Compressive strength and yield strength determination is one of the most critical mechanical performance tests for thermal insulation materials, directly governing their suitability for load-bearing applications such as roof boards, floor insulation, and under-slab installations. This article provides a comprehensive technical overview of the determination of compressive strength and yield strength of thermal insulation materials in accordance with internationally recognized standards such as ISO 844, ASTM D1621, EN 826, and GB/T 8813, covering sample requirements, testing procedures, calculation methods, and practical applications for manufacturers, engineers, and accredited third-party laboratories.
Technical Overview
Thermal insulation materials in building and industrial applications are frequently subjected to sustained and transient compressive loads. Whether installed beneath concrete floor slabs, on flat roofs under ballast and membrane systems, or within cold-chain freezer floors, the material must resist deformation and structural failure over decades of service. The determination of compressive strength and yield strength is therefore a fundamental mechanical test used for product qualification, factory production control, and conformity assessment.
Compressive strength is generally defined as the maximum compressive stress carried by the specimen, either at a specified relative deformation (commonly 10% of the initial thickness) or at the yield point if the material yields or crushes before reaching that deformation. Yield strength, in turn, refers to the stress at the yield point — the first point on the force-deformation curve at which the specimen continues to deform without a further increase in force. For ductile-behaving cellular plastics such as expanded polystyrene (EPS), a distinct yield point is typically absent, so strength is reported at 10% deformation. For rigid foams such as polyisocyanurate (PIR) or phenolic foam, brittle failure or a pronounced yield plateau may occur well before 10% deformation, and the yield stress then becomes the governing value.
The principal test methods for this determination include:
- ISO 844 — Rigid cellular plastics: Determination of compression properties
- GB/T 8813 — Determination of compression properties of rigid cellular plastics (harmonized with ISO 844)
- ASTM D1621 — Standard Test Method for Compressive Properties of Rigid Cellular Plastics
- EN 826 — Thermal insulating products for building applications: Determination of compression behaviour
- ASTM C165 — Standard Test Method for Measuring Compressive Properties of Thermal Insulations
Results are typically expressed in kilopascals (kPa) and may be declared under European product standards using the classification format CS(10\Y)xxx, which denotes the compressive stress at 10% deformation with or without yielding. Testing is routinely performed by manufacturers' in-house laboratories and by accredited third-party laboratories for certification and regulatory compliance purposes.
Testing Samples
The determination applies to a wide range of rigid and semi-rigid thermal insulation products. Specimen preparation and geometry have a decisive influence on the measured values, so the requirements of the governing standard must be followed strictly.
Rigid Cellular Plastic Foams
This category includes expanded polystyrene (EPS), extruded polystyrene (XPS), rigid polyurethane (PUR), polyisocyanurate (PIR), and phenolic foam boards. These materials exhibit an elastic region followed by cell collapse, densification, or brittle fracture. Specimens are typically cut with square faces (for example, 50 mm × 50 mm or 100 mm × 100 mm depending on the standard) with the thickness taken either as the product thickness or a standardized value such as 50 mm. All faces must be flat, mutually perpendicular, and free of visible defects, melt skin damage, or dust.
Mineral Fiber Insulation Products
Rock (stone) wool and glass wool boards intended for load-bearing service, such as roof boards, are tested in compression to characterize their short-term compressive behavior. Because fibrous products deform progressively without a distinct yield point, strength is normally evaluated at a specified deformation, and long-term behavior is additionally assessed using creep testing under sustained load as a separate procedure.
Cellular Glass and Other Rigid Insulation
Cellular (foam) glass insulation behaves elastically up to a sharply defined brittle failure. The maximum compressive stress at fracture is reported as the compressive strength, making this material class a classic example of yield- or fracture-governed behavior rather than deformation-governed behavior.
Specimen Conditioning
Unless otherwise specified, specimens shall be conditioned to equilibrium in a standard atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity prior to testing. A minimum conditioning period of 24 to 72 hours, depending on the standard and material density, is typically required. Specimens shall not be stacked or exposed to loads during conditioning, and the loading faces shall remain in their as-manufactured state whenever possible.
Testing Items
A complete compression test program for thermal insulation materials generally encompasses the following measured and derived parameters:
- Compressive strength at 10% relative deformation (σ10) — the stress corresponding to a reduction of the specimen thickness by 10%
- Compressive strength at yield (σy or σm) — the maximum stress sustained when the specimen yields or fails before 10% deformation
- Yield point identification — the first point on the force-deformation curve where deformation increases without an increase in force, or where the tangent slope first reaches zero
- Relative deformation at yield or at maximum force
- Compressive modulus (secant or chord modulus within the elastic region), where required by specification
- Compressive stress at other specified deformations (for example, 2% or 5%) for special engineering applications
For product declarations under the harmonized European system, the results are converted into declared levels such as CS(10\Y)100, CS(10\Y)300, or CS(10\Y)500, indicating kilopascals at the specified deformation with or without yield. Similar declared-value logic is applied under Chinese national standards (GB) for product conformity marking.
Testing Methods
Apparatus
The test is performed on a universal compression testing machine fitted with two rigid, parallel platens, at least one of which is mounted on a self-aligning (spherical) seat to ensure uniform load distribution. The force-measuring system shall meet accuracy requirements of the applicable standard, commonly class 1 or better, and deformation shall be recorded either through crosshead displacement or, preferably, through a direct-contact extensometer or displacement transducer to exclude machine compliance. A calibrated means for measuring specimen dimensions — typically a vernier caliper or micrometer with appropriate resolution — is required for the cross-sectional area calculation.
Test Procedure
After conditioning, the specimen dimensions are measured at multiple locations and the initial thickness d0 and load-bearing area A0 are determined. The specimen is centered between the platens with its load axis aligned with the machine axis. Compressive force is then applied at a constant rate of crosshead movement equal to 10% of the initial specimen thickness per minute until one of the following occurs:
- The relative deformation reaches 10%, for materials that do not yield beforehand
- The specimen yields, fails, or the force reaches a clear maximum before 10% deformation
- The force drops significantly below the maximum, indicating structural collapse
Force and deformation are recorded continuously to construct the force-deformation curve. For a 50 mm thick specimen, the standard test speed of 10% per minute corresponds to 5 mm/min, ensuring consistent strain-rate conditions across laboratories.
Calculation and Expression of Results
The compressive stress at any point is calculated by dividing the recorded force F by the initial cross-sectional area A0, and the relative deformation X is calculated as the change in thickness divided by the initial thickness d0, expressed as a percentage. The compressive strength is then reported as follows:
- When the maximum force occurs at or beyond 10% deformation, the compressive strength is the stress at 10% deformation (σ10)
- When the specimen yields or fails before 10% deformation, the compressive strength is the maximum stress recorded (σm), together with the deformation at which it occurred
- The yield point is identified from the force-deformation curve as the point of zero tangent slope or the first departure from substantially linear behavior, as defined by the governing standard
Results are expressed in kPa to three significant figures. The standard deviation and coefficient of variation are calculated, and the number of specimens (typically a minimum of five) is reported. The test report shall also state the standard used, specimen orientation (parallel or perpendicular to the faces), conditioning details, test speed, and a full graphical record of the force-deformation curve.
Key Influencing Factors
Several factors must be controlled to obtain reproducible results:
- Specimen geometry and thickness — thin specimens cut from thick boards can overestimate strength due to skin effects and constraint from the platens
- Test speed — higher strain rates produce higher apparent strength values in viscoelastic polymer foams
- Temperature — polymeric foams soften at elevated temperatures, so testing must occur at the standard atmosphere unless a specific service temperature is being evaluated
- Specimen orientation — foams with facers, skins, or anisotropic cell structures show direction-dependent strength
- Platen alignment — non-parallel loading introduces stress concentrations and premature local failure
Applications
The determination of compressive strength and yield strength supports a broad spectrum of engineering, regulatory, and commercial activities:
- Product qualification and conformity marking of insulation products under national and regional building regulations, including CE marking under harmonized European standards and GB conformity assessment in China
- Design verification for heavily loaded applications such as inverted roofs, plaza decks, parking deck insulation, freezer floor construction, and under-slab insulation
- Factory production control, where regular compression testing validates process consistency in foam extrusion, molding, and lamination lines
- Type testing and surveillance testing performed by accredited third-party laboratories as part of product certification schemes
- Comparative material selection, allowing designers to choose between EPS, XPS, PIR, mineral wool, and cellular glass products with declared compressive performance levels matched to the applied loads
- Input for long-term design: short-term compressive strength values are combined with creep test data to establish allowable long-term loads, typically limited to a small fraction of the short-term strength for sustained loading
- Research and development of high-load-bearing foam formulations, facer systems, and density optimization
For example, an XPS board intended for use beneath a concrete slab may be declared at CS(10\Y)300 or higher, whereas general-purpose EPS wall insulation may only require CS(10\Y)70 to CS(10\Y)100. Selecting a material with verified compressive strength is essential to prevent long-term settlement, cracking of finishes, and loss of thermal performance due to thickness reduction.
FAQ
What is the difference between compressive strength at 10% deformation and yield strength?
Compressive strength at 10% deformation is the stress recorded when the specimen thickness has been reduced by exactly 10% and no earlier maximum occurred. Yield strength is the stress at which the material begins to deform permanently without additional force — often visible as a peak or plateau on the force-deformation curve. If yielding or crushing occurs before 10% deformation, the maximum stress up to that point is reported as the compressive strength instead of the stress at 10% deformation.
Which standard should be used for testing?
ISO 844 and its Chinese equivalent GB/T 8813 govern rigid cellular plastics; ASTM D1621 is the corresponding North American method; EN 826 applies to thermal insulating products for building applications in the European system; and ASTM C165 is used for a wider range of thermal insulation types. The choice depends on the market of sale, the product certification scheme, and the specification referenced in the project documents.
What specimen size and how many specimens are required?
Typical specimens have square loading faces of 50 mm × 50 mm or 100 mm × 100 mm with a thickness of 50 mm, or the full product thickness where it is less, always in accordance with the specific standard. A minimum of five specimens per sample is generally required, and more may be needed for heterogeneous or anisotropic products.
Why is the test speed set to 10% of thickness per minute?
Cellular polymer foams are strain-rate sensitive: faster loading yields higher apparent strength. Fixing the crosshead speed proportionally to specimen thickness normalizes the strain rate so that results from different laboratories, and from specimens of different thicknesses, remain comparable.
Do mineral wool products have a yield strength?
Mineral fiber products generally do not exhibit a distinct yield point in short-term compression; instead they deform progressively as fibers buckle and rearrange. Their compressive behavior is therefore characterized by the stress at a specified deformation, while their long-term load capacity is evaluated through separate sustained-load creep testing.
How are results used in structural design?
Short-term compressive strength is not applied directly as an allowable load. Design values incorporate safety and creep factors, and sustained loads are typically limited to a small percentage of the short-term strength to ensure that creep deformation over the service life remains within acceptable limits. Declared values such as CS(10\Y)xxx provide the standardized basis for these design conversions.
Can testing be performed at other temperatures or after aging?
Yes. Compressive strength can be determined after thermal conditioning, water immersion, freeze-thaw exposure, or accelerated aging to assess durability, provided the conditioning regime and any deviations from the standard atmosphere are clearly documented in the test report.
Where can this test be performed?
Testing may be carried out by manufacturers' laboratories for factory production control or by accredited third-party laboratories for independent verification, type testing, and certification purposes. Independent testing is strongly recommended whenever results will be used for regulatory conformity marking, public tender submissions, or dispute resolution.