Heat release rate (HRR) testing is a critical fire performance evaluation for thermal insulation materials, quantifying the amount of thermal energy released during combustion and serving as a key indicator of fire hazard. This testing is typically performed using cone calorimeter methods in accordance with ISO 5660-1, ASTM E1354, and related international standards. This article provides a comprehensive overview of sample types, test parameters, methodologies, and applications of heat release rate testing for insulation products.
Technical Overview
Heat release rate (HRR) is widely recognized as the single most important quantitative parameter in fire hazard assessment. It represents the rate at which thermal energy is released by a burning material, expressed in kilowatts (kW) or normalized per unit area as kilowatts per square meter (kW/m²). For thermal insulation materials—which are installed in large quantities on building envelopes, roofs, and internal partitions—the heat release characteristics directly influence fire growth, flashover potential, and the safety of occupants and firefighting personnel.
Modern heat release rate measurement is predominantly based on the oxygen consumption principle, which establishes that approximately 13.1 MJ of heat is released per kilogram of oxygen consumed during combustion, regardless of the fuel type. By measuring the oxygen concentration, gas flow rate, temperature, and pressure of combustion products in an exhaust duct, laboratories can calculate heat release rate with high accuracy and repeatability.
Insulation materials present unique fire behavior challenges because many high-performance products, such as expanded polystyrene (EPS), extruded polystyrene (XPS), and polyurethane (PU) foams, are organic and combustible. Heat release rate testing allows manufacturers, regulators, and fire safety engineers to classify materials, optimize formulations with flame retardants, and demonstrate compliance with building codes such as GB 8624, EN 13501, and various international regulatory frameworks.
Testing Samples
Organic Foam Insulation Materials
This category includes expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PU), polyisocyanurate (PIR), and phenolic foam. These materials offer excellent thermal performance but are inherently combustible, making heat release rate characterization essential. Samples are typically conditioned to constant mass and tested at standard dimensions of 100 mm × 100 mm with thicknesses up to 50 mm for cone calorimeter testing.
Inorganic Fiber Insulation Materials
Rock (mineral) wool and glass wool are non-combustible or limited-combustibility materials composed of molten rock or glass fibers. Although their contribution to heat release is minimal, testing is still performed to verify classification claims and to evaluate any organic binders that may contribute small amounts of combustible mass.
Composite and Sandwich Panel Systems
Metal-faced sandwich panels, composite boards with facer materials, and insulation cores bonded to substrates are tested to evaluate the combined fire response of the system. Sample preparation may involve cutting through the full panel cross-section to expose the insulation core to the radiant heat source.
Specialty and Emerging Insulation Products
Aerogel blankets, vacuum insulation panels (VIPs), calcium silicate boards, perlite boards, cellulose insulation, and foam glass are also evaluated. For products with outer barrier films or facings, testing protocols address whether the specimen is tested as-received or with protective layers removed.
Testing Items
Heat release rate testing of insulation materials encompasses a comprehensive set of measurable fire performance parameters:
- Peak heat release rate (pHRR): the maximum instantaneous heat release rate recorded during the test, a key indicator of fire intensity and flashover propensity.
- Total heat release (THR): the cumulative heat energy released over the entire test duration, reflecting the total fuel contribution of the specimen.
- Average heat release rate: measured over defined intervals such as 180 seconds and 300 seconds after ignition, used for regulatory comparison.
- Time to sustained ignition (TTI): the period of radiant exposure before sustained flaming occurs, indicating ease of ignition.
- Fire growth rate index (FIGRA): the ratio of maximum heat release rate to the elapsed time at which it occurs, used in EN 13823 and GB/T 20284 classifications.
- Effective heat of combustion (EHC): heat released per unit mass of material consumed, expressed in MJ/kg.
- Mass loss rate (MLR): the rate of specimen mass reduction during combustion, recorded by load cells integrated into the test apparatus.
- Smoke production rate (SPR) and total smoke production (TSP): measured optically across the exhaust duct, since smoke obscuration is a major life-safety factor.
- Carbon monoxide and carbon dioxide yields: measured by gas analyzers to assess toxic gas generation.
Testing Methods
Cone Calorimeter Method (ISO 5660-1 / ASTM E1354)
The cone calorimeter is the benchmark bench-scale technique for heat release rate measurement. A square specimen of 100 mm × 100 mm is placed on a load cell beneath a conical heater that exposes the sample to a controlled radiant flux, typically 25, 35, 50, or 75 kW/m², simulating various fire exposure severities. An electric spark igniter initiates piloted ignition, and combustion products are collected in an exhaust duct where oxygen, CO, and CO₂ analyzers, together with smoke opacity measurement devices, enable calculation of heat release rate, mass loss, and smoke data. Each test typically runs for a minimum of 10 minutes or until flameout, and a minimum of three replicate specimens is required for statistical validity.
Bomb Calorimeter Method (ISO 1716 / GB/T 14402)
This method determines the gross heat of combustion (PCS) of a material under complete combustion conditions in an oxygen bomb at constant volume. While it does not provide a time-based heat release rate, the total potential heat value is used in classification schemes, such as determining whether a product meets the criteria for non-combustible (A1) or limited combustibility (A2) classes.
Single Burning Item Test (EN 13823 / GB/T 20284)
This intermediate-scale method exposes two vertically mounted wing panels to a propane burner with a nominal 30 kW heat output. It measures FIGRA, THR600s (total heat release within the first 600 seconds), smoke growth rate (SMOGRA), and lateral flame spread, forming the basis for Euroclass B, C, and D classifications under EN 13501 and the corresponding GB 8624 classes.
OSU Heat Release Rate Apparatus (ASTM E906)
This method, widely used in the transportation sector, measures heat release using a thermopile-based sensible enthalpy approach in an enclosed chamber. It is frequently applied to insulation and lining materials intended for aircraft and rail applications under FAA and other regulatory frameworks.
Room Corner Test (ISO 9705)
As a large-scale reference scenario, the room corner test evaluates the real-room fire performance of wall and ceiling insulation linings using a gas burner of defined heat output, validating whether flashover occurs and correlating bench-scale heat release data with full-scale behavior.
Applications
Heat release rate testing of insulation materials supports a broad spectrum of engineering, regulatory, and commercial objectives:
- Building code compliance: demonstrating conformance with combustion performance classifications such as GB 8624 (China), EN 13501 (Europe), and comparable national schemes.
- Product development: comparing flame-retardant formulations, optimizing additive loading, and evaluating the effect of density, thickness, and facer types on fire behavior.
- Fire safety engineering: supplying HRR input data for computational fluid dynamics (CFD) fire modeling, egress analysis, and performance-based building design.
- Quality control: verifying batch-to-batch consistency during production and validating raw material substitutions.
- Facade and cladding system assessment: evaluating insulation contribution within rainscreen, ETICS (external thermal insulation composite systems), and sandwich panel assemblies.
- Certification and market access: providing technical evidence for CE marking, voluntary product certification, and technical files required by regulators and specifiers.
- Incident investigation: characterizing materials after fire events to support root-cause analysis and liability assessment.
FAQ
What is heat release rate and why is it important for insulation materials?
Heat release rate measures how quickly burning material releases energy. For insulation materials, which cover large building surfaces, a high heat release rate accelerates fire growth and increases flashover risk, making HRR one of the most decisive indicators of fire hazard.
What sample size is required for cone calorimeter testing?
Standard cone calorimeter specimens measure 100 mm × 100 mm, with thickness up to 50 mm. Specimens must be conditioned to constant mass at a controlled temperature and humidity, typically 23 °C and 50% relative humidity, prior to testing.
Which radiant heat flux should be selected?
The commonly selected irradiance levels are 25, 35, 50, and 75 kW/m². A flux of 50 kW/m² is frequently used for building products as it represents a well-developed fire scenario, while 25 kW/m² may simulate developing fire conditions. The selected level should reflect the intended application and reference specification.
How many specimens are needed, and how long does testing take?
ISO 5660-1 and ASTM E1354 generally require a minimum of three replicate specimens per condition. Each test runs for at least 10 minutes or until combustion ceases. Including setup, calibration, conditioning, and reporting, a complete test program typically takes several working days.
Can heat release rate data be used for regulatory classification directly?
Cone calorimeter results primarily serve as research, benchmarking, and fire modeling inputs. Regulatory classification under GB 8624 and EN 13501 generally relies on combined methods such as ISO 1716, EN ISO 1182, and EN 13823. However, HRR data are integral to several classification parameters and are often required in technical dossiers.
How should melting or shrinking insulation materials be tested?
Thermoplastic foams such as EPS and XPS may melt, drip, or shrink during exposure. Test protocols address this by using retainer frames, grid supports, or substrate backing to maintain specimen geometry, ensuring reproducible and comparable heat release measurements.