Reaction to fire

Reaction to fire describes how a material or product contributes to the development of a fire under specified exposure conditions. It includes the behaviour leading to ignition and the subsequent release of heat and combustion products. Heat release rate is a central measure because it links burning material to the thermal conditions developing around it.

The response depends on more than the material's name. Thickness, orientation, mounting, incident heat and the surrounding enclosure can change the observed behaviour. Results from a small specimen therefore need interpretation before they are applied to a complete product or room. A test classification retains meaning through the conditions and application rules attached to it.

Reaction to fire is distinct from the ability of an assembly to maintain its function during fire exposure. A combustible material can form part of a fire-resisting assembly, while a noncombustible material can lose mechanical performance when heated. The two assessments describe different parts of building fire safety.

Contribution to a developing fire

A material can have several roles in the same building. It may carry loads, insulate another component, and supply fuel when heated. Kodur and Harmathy distinguish these functions because a useful structural material can become an additional fire source. A description of its room-temperature strength therefore says little about its contribution to fire development.

Combustibility concerns whether a material can burn. Reaction to fire examines a wider sequence, including the exposure needed for ignition and the subsequent rate of energy release. A combustible product that releases energy slowly can produce a different fire from one that burns rapidly. Material mass alone cannot describe that difference.

Fire resistance addresses another question: how long a building assembly maintains specified functions during a defined fire exposure. These functions include carrying loads and limiting heat transmission or the passage of flames. Reaction-to-fire evidence characterises the product's contribution to the fire; it does not establish the endurance of a complete wall, floor or protected member.

Heating, ignition and flame spread

Before a solid ignites, incoming heat raises its surface temperature and penetrates its interior. Some energy returns to the surroundings through radiation and convection. Moisture evaporation can absorb further energy before substantial decomposition occurs. The resulting balance depends on the exposure, material thickness and thermal properties.

Heating can produce combustible gases through thermal decomposition, also called pyrolysis. These gases mix with surrounding air and may ignite when suitable conditions develop. Piloted ignition involves an external ignition source, such as a small flame or spark. Ignition without such a pilot requires the gas mixture itself to reach conditions that sustain the reaction.

Flame spread extends burning into previously unburned material. Heat from the advancing flame prepares the next surface region to release combustible gases. The flame front and the boundary of the decomposing solid are related, but they are not identical physical boundaries. Surface orientation and airflow affect the heat transferred ahead of the flame.

Spread and burnout compete during fire growth. If new surface becomes involved before the earlier region stops burning, the active burning area can increase. If the earlier fuel burns out faster than the flame advances, growth can slow or stop. This explains why a spread measurement requires an account of both material behaviour and the surrounding exposure.

Reaction to fire covers several material responses. Based on Janssens (2002), pp. 3-38–3-39 and Quintiere (2002), p. 2-247.

Heat release and mass loss

Heat release rate measures the rate of thermal energy production, expressed in kilowatts. Mass loss rate measures how quickly the specimen loses mass, expressed in kilograms per second. The effective heat of combustion connects these quantities under the particular burning conditions. It represents the energy released per unit mass lost, rather than a complete description of the fuel's chemistry.

Multiplying the mass loss rate by the effective heat of combustion gives the corresponding heat release rate. The following form uses kilojoules per kilogram, so that the result is directly in kilowatts. Both quantities must refer to the same specimen and stage of burning.

$$\dot{Q}=\dot{m}\mathrm{\Delta }{h}_{c,\mathrm{e}\mathrm{f}\mathrm{f}}$$

Symbol	Description	Unit
$\dot{Q}$	Heat release rate	kW
$\dot{m}$	Mass loss rate	kg/s
$\mathrm{\Delta }{h}_{c,\mathrm{e}\mathrm{f}\mathrm{f}}$	Effective heat of combustion	kJ/kg

The effective heat of combustion need not remain constant during a test. Babrauskas describes a 17 mm Western red cedar specimen exposed to an irradiance of 65 kW/m². Its effective heat of combustion was approximately 12 MJ/kg during the earlier period and approximately 30 MJ/kg during later char burning. Using one fixed value throughout would conceal this change.

A unit-rate calculation illustrates the consequence without reconstructing that experiment. At an assumed mass loss rate of 0.001 kg/s, the two source values give 12 kW and 30 kW. The second result is 2.5 times the first, despite identical assumed mass loss rates. These calculated powers are illustrative normalisations, not measured powers from the cedar test.

The effective value also differs from a calorific value measured in an oxygen bomb calorimeter. That apparatus burns a small specimen under high-pressure oxygen. Janssens explains that these conditions differ from practical fires, and that effective values for many solids are lower. Substituting a bomb-calorimeter value into a fire model can therefore change the predicted energy release.

Quintiere gives a simple illustration of surface spread on an upholstered chair. A burning region initially 6 cm across grows to 18 cm across after one minute at the stated radial spread rate. The corresponding circular area becomes nine times larger. This is a geometric illustration using the source's dimensions, rather than a prediction for every upholstered product.

The distinction between spread distance and burning area matters when interpreting such an example. A front advances along a boundary, while energy release comes from the active burning surface. Growth in that surface can provide more heat feedback and accelerate further involvement. The material's burning duration determines whether the earlier regions remain active as the boundary advances.

What tests measure

Fire testing can examine a small specimen, a complete product or an assembly in a room. These scales expose different parts of the burning process. A specimen test offers controlled exposure and detailed measurements, while a room test includes interactions with the enclosure. Neither description alone establishes that one test can replace the other.

In a room-corner test, lining materials are mounted on the walls or ceiling of a small enclosure. A burner near a rear corner provides the ignition exposure. Combustion products leave through the opening and enter a collection hood. Measurements in the exhaust can include gas concentrations, flow and smoke obscuration, alongside the information needed to determine heat release.

The Single Burning Item test uses two wall panels arranged as a corner without a ceiling. Babrauskas describes it as a European regulatory heat-release test in the 2002 handbook. Its geometry differs from the enclosed room-corner arrangement. The Euroclass system is the related classification topic; the historical test description alone does not provide a complete classification procedure.

Calorimeter results depend on specimen mounting, orientation, geometry and exposure. Janssens reports comparisons in which different apparatuses produced different peaks or time-dependent heat-release behaviour. Agreement for an average value did not necessarily imply agreement for the peak. The measured quantity and averaging period therefore belong with any reported result.

Reaction-to-fire tests address different scales and feedback. Based on Janssens (2002), p. 3-39 and Babrauskas (2002), p. 3-33.

Oxygen-consumption calorimetry estimates heat release through the oxygen consumed during combustion. Babrauskas reports a commonly used conversion of 13.1 MJ per kilogram of oxygen consumed. He also notes that some substances differ substantially from that convention. Knowing the composition can support a correction, so the conversion is an approximation with a physical basis rather than an exact constant for every fuel.

The gas measurement belongs to a complete collection and flow-measurement arrangement. In the room-corner description, the exhaust carries combustion products into the instrumented duct. Gas concentration and total flow are therefore connected parts of the measurement. A concentration by itself does not specify how much material or energy passes through the system per unit time.

Why total energy and peak power differ

The total heat released describes energy over the complete burning period. Peak heat release rate describes the largest rate observed during that period. A large total energy does not necessarily imply the highest peak power. Babrauskas illustrates this distinction through wardrobe tests involving different panel materials and thicknesses.

One tested wardrobe used unpainted plywood panels 3.2 mm thick, with 36.0 kg of combustible construction and 1.93 kg of contents. Its reported peak was 6,400 kW and its total heat release was 590 MJ. A 19 mm particleboard wardrobe contained 120.3 kg of combustible construction and 0.81 kg of contents. Its reported peak was 1,900 kW, while its total heat release was 1,349 MJ.

The thinner plywood package therefore produced approximately 3.4 times the peak power of the particleboard package. The particleboard package nevertheless released approximately 2.3 times as much total energy. These ratios are calculated from the reported tests, whose constructions and contents differed. They illustrate package behaviour and cannot establish a universal ranking of plywood against particleboard.

The enclosure can further alter burning. Radiation from hot gases and surrounding surfaces supplies additional energy to the fuel, while ventilation can restrict combustion. A product burning freely in an open calorimeter may therefore behave differently inside a compartment. Babrauskas specifically cautions against extrapolating a room-test result to rooms with larger ventilation openings.

Classification and use in buildings

Classification connects specified test results with a defined regulatory framework. The intended use matters because a wall lining, floor covering and structural member serve different functions. New Zealand's C/VM2, second edition of 2025, treats rapid spread over internal surface linings as a specific design scenario. Its criteria depend on location, building use and importance level.

In that New Zealand method, exposed internal wall and ceiling surfaces receive material Group Numbers. The commentary associates ordinary timber products with Group 3 and many fire-retardant-treated timbers with Group 2. It identifies exposed polyurethane foams as examples associated with Group 4. These are explanatory examples, rather than classifications transferable to every product bearing the same material name.

Flooring is assessed through critical radiant flux in the same document. A higher critical flux corresponds to better performance in the specified flooring assessment. For sleeping areas and exitways in care or detention buildings, the 2025 table gives 4.5 kW/m² without sprinklers and 2.2 kW/m² with sprinklers. These are New Zealand scenario requirements, not general ignition thresholds for all materials.

C/VM2 critical radiant flux depends on occupancy and sprinklers. Based on Ministry of Business, Innovation and Employment (2025), p. 52.

The distinctions reflect the role of the space. C/VM2 gives greater emphasis to linings along escape routes, where they may form a substantial part of the available fuel. Occupants' ability to evacuate and the presence of sprinklers also affect the specified criteria. Applying one classification requirement throughout every occupancy would remove these distinctions.

Smoke production and treatment effects

Heat release and smoke-related hazards are connected but are not interchangeable measurements. Janssens expresses the production rate of a toxic gas as its yield multiplied by the mass loss rate. A high yield alone does not establish the largest total release. The burning rate and its variation during the exposure are also part of the assessment.

A fire-retardant treatment can change more than one measured property. Janssens discusses treatments that increase a toxic-gas yield while reducing mass loss sufficiently to reduce the assessed hazard. That example explains why treatment performance cannot be judged from one isolated number. Its interpretation depends on the complete combination of burning rate, yields and exposure conditions.

Smoke obscuration is another measurement available in calorimetry. It describes the effect of suspended products on transmitted light, rather than directly measuring structural endurance or the complete toxic hazard. A test reporting smoke obscuration therefore answers a different question from a furnace test of a separating assembly. The measurements become useful when each remains connected to its own physical quantity.

Evidence and uncertainty

Heat-release measurements contain both systematic bias and random variation. Babrauskas describes calibration with a metered gas supply as one means of addressing bias. Comparisons between laboratories reveal variability that a single test cannot show. The historical interlaboratory results in the handbook are evidence about those apparatuses and trials, rather than permanent uncertainty allowances for every later test.

A model that uses small-specimen results introduces a further step between measurement and application. Material properties derived from those results must reproduce the processes relevant to the larger fire. Janssens describes approaches ranging from full-scale experiments to calculations using bench-scale data. Their suitability depends on the fire scenario, the available measurements and the treatment of enclosure feedback.

Even the same product can present different exposed areas and orientations in different applications. A room-corner arrangement provides different heat feedback from an isolated specimen, while a mounting method can alter burning. A defensible comparison consequently identifies the product, construction, exposure, measured quantity and test configuration together. Removing those conditions can turn a valid measurement into an unsupported prediction.

See also

• Flame retardant
• Heat release rate
• Ignition

References

• Babrauskas, V. (2002). Heat release rates. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-1–3-37). National Fire Protection Association.
• Janssens, M. (2002). Calorimetry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-38–3-62). National Fire Protection Association.
• Kanury, A. M. (2002). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-229–2-245). National Fire Protection Association.
• Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-155–1-181). National Fire Protection Association.
• Milke, J. A. (2002). Analytical methods for determining fire resistance of steel members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-209–4-238). National Fire Protection Association.
• Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.).
• Quintiere, J. G. (2002). Surface flame spread. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-246–2-257). National Fire Protection Association.