere before ignition.
• Apply the ignition flame to the top of the specimen in accordance with Method A (top-surface ignition) or Method B (propagation ignition), following the prescribed application and pause intervals.
• Observe the response: the specimen "passes" at a given concentration if burning extinguishes before 180 seconds of afterflame or before the flame reaches the 50 mm mark below the top; otherwise it "fails".
• Adjust the oxygen concentration upward or downward in defined steps to bracket the critical concentration, then confirm the result with additional specimens at the bracketing concentrations.

Calculation and Expression of Results

The limiting oxygen index is calculated as the volume percentage of oxygen in the total gas mixture at the critical concentration: LOI = 100 × V(O2) / [V(O2) + V(N2)]. The final value is reported to one decimal place, together with the specimen dimensions, ignition method, conditioning history, and a description of any dripping or flaming debris. Repeatability within a laboratory is typically on the order of a few tenths of a percent oxygen when apparatus calibration and gas metering are properly controlled.

Applications

Building Product Classification and Specification Compliance

Oxygen index results support the evaluation of insulation materials against flame-retardant requirements in building codes and product standards. In the Chinese market, for example, product standards for polystyrene insulation boards commonly require a minimum oxygen index of 30% for flame-retardant grades as part of demonstrating compliance with GB 8624 fire performance classes. Similar thresholds appear in specifications for elastomeric foam used in HVAC and refrigeration pipework.

Factory Quality Control

Because the test is fast, repeatable, and requires only a small sample, it is widely used by insulation manufacturers for routine batch release testing and incoming inspection of flame-retardant raw materials such as halogenated or phosphorus-based additives. A downward drift in LOI provides an early warning of under-dosed flame retardant or a raw material substitution.

Research, Development, and Sector-Specific Requirements

In formulation development, the LOI value is a convenient metric for comparing flame-retardant packages, foam densities, and polymer chemistries. The test is also referenced in sector-specific material selection schemes, including railway applications under EN 45545-2, where LOI data supplement larger-scale reaction-to-fire tests, and in screening programs for marine, aerospace, and wire and cable components.

FAQ

What oxygen index value qualifies an insulation material as flame retardant?

There is no single universal threshold, but by convention materials with an LOI of 26% or above are considered flame retardant, since they will not easily sustain burning in air. Many building insulation specifications set the limit higher, commonly at 30% or 32%, particularly for polystyrene boards and elastomeric foam products intended for B1-class applications.

Are ISO 4589-2, ASTM D2863, and GB/T 2406.2 results directly comparable?

The three standards are based on the same principle and yield very similar values, but they differ in details such as specimen dimensions, chimney geometry, and ignition criteria. For regulatory or contractual purposes, the standard cited in the applicable specification should be followed exactly, and accredited third-party laboratories can confirm which method a given market requires.

How many specimens are needed for one test?

A complete determination normally consumes 15 or more specimens because the bracketing procedure requires repeated trials at different oxygen concentrations plus confirmation runs. Specimens should all be cut from representative material and conditioned identically.

Is a high LOI sufficient to declare a material safe for building use?

No. The oxygen index test is a small-scale ignitability screening method. Formal reaction-to-fire classification for construction products additionally relies on larger-scale tests such as the single burning item test and flame spread tests under EN 13501-1, GB 8624, or equivalent national frameworks. LOI data are best used for material comparison, quality control, and as supporting evidence within a broader fire performance dossier.