Determination of the Limiting Oxygen Index (LOI) of Thermal Insulation Materials

Oxygen index testing determines the minimum oxygen concentration, expressed as a volume percentage in an oxygen–nitrogen mixture, that will just sustain flaming combustion of a thermal insulation material, providing a reproducible measure of ignitability and flame resistance. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the limiting oxygen index (LOI) is a core fire-safety indicator for EPS, XPS, PUR/PIR, phenolic, and elastomeric foam insulation products. This article explains the test principle, apparatus, specimen preparation, procedure, calculation, acceptance criteria, and typical applications of oxygen index determination for insulation materials.

Technical Overview

The oxygen index (OI), also known as the limiting oxygen index (LOI) or critical oxygen index, is defined as the minimum concentration of oxygen in a flowing mixture of oxygen and nitrogen, expressed as a percentage by volume, that will just support flaming combustion of a vertically mounted specimen initially at room temperature. Because ordinary air contains approximately 20.9% oxygen, the LOI provides an intuitive benchmark of flammability: a material with an LOI below 21% will continue to burn in normal air, whereas a material with a higher LOI requires an oxygen-enriched atmosphere to sustain combustion and is therefore more difficult to ignite and less likely to propagate flame.

Thermal insulation materials are frequently installed in concealed spaces such as wall cavities, roof assemblies, and around pipes and HVAC ducts, where fire performance is critical. The oxygen index test is a small-scale, economical, and highly reproducible method widely used for product development, quality control, conformity verification against product standards, and supporting evidence in fire-classification programs such as GB 8624 and EN 13501-1. Typical interpretation of LOI results is summarized below:

• LOI below 21%: readily combustible; the material will burn freely in ambient air.
• LOI between 21% and 27%: combustible; the material burns in air but with reduced vigor.
• LOI of 27% to 28% and above: generally regarded as self-extinguishing or flame-retardant.
• LOI of 32% and above: high flame retardancy, frequently specified for Class B1 flexible elastomeric foam insulation and similar products.

Testing Samples

Oxygen index testing is applicable to most polymeric insulation materials in board, tube, sheet, or block form. Specimens are cut from representative production samples, and facings, skins, and cores may be tested separately when the product construction warrants it.

Rigid Foam Insulation

This category includes expanded polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PUR) and polyisocyanurate (PIR) foams, and phenolic foam. Specimens are typically cut as vertical bars 80 to 150 mm in length, 10 mm in width, and 4 to 10 mm in thickness in accordance with ISO 4589-2 and GB/T 2406.2, while ASTM D2863 commonly uses bars 6.5 mm wide and 3 mm thick. Dense surface skins are usually removed so that the measured value reflects the foam core, unless the facing itself is the object of the test.

Flexible Foam Insulation

Flexible elastomeric foams based on nitrile-butadiene rubber or EPDM, as well as polyethylene and other polyolefin foams, are tested in larger cross-sections (for example, 50 mm by 13 mm) to limit distortion and shrinkage away from the ignition flame. Specimens that melt, drip, or collapse require careful observation, and the operator records any behavior that departs from standard burning criteria so it can be noted in the test report.

Fibrous and Mineral-Based Insulation

Glass wool, rock (stone) wool, and ceramic fiber products are fundamentally non-combustible, and their LOI values often exceed the practical range of the apparatus. Oxygen index testing of these products is mainly applied to combustible binders, facings, or composite layers, and the result is used for comparative purposes rather than classification. Thin facing films follow the specific specimen provisions for films and sheeting in the applicable standard.

Specimen Conditioning

Unless otherwise specified by the governing product standard, specimens are conditioned at 23 ± 2°C and 50 ± 5% relative humidity for a minimum of 88 hours in accordance with ISO 291. A test typically consumes 15 to 20 specimens, and additional backup specimens should be prepared because each trial uses a fresh specimen. For methods based on ISO 4589-2 and GB/T 2406.2, a reference mark is drawn 50 mm below the top edge of each specimen.

Testing Items

Oxygen index determination for thermal insulation materials typically covers the following laboratory services, which are available at accredited third-party laboratories as well as in manufacturers' in-house quality facilities:

• Ambient-temperature limiting oxygen index (LOI) determination per ISO 4589-2, EN ISO 4589-2, GB/T 2406.2, and ASTM D2863.
• Elevated-temperature oxygen index determination per ISO 4589-3 for materials intended for higher service temperatures.
• Temperature index (TI) determination, defined as the temperature at which the LOI falls to the oxygen concentration of ambient air, approximately 20.9%.
• Verification of declared LOI minimum values referenced by product standards for EPS and XPS boards, flexible elastomeric cellular insulation, and related products.
• Comparative LOI screening to support flame-retardant formulation development, additive loading optimization, and aging or durability studies.

Testing Methods

Test Principle and Apparatus

The apparatus consists of a vertical borosilicate glass chimney, typically at least 450 mm high with an internal diameter of 75 to 100 mm, mounted above a gas-distribution bed of glass beads that homogenizes the upward flow. Oxygen and nitrogen of at least 98% purity are supplied through pressure regulators, needle valves, and calibrated flow meters or mass-flow controllers, which are adjusted to deliver a total linear gas velocity of 40 ± 10 mm/s through the column. The specimen is clamped vertically in a holder at the base of the chimney and ignited at its top edge using a small propane diffusion flame. A timing device and, where required, an oxygen analyzer complete the setup.

Test Procedure

• Estimate an initial oxygen concentration based on the known or expected behavior of the material, set the oxygen and nitrogen flows accordingly, and allow the gas stream to purge and stabilize the chimney atmosphere for at least 30 seconds.
• Mount a conditioned specimen vertically, apply the ignition flame to the top edge for up to 30 seconds, briefly withdrawing the flame at intervals to observe whether ignition has occurred.
• After removal of the igniter, record whether the flame front reaches the 50 mm reference mark or the specimen continues to burn beyond the limiting time specified by the standard (commonly 180 seconds), which constitutes a "burning" response; if the specimen extinguishes before either criterion is met, the response is "no burning."
• If the specimen does not burn, increase the oxygen concentration; if it burns, decrease the oxygen concentration, using a fresh specimen for every trial and progressively narrowing the step size to 0.5% or 0.2%.
• Continue the series until the transition between burning and no-burning responses is bracketed, then calculate the LOI using the nearest-tail-mean (NTM) method from the final confirmatory series of trials.
• Verify the calculated result by testing one additional specimen at the calculated LOI; the specimen must not satisfy the burning criteria for the value to be accepted.

Calculation and Expression of Results

The oxygen concentration of the gas mixture is calculated as the volume percentage of oxygen, that is, the oxygen flow divided by the total oxygen plus nitrogen flow, corrected according to the calibration of the flow measurement devices. The LOI is expressed to one decimal place as a volume percent. Based on interlaboratory data, repeatability within a single laboratory is typically within about 0.5% oxygen, while reproducibility between laboratories is on the order of 1% to 1.5% oxygen. The test report should identify the standard used, the material and its identification, specimen type and dimensions, conditioning history, the calculated LOI, any deviations from the burning criteria such as dripping or shrinking, and the date of testing.

Elevated-Temperature Testing

ISO 4589-3 extends the oxygen index method to elevated temperatures using a heated chimney assembly. The LOI is measured at a series of temperatures, and the temperature index is derived as the temperature at which the LOI equals the oxygen content of ambient air. This characterization is valuable for insulation materials used near heat sources, such as pipe insulation on hot services, because flame retardancy measured at room temperature may decrease significantly as temperature rises.

Applications

Oxygen index determination supports a wide range of engineering, regulatory, and commercial activities involving thermal insulation materials:

• Conformity demonstration against minimum LOI requirements cited in product standards for EPS and XPS boards, flexible elastomeric foam insulation, phenolic foam, and related products, and supporting data packages for GB 8624 fire-classification files.
• Routine quality control, incoming inspection, and batch release testing of flame-retarded insulation materials in manufacturing plants.
• Research and development of flame-retardant systems, including halogenated, phosphorus-based, intumescent nitrogen–phosphorus, and mineral-filler formulations, and optimization of additive loading levels.
• Assessment of the durability of flame retardancy after thermal aging, humidity exposure, or UV conditioning, which is important because blowing agents and additive flame retardants can migrate or degrade over time.
• Comparative material selection by building designers, HVAC and refrigeration engineers, and OEM specifiers choosing between competing insulation products.
• Failure analysis and post-incident fire investigations where the residual combustibility of installed insulation must be characterized.

FAQ

The following questions address the most common technical and commercial inquiries received regarding oxygen index testing of thermal insulation materials.

What oxygen index value qualifies insulation as flame retardant?

As a widely accepted convention, materials with an LOI of 27% to 28% and above are considered self-extinguishing or flame retardant. In practice, many procurement specifications for building insulation demand higher values: minimum LOI values of 30% or 32% are common for foam insulation boards, and high-grade flexible elastomeric insulation products are often specified at 35% or higher.

Which standards govern oxygen index testing?

ISO 4589-2 (adopted in Europe as EN ISO 4589-2) and GB/T 2406.2 govern ambient-temperature oxygen index determination for plastics, ISO 4589-3 covers elevated-temperature testing, and ASTM D2863 is the equivalent North American method. GB/T 5454 applies to textile fabrics used as insulation wraps and jacketing. UL 94 is a related but distinct flammability test that evaluates burning behavior in normal air rather than measuring the critical oxygen concentration.

Does a high LOI guarantee compliance with fire classification grades?

No. The LOI is a small-scale screening indicator of ignitability. Formal classification under GB 8624 or EN 13501-1 requires a full suite of fire tests, including single-flame-source tests, the single burning item (SBI) test, and cone calorimetry. A high LOI is strong supporting evidence of flame retardancy but cannot, by itself, establish a fire classification grade.

How many specimens are required for a test?

A typical determination consumes 15 to 20 conditioned specimens, because each individual trial uses a fresh specimen and the binary search procedure, followed by the confirmatory tail series and a verify specimen, requires multiple trials. Materials with variable or borderline behavior may require additional specimens.

What factors can influence the test result?

Specimen geometry and density, the presence or absence of skins and facings, conditioning history, aging of flame retardants, gas purity, calibration of flow measurement devices, chimney flow velocity, and operator judgment at the burning transition all affect the outcome. Consistent specimen preparation and strictly controlled laboratory conditions are therefore essential for obtaining comparable results.

Can faced or laminated insulation products be tested?

Yes. Composite products are normally tested by separating the foam core and the facing materials, since thin films and facings are governed by different specimen provisions in the standards. Testing the components separately provides clearer insight into which element governs the fire behavior of the finished product, and both results should be documented in the report.