Determination of the combustion heat value of thermal insulation materials measures the total energy released during complete combustion and is a cornerstone of reaction-to-fire assessment for building products. Performed with an oxygen bomb calorimeter in accordance with EN ISO 1716 and GB/T 14402, the test underpins fire classification under EN 13501-1 and GB 8624. This article explains the test principle, sample requirements, methodology, calculation, and practical applications for manufacturers, specifiers, and accredited third-party laboratories.
Technical Overview
The combustion heat value of a thermal insulation material, formally known as the gross heat of combustion (PCS) or gross calorific value, is the total thermal energy released when a unit mass of the material is burned completely in oxygen at constant volume, with the water formed by combustion condensed to the liquid state. It is expressed in megajoules per kilogram (MJ/kg) or kilojoules per gram (kJ/g). Because the test quantifies the maximum potential energy contribution of a product to a fire, it is a decisive parameter in reaction-to-fire classification systems worldwide.
The determination is carried out using oxygen bomb calorimetry in accordance with EN ISO 1716 (identical to ISO 1716) and, for the Chinese market, GB/T 14402. Under the European classification standard EN 13501-1 and the Chinese standard GB 8624-2012, the gross heat of combustion is a mandatory criterion for the highest fire performance classes: class A1 requires a total PCS of no more than 2.0 MJ/kg, while class A2 permits up to 3.0 MJ/kg for the total product, with individual substantial components of non-homogeneous products limited to 4.0 MJ/kg in class A2.
- Core method: oxygen bomb calorimetry under EN ISO 1716 / ISO 1716 / GB/T 14402
- Reported quantity: gross heat of combustion (PCS), in MJ/kg
- Classification thresholds: 2.0 MJ/kg (A1) and 3.0 MJ/kg (A2, total product)
- Companion tests for A1/A2 classification: non-combustibility furnace test (EN ISO 1182 / GB/T 5464) and single burning item test (EN ISO 13823) as applicable
Testing Samples
Organic Foam Insulation Materials
Expandable polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PUR), polyisocyanurate (PIR), and phenolic foam boards exhibit high organic content and correspondingly high gross heat values, typically in the range of 15 to 40 MJ/kg. Specimens are taken from representative locations of the board, excluding facings unless the facing is part of the component under evaluation, and are comminuted to a fine, homogeneous powder before testing.
Inorganic and Fibrous Insulation Materials
Glass wool, rock (stone) wool, foam glass, calcium silicate, expanded perlite, and vermiculite products contain little or no combustible matrix. Their PCS is governed mainly by the organic binder, dust-depositing oils, and hydrophobic agents, and is usually below 2 to 3 MJ/kg. Even small increases in binder content can push such products across a classification limit, making precise calorimetry essential.
Non-Homogeneous and Faced Products
Laminated boards, faced panels, and multilayer systems are non-homogeneous products. Each substantial component, such as the insulation core, mineral wool fleece facing, aluminum foil, or coating layer, is separated mechanically and tested individually. The results are then combined arithmetically using the mass fractions of the components in the finished product to obtain the total PCS.
Ancillary Materials
Adhesives, primers, binders, sealants, and decorative facings used in insulation systems may be tested separately when they form part of the product as placed on the market or when classification rules require their energy contribution to be accounted for.
Testing Items
The scope of a typical combustion heat value determination for thermal insulation materials includes the following reported items:
- Gross heat of combustion (PCS) of homogeneous insulation products, in MJ/kg
- Gross heat of combustion of each substantial component of non-homogeneous products
- Mass-weighted total PCS of the complete non-homogeneous product
- PCS per unit area (MJ/m²) derived from PCS and mass per unit area, where the classification or specification requires it
- Evaluation against the acceptance thresholds of EN 13501-1 and GB 8624-2012 for classes A1 and A2, stated as a pass or fail conclusion
Testing Methods
Governing Standards
The principal test standard is EN ISO 1716, Reaction to fire tests for products for the building industry — Determination of the gross heat of combustion (calorific value), which is internationally identical to ISO 1716. In China, GB/T 14402, Test method of heat of combustion for building materials, is technically aligned with ISO 1716 and is invoked by the classification standard GB 8624-2012. Related calorimetry methods such as ASTM D5865 and ASTM D5468 for solid fuels and ASTM D240 for liquids may serve as technical references but are not used for building product classification.
Apparatus: Oxygen Bomb Calorimeter
The test employs a sealed oxygen bomb, typically with a capacity of approximately 300 mL and constructed of corrosion-resistant steel, in which the specimen is burned in a crucible. The bomb is charged with oxygen to a pressure of about 3.0 MPa (30 bar) and immersed in a calibrated calorimeter vessel containing a weighed quantity of distilled water. A stirring system ensures uniform temperature distribution, and a high-resolution temperature sensor, capable of resolving changes of the order of a few millikelvin, records the temperature rise of the water jacket. An electrical ignition circuit with a fuse wire and a short length of cotton thread initiates combustion.
Calibration
Before testing, the energy equivalent (effective heat capacity) of the calorimeter is determined by burning certified reference grade benzoic acid, whose massic energy of combustion of approximately 26.4 kJ/g is traceable to a national metrology institute. Calibration is repeated at defined intervals and verified with each test series to ensure the accuracy of the temperature-to-energy conversion.
Specimen Preparation and Conditioning
Representative material is ground to a fine, homogeneous powder, typically passing a sieve of about 0.5 mm aperture, to ensure complete and reproducible combustion. Specimens are conditioned to constant mass prior to testing. A test portion of approximately 0.5 g, adjusted according to the expected energy release, is weighed to the nearest 0.1 mg, and a minimum of three valid determinations are performed on each material or component.
Test Procedure
- Weigh the test portion in the crucible and connect the ignition fuse with cotton thread contacting the specimen.
- Assemble and seal the bomb, purge and charge it with oxygen to about 3.0 MPa.
- Place the bomb in the calorimeter vessel filled with the calibrated mass of water and allow the system to reach thermal equilibrium.
- Initiate ignition and record the complete temperature–time curve through the main period until the after period is stabilized.
- Correct the observed temperature rise for heat exchange with the environment using the extrapolation method prescribed by the standard.
- Open the bomb after the run, inspect for complete combustion, and analyze the washings where sulfur or nitric acid corrections apply.
Calculation and Expression of Results
The gross heat of combustion of each determination is calculated as the calorimeter energy equivalent multiplied by the corrected temperature rise, minus the energy contributions of the ignition wire, cotton thread, and any acid corrections, divided by the dry mass of the specimen. The arithmetic mean of the valid determinations is reported as the PCS of the material, customarily rounded to 0.1 MJ/kg for classification purposes. For non-homogeneous products, the total PCS is obtained by summation of the component PCS values multiplied by their respective mass fractions, and the per-unit-area value, where required, is derived from the areal density of the product. Individual determinations must agree within the repeatability limits stated in the standard, and the full calculation chain is documented in the test report together with specimen conditioning, calibration status, and any deviations.
Applications
Combustion heat value determination is applied throughout the life cycle of thermal insulation products. Manufacturers use it for reaction-to-fire classification and CE marking under the European Construction Products Regulation, for compliance with GB 8624 grading in the Chinese market, and for demonstrating A1 or A2 performance in projects governed by fire codes such as GB 50016 for high-rise and public buildings. During product development, PCS data guide the optimization of binder content and additive formulations so that mineral wool and other inorganic insulations remain below the 2.0 or 3.0 MJ/kg thresholds. Production quality control programs track PCS periodically to detect binder drift or raw material changes. Fire safety engineers also use gross heat of combustion values as input data for fire load calculations and zone modeling, and accredited third-party laboratories rely on the method for type testing, surveillance, and certification of insulation systems used in facades, roofing, curtain walls, and technical insulation.
FAQ
What is the difference between gross and net heat of combustion?
The gross heat of combustion (PCS, higher heating value) assumes that all water formed during combustion condenses to liquid, thereby including the latent heat of condensation in the measured energy. The net heat of combustion (PCI, lower heating value) excludes this latent heat. Reaction-to-fire classification of building products is based exclusively on the gross value.
Which fire performance classes require a combustion heat value test?
Under EN 13501-1 and GB 8624-2012, classes A1 and A2 require a PCS determination. Class A1 demands a total PCS of no more than 2.0 MJ/kg, and class A2 allows up to 3.0 MJ/kg for the total product with individual substantial components of non-homogeneous products limited to 4.0 MJ/kg.
How much sample is required for testing?
Each determination consumes roughly half a gram of powdered material, but a representative laboratory sample of at least 50 to 100 g is recommended to allow grinding, homogenization, conditioning, and repeat determinations. Non-homogeneous products require separate sampling of every substantial component, typically several grams to tens of grams per component.
Why can a mineral wool product fail the 2.0 MJ/kg limit?
Although the fiber itself is inorganic and non-combustible, the cured resin binder, mineral oil dust suppressants, and hydrophobic additives are combustible. If their combined content rises slightly, the mass-weighted PCS can exceed 2.0 MJ/kg, which excludes the product from class A1 even if it passes the non-combustibility furnace test.
How are faced or laminated insulation boards evaluated?
The product is treated as non-homogeneous: the core, facings, adhesives, and coatings are separated and tested individually, and the total PCS is computed from the mass fractions of all components. This prevents a thin combustible facing from being masked by a large inorganic core.
How long does a determination take?
A single calorimeter run lasts less than one hour, but a complete assignment including drying, grinding, conditioning, calibration verification, three determinations, calculation, and reporting typically requires several working days in an accredited laboratory, longer for multi-component products where every layer must be characterized separately.