This compressive strength evaluation scheme for thermal insulation materials provides a systematic framework for determining the load-bearing capability of rigid foams, mineral wools, and other insulation products in accordance with internationally recognized standards such as ISO 844, ASTM D1621, EN 826, and GB/T 8813. The scheme covers sample selection, conditioning, test procedures, calculation methods, classification criteria, and reporting requirements. It is intended for manufacturers, design engineers, contractors, and accredited third-party laboratories that require reliable data for product qualification, factory quality control, certification, and structural design verification.
Technical Overview
Compressive strength describes the capacity of a thermal insulation material to resist crushing loads without excessive deformation or structural failure. For rigid cellular plastics, it is conventionally defined as the compressive stress corresponding to 10% relative deformation, or as the yield stress when the specimen yields before the 10% deformation level is reached. For mineral wool and other fibrous products, the characteristic value is typically the compressive stress at 10% deformation, declared in kilopascals (kPa).
Compressive performance is a decisive property whenever insulation carries sustained or transient loads: floor slabs, inverted roofs, plaza decks, road and railway subgrades, cold-storage floors, and load-bearing pipe supports. Insufficient compressive strength leads to progressive crushing, thickness loss, thermal bridging, surface cracking, and premature system failure. A rigorous evaluation scheme therefore combines short-term compressive testing with, where required, long-term compressive creep assessment and deformation-under-load verification.
- ISO 844 – Rigid cellular plastics, determination of compression properties
- ASTM D1621 – Compressive properties of rigid cellular plastics
- EN 826 – Thermal insulating products for building applications, determination of compression behavior
- GB/T 8813 – Determination of compressive strength of rigid cellular plastics (aligned with ISO 844)
- ASTM C165 – Measuring compressive properties of thermal insulations (fibrous and blanket-type products)
- GB/T 5486 – Test methods for inorganic rigid thermal insulation products
- ISO 7850 / EN 1606 – Determination of compressive creep for extended load-duration evaluation
Testing Samples
Rigid Cellular Plastic Insulation (EPS, XPS, PUR/PIR, Phenolic Foam)
Specimens are cut from representative production boards or pipe sections, free of visible defects and skin damage unless the skin is part of the declared product geometry. The preferred specimen is a square prism or cube with a thickness of approximately 50 mm or the full product thickness where this is less, and lateral dimensions not smaller than the thickness. Load-bearing faces shall be flat, mutually parallel, and perpendicular to the loading axis within 1% of the thickness, as required by ISO 844, ASTM D1621, and GB/T 8813.
Mineral Wool Boards and Slabs (Rock Wool, Glass Wool)
For factory-made mineral wool products, test pieces of 200 mm × 200 mm with the product thickness as supplied are tested in accordance with EN 826 and the relevant harmonized product standards (EN 13162 series). The load is applied perpendicular to the board face, replicating the service condition. Where facings are integral to the product, they are retained unless otherwise specified.
Inorganic Rigid and Cellular Glass Products
Calcium silicate, expanded perlite, and microporous boards are sampled in accordance with GB/T 5486, using machined blocks with ground parallel faces. Cellular glass blocks are tested per ASTM C240 or EN 826 as applicable, with specimen dimensions taken from the governing product specification.
Sampling and Batch Requirements
- Draw samples from production lots using the statistical sampling plan defined in the applicable product standard or certification scheme.
- Prepare a minimum of five specimens per condition; a larger number (up to ten) is recommended for anisotropic or heterogeneous products.
- Mark the loading direction, particularly for extruded products where orientation influences results.
- Do not stack thin specimens to reach the reference thickness unless the applicable standard explicitly permits it.
Testing Items
The core testing items of this evaluation scheme are listed below and may be combined into a single test program or selected individually according to the specification, product family, and end-use exposure.
- Compressive strength at 10% relative deformation (the declared characteristic value for most insulation standards)
- Compressive yield strength, where the force–deformation curve exhibits a yield point before 10% deformation
- Compressive stress at specified deformation levels (for example 2% and 5%) for deformation-sensitive applications
- Compressive modulus, determined from the slope of the linear region of the force–deformation curve
- Deformation under a specified compressive stress, such as 20 kPa, 40 kPa, or 80 kPa
- Compressive creep behavior under sustained load per ISO 7850 or EN 1606, including extrapolated long-term deformation
- Supporting measurements: specimen dimensions, density, and moisture content, which are recorded as conditioning and traceability data
Testing Methods
Specimen Conditioning and Test Environment
Specimens are conditioned to equilibrium at 23 ± 2°C and 50 ± 5% relative humidity in accordance with ISO 291 or the conditioning clause of the governing product standard, typically for a minimum of 24 hours for cellular plastics and mineral wools. Specimen dimensions are measured after conditioning and immediately before testing. Tests are performed in the same controlled environment, or the time out of conditioning is limited so that no measurable change in properties occurs.
Apparatus and Calibration
- A universal compression testing machine with force measurement verified to ISO 7500-1, accuracy class 1 or better.
- Two rigid, parallel compression platens, at least as large as the specimen faces, with a fixed platen and a self-aligning or spherically seated platen as required by the standard.
- Displacement measurement by crosshead movement or, where higher accuracy is required for modulus determination, by an extensometer directly on the specimen.
- Calibration of the load cell, displacement system, and environmental monitoring instruments under a documented traceability program.
Test Procedure
- Measure and record the initial length, width, and thickness of each conditioned specimen to calculate the original loaded area.
- Center the specimen between the platens and apply a small preload, typically 100 Pa as specified in ISO 844 and EN 826, to ensure full face contact.
- Compress the specimen at a constant rate, normally 10% of the initial thickness per minute (for example, 5 mm/min for a 50 mm specimen), or at the rate stipulated by the applicable standard so that 10% deformation is reached within the specified time window.
- Record the force–deformation curve continuously until 10% relative deformation is exceeded, or until the standard-defined termination point.
- Where compressive creep is evaluated, apply and maintain the designated sustained stress for the required duration per ISO 7850 or EN 1606, recording deformation at prescribed intervals.
Calculation and Expression of Results
Compressive stress is calculated as the recorded force divided by the original cross-sectional area of the specimen and expressed in kPa. The characteristic result is the compressive strength at 10% deformation, or the yield strength if yield occurs earlier. Individual values, the arithmetic mean, standard deviation, and coefficient of variation are reported for each sample set. For modulus determination, the slope of the straight-line portion of the stress–strain curve is used, with corrections for machine compliance where necessary. When testing is performed by accredited third-party laboratories for certification purposes, reports include specimen identification, sampling origin, conditioning history, test direction, machine settings, full force–deformation data, and statements of conformity to the declared classification.
Acceptance Criteria and Classification
Acceptance is assessed against the compressive class declared in the product specification. Representative levels under the EN 13162 to EN 13166 series include CS(10) classes ranging from 40 kPa for light mineral wool boards up to 500 kPa or higher for high-load XPS and EPS grades; GB, ASTM, and JIS product standards define analogous minimum values. For sustained-load designs, the allowable stress is additionally limited by long-term creep criteria, commonly expressed as a maximum extrapolated deformation, such as 2% over a 50-year design life, and by the partial safety factors defined in the structural design code applicable to the project.
Applications
The results of this compressive strength evaluation scheme directly support material selection, system design, and regulatory compliance across multiple construction and industrial sectors.
- Building envelopes: external wall insulation systems, floor and under-slab insulation, flat and inverted roofs, green roofs, and terrace decks subject to maintenance or pedestrian traffic.
- Infrastructure: frost-protection layers under roads, railways, airport pavements, and bridge approaches, where insulation must resist soil pressure and cyclic traffic loads.
- Industrial and energy facilities: cold-storage and freezer floors, cryogenic equipment supports, district heating pipe insulation, and load-bearing pipe support blocks.
- Prefabrication: sandwich panels and precast concrete insulation panels, where the insulation layer participates in load transfer during manufacture, transport, and service.
- Product development and compliance: research and development benchmarking, factory production control, type testing, and certification by accredited third-party laboratories for CE marking, GB marking, and international market access.
In every application, the short-term compressive strength value must be derated for long-term loading, elevated or reduced service temperature, and moisture exposure before being used as a design allowable, and the evaluation scheme provides the data basis for these conversions.
FAQ
What is meant by compressive strength at 10% deformation?
It is the compressive stress, in kPa, recorded when the specimen has been shortened by 10% of its original thickness. For rigid cellular plastics, if the material yields before reaching 10% deformation, the yield stress is reported as the compressive strength instead, as defined in ISO 844, ASTM D1621, and GB/T 8813.
Which test standard should be selected for a given insulation product?
Cellular plastics such as EPS, XPS, PUR/PIR, and phenolic foam are tested to ISO 844, ASTM D1621, or GB/T 8813. Mineral wool boards are tested to EN 826 with the corresponding EN 13162 series product standard. Fibrous and compressible insulations may additionally reference ASTM C165, while inorganic rigid products follow GB/T 5486 or the applicable ASTM and EN methods. The choice follows the market region and the certification scheme under which the product is declared.
How many specimens are required, and does direction matter?
A minimum of five specimens per condition is standard, with ten recommended where results are used for certification of variable products. Direction matters for anisotropic materials such as extruded polystyrene, where the foaming or extrusion direction alters the cellular structure; specimens are tested in the through-thickness direction corresponding to the service loading, and this direction is reported.
How does short-term compressive strength differ from compressive creep?
Short-term compressive strength is measured in a test lasting a few minutes and characterizes instantaneous load capacity. Compressive creep, evaluated under sustained stress per ISO 7850 or EN 1606, measures time-dependent deformation and is extrapolated to the design service life. Load-bearing designs are governed by the creep-derived allowable stress, which is significantly lower than the short-term strength.
What are typical declared compressive strength levels?
Indicative ranges are 30 to 200 kPa for mineral wool boards, 70 to 500 kPa for EPS grades, 150 to 700 kPa and above for XPS boards, and roughly 100 to 350 kPa for PUR/PIR boards. Cellular glass and calcium silicate products can achieve substantially higher values. The declared value must always be verified against the specific product certification, since density, facings, and manufacturing process cause wide variation within each material family.
Can testing be performed at non-ambient temperatures?
Yes. For cryogenic supports, hot equipment, or high-temperature service, the scheme can be extended with temperature-conditioned compression testing using environmental chambers, provided that specimen conditioning, preload, and loading rate are documented and the deviations from the standard reference temperature are clearly reported.