Fibre Reinforced Facade Encyclopedia: Testing and standards
Heat release rate is the rate at which a burning material or object gives off energy, measured in kilowatts, or in kilowatts per square metre of burning surface when the specimen area is known. It is the single most useful number in fire science: almost everything else that matters in a fire, including temperature, smoke production, flame height and the time available to escape, follows from how fast energy is being released. Heat release rate is a reaction-to-fire quantity. It describes how a material burns, not how long an assembly holds a fire back.
A burning wastepaper basket releases roughly 50 to 150 kW. A single upholstered armchair peaks around 500 kW to 1 MW. Flashover in a small room happens at about 1 MW. The classification tests all work in this range and report it in the same units.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Symbol and unit | HRR, in kilowatts (kW) | Per unit area in kW/m² for bench tests |
| Measurement principle | Oxygen consumption calorimetry | About 13.1 MJ of heat per kilogram of oxygen consumed |
| Bench test | AS/NZS 3837 cone calorimeter (ISO 5660-1, ASTM E1354) | Specimen 100 by 100 mm at 50 kW/m² irradiance |
| Intermediate test | EN 13823 single burning item | 30.7 kW burner in a corner, 20 minutes |
| Full room test | AS ISO 9705 room corner test | Burner 100 kW then 300 kW; flashover taken as 1 MW |
| Peak HRR | Highest instantaneous value, kW/m² | Governs the worst moment of the fire |
| Total heat release | Integral of HRR over time, in MJ/m² | THR600s in MJ is used in EN 13501-1 |
| Fire growth rate index | FIGRA, in W/s | HRR divided by time, used for Classes A2 to D |
| Flashover threshold | About 1 MW in the AS ISO 9705 room | Sets the group number boundaries |
How it is measured
Heat release rate is almost never measured directly. It is calculated from the oxygen consumed by the fire, using the observation that burning most common organic materials releases close to 13.1 megajoules of heat for every kilogram of oxygen consumed, within about five percent regardless of the fuel. That constant is what makes the whole family of modern fire tests possible.
In practice the specimen burns under a hood, all the combustion products are drawn into an exhaust duct, and instruments in the duct measure the flow rate and the oxygen, carbon dioxide and carbon monoxide concentrations. Comparing the oxygen in the exhaust with the oxygen in the incoming air gives the mass of oxygen consumed per second, and multiplying by 13.1 MJ/kg gives the heat release rate. The same technique scales from a 100 millimetre square specimen in a cone calorimeter to a full room in AS ISO 9705.
Peak, total and rate of growth
A heat release curve is more informative than any single number, and the classification systems each pull a different feature out of it.
- Peak HRR, in kW/m², is the worst instantaneous output. It sets the maximum flame height and radiant flux from the burning surface
- Total heat release, in MJ/m², is the area under the curve. It measures how much fuel the product actually contributes
- Time to ignition, in seconds, is where the curve starts. A long ignition delay is worth as much as a low peak
- Rate of growth, expressed as FIGRA in watts per second, is how steeply the curve rises. Fire growth, not fire size, is what outruns detection and escape
- MARHE, the maximum average rate of heat emission, combines size and growth into one number and is used in some transport and marine classifications
A thin panel and a thick one of the same material can have very similar peak values and very different total heat release, which is why every reaction-to-fire result belongs to the thickness and substrate tested.
Where heat release rate appears in the codes
In Australia the group number under AS 5637.1 is either measured in the AS ISO 9705 room, where the criterion is the time to reach 1 MW of total heat release, or predicted from AS/NZS 3837 cone calorimeter data. Group 1 means flashover was not reached within twenty minutes. The prediction route works because the room result correlates with the bench scale heat release curve.
In Europe, EN 13501-1 Classes A2 to D are decided by the single burning item test, and both criteria are heat release quantities: the fire growth rate index FIGRA in watts per second and the total heat released in the first 600 seconds in megajoules. Class B requires FIGRA not exceeding 120 W/s and THR600s not exceeding 7.5 MJ; Class C allows 250 W/s and 15 MJ; Class D allows 750 W/s.
In the United States the Steiner tunnel test ASTM E84 does not measure heat release at all. It reports a comparative flame spread index instead, which is one reason American and European classifications cannot be mapped onto one another.
What it means for facade materials
A non-combustible material has no meaningful heat release at all, which is why glass fibre reinforced concrete, fibre cement, stone and metal never need a heat release argument. ShapeShell-RC is Not Deemed Combustible under AS/NZS 1530.1, tested by Ignis Labs, and the question does not arise for it.
For a polymer composite the heat release curve is the whole argument. Fibre reinforced polymer is combustible, cannot pass AS 1530.1, EN ISO 1182 or ASTM E136, and is instead judged by how much energy it releases and how quickly. Resin chemistry, fire retardant loading, glass content and laminate thickness all move the curve. ShapeShell-RT holds an AS 5637 Group 1 classification from AWTA Product Testing and is classified B-s2,d0 under EN 13501-1 by Intertek Testing Services, both of which are heat release derived results, and it passed the full-scale BS 8414 and AS 5113 test at BRE Global.
Bench scale heat release is a good predictor and a poor guarantee. It cannot reproduce a joint, a cavity, a fixing or a melting substrate, which is why codes require a full-scale facade test for tall buildings rather than accepting a cone calorimeter result on its own.
Standards and references
- AS/NZS 3837 Method of test for heat and smoke release rates for materials and products using an oxygen consumption calorimeter (ISO 5660-1, ASTM E1354)
- AS ISO 9705 Fire tests: full-scale room test for surface products
- AS 5637.1:2015 Determination of fire hazard properties: wall and ceiling linings
- EN 13823 Single burning item test; EN 13501-1 classification criteria (FIGRA, THR600s)
- ISO 13943 Fire safety vocabulary (definitions of heat release rate and related terms)
- NCC 2022 Volume One Specification 7 (fire hazard properties) (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
What units is heat release rate measured in?
Kilowatts for an object or a room fire, and kilowatts per square metre for a bench test where the exposed specimen area is known. Total heat release is reported in megajoules, or megajoules per square metre.
How is heat release rate measured without measuring heat?
By oxygen consumption. Burning most organic materials releases about 13.1 megajoules for every kilogram of oxygen consumed, so measuring the oxygen depletion in the exhaust stream gives the heat release rate directly.
Is a low heat release rate the same as non-combustible?
No. Non-combustibility is decided by a furnace test such as AS 1530.1, EN ISO 1182 or ASTM E136. A combustible material can have a low heat release rate and still fail those tests, which is the case for every fibre reinforced polymer.
What is peak heat release rate?
The highest instantaneous value on the heat release curve, usually in kW/m². It sets the worst moment of the fire, but the rate of growth and the total heat released matter just as much for classification.
Does heat release rate affect a fire resistance level?
No. An FRL is a time in minutes for which a tested wall, floor or door assembly resists a standard furnace fire under AS 1530.4. Heat release rate is a material property measured by a different family of tests.
ShapeShell fire data is published test by test: AS/NZS 1530.1 for RC, and AS 5637 Group 1, EN 13501-1 and the full-scale AS 5113 result for RT. See ShapeShell fire test data
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.