Compressive strength is one of the most decisive performance indicators for thermal insulation materials, directly determining their suitability for load-bearing applications such as floor insulation, inverted roofs, and cold storage construction. This article provides a systematic analysis of compressive strength test results for common insulation products, including EPS, XPS, PUR/PIR foam, phenolic foam, and mineral wool, with reference to recognized standards such as ISO 844, ASTM D1621, EN 826, and GB/T 8813. Readers will learn how test data are generated, which material and testing factors influence the results, and how to interpret them correctly for product classification and engineering design.
Technical Overview
Compressive strength describes the ability of an insulation material to resist deformation and damage under perpendicular compressive loading. Unlike dense structural materials, cellular insulation products such as polystyrene foams and polyurethane foams rarely fail by sudden fracture. Instead, their force–deformation curve typically shows an initial linear-elastic region, followed by a plateau caused by cell wall buckling and collapse, and finally a steep rise as the collapsed cell structure densifies. For this reason, the relevant standards define compressive strength in a specific way: it is the compressive stress at the yield point if yield occurs before 10% relative deformation, or the compressive stress at 10% deformation if no yield point appears.
Several internationally recognized test standards govern this measurement. ISO 844 and its Chinese counterpart GB/T 8813 cover rigid cellular plastics, ASTM D1621 specifies the compressive properties of rigid cellular materials, and EN 826 addresses the compression behavior of thermal insulating products for building applications. For mineral wool products, EN 826 and GB/T 13480 are commonly applied. Although the test principles are very similar, differences in specimen geometry, conditioning, and loading conditions mean that results obtained under different standards should be compared with care.
- Compressive strength or compressive stress is expressed in kPa or MPa.
- The result is strongly correlated with material density and cell structure.
- Product standards assign declared levels, for example CS(10) values in the EN 13162 to EN 13167 series.
- Short-term compressive strength is distinct from long-term compressive creep, which must be assessed separately for sustained loads.
Testing Samples
Expanded Polystyrene (EPS)
EPS is a molded closed-cell foam with densities typically ranging from 15 to 35 kg/m³. Its compressive stress at 10% deformation generally falls between 60 and 300 kPa depending on density, with higher density grades such as EPS 150 and EPS 200 commonly used under floor screeds and light industrial loads. Test results usually show a well-defined plateau, and the measured value depends noticeably on the loading direction relative to the molding direction and on the curing time after molding.
Extruded Polystyrene (XPS)
XPS is produced by continuous extrusion and features a fine, uniform closed-cell structure with dense skin layers. Densities typically range from 25 to 45 kg/m³, and compressive strength classes commonly extend from 150 kPa to 500 kPa and above. Result analysis should account for the pronounced anisotropy of XPS: values measured parallel to the extrusion direction can differ measurably from those measured perpendicular to the board faces, and the integrity of the skin layers at cut edges influences the outcome.
Rigid Polyurethane and Polyisocyanurate (PUR/PIR) Foam
PUR and PIR boards typically have densities of 30 to 60 kg/m³ with compressive strength in the range of 100 to 300 kPa. The foam rises during manufacture, creating elongated cells, so the direction of loading relative to the rise direction is a key analytical variable. In addition, because air gradually diffuses into the cells after production and reduces the internal gas overpressure, freshly produced boards can show higher values than aged ones. Product specifications and accredited third-party laboratories therefore often require a defined aging or conditioning period before testing.
Phenolic Foam
Phenolic foam offers excellent fire performance with densities of roughly 40 to 80 kg/m³. Compressive results are commonly in the range of 100 to 250 kPa, but the relatively friable cell structure means that specimens must be cut carefully to avoid crumbling edges, which would otherwise underestimate the true strength. Yield behavior is frequently observed before 10% deformation and must be documented in the report.
Mineral Wool (Rock Wool and Glass Wool)
Mineral wool products achieve their compressive resistance through fiber interlock and binder stiffness rather than closed cells. Compressive stress at 10% deformation can span from about 5 kPa for soft rolls to 200 kPa for high-density rock wool boards used in curtain walls and floor systems. Result analysis is dominated by fiber orientation, product thickness, and specimen cross-sectional area, and many rock wool boards exhibit a visible yield point, which is declared in EN 13162 as a yield-classified compressive stress level.
Testing Items
A complete compressive evaluation of insulation materials normally includes the following reported items, all derived from a single continuous force–deformation record:
- Compressive strength at yield, reported when the force reaches a maximum and then drops or plateaus below 10% deformation.
- Compressive stress at 10% relative deformation, designated CS(10) in EN product standards, reported when no yield occurs earlier.
- Compressive modulus, calculated from the slope of the initial linear portion of the stress–deformation curve.
- Deformation at yield, useful for distinguishing ductile plateau behavior from brittle response.
- Measured compressive force, specimen dimensions, and apparent density for traceability.
- Direction of loading relative to the product faces and, where applicable, to the foam rise or extrusion direction.
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut from finished products with flat, parallel faces and sharp, undamaged edges. Common geometries include 50 mm by 50 mm to 100 mm by 100 mm sections for rigid cellular plastics, with thickness equal to the product thickness, while mineral wool products under EN 826 are typically tested on larger rectangular sections such as 200 mm by 100 mm. Facings and composite layers are removed unless the product standard requires testing the complete assembly. Before testing, specimens are conditioned in a standard atmosphere, typically 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291, for a period defined by the relevant product specification.
Apparatus and Loading Conditions
Testing is performed on a universal testing machine fitted with rigid parallel compression platens, using a force measuring system accurate to at least 1% of the measured value. Deformation is recorded between the platens or with a contacting gauge. An initial preload corresponding to a stress of no more than approximately 100 Pa is applied to seat the specimen, after which the load is applied at a constant rate of about 10% of the initial thickness per minute until yield or 10% deformation is reached.
Test Procedure
- Measure and record the initial thickness and loaded cross-sectional area of each specimen.
- Center the specimen between the platens to ensure uniform load distribution.
- Apply the preload and zero the deformation measurement.
- Compress at the specified constant rate while continuously recording force and deformation.
- Terminate the test at yield or at 10% relative deformation, whichever occurs first.
- Test at least five specimens per sample and calculate individual and mean results.
Calculation of Results
Compressive stress is calculated by dividing the relevant force by the initial loaded area, and relative deformation by dividing the thickness change by the initial thickness. If the force–deformation curve shows a yield point before 10% deformation, the maximum force up to that point defines the compressive strength; otherwise the force at 10% deformation is used. The compressive modulus is derived from the slope of the linear region below approximately 2% strain. Results are reported in kPa, normally rounded to three significant figures, together with the mean, the individual values, and the coefficient of variation.
Result Analysis and Interpretation
Sound interpretation of compressive data requires attention to the following influencing factors:
- Density: for polymer foams, compressive strength increases disproportionately with density, so density verification should accompany every result review.
- Anisotropy: loading direction relative to the rise or extrusion direction can shift results by a noticeable margin, and the tested direction must always be stated.
- Specimen size and edge effects: cut-open cells at machined edges reduce apparent strength, particularly for fine-celled XPS and skin-bearing products.
- Strain rate: cellular plastics are viscoelastic, so higher loading rates produce higher apparent values; the standardized rate must be strictly observed.
- Temperature and moisture: elevated temperatures soften polymer foams and lower results, while moisture in facings or boards can alter measured behavior.
- Material age: PUR/PIR foams lose cell gas overpressure during the first weeks after production, so early results may not represent long-term values.
- Statistical spread: interlaboratory comparisons typically show deviations of several percent even under controlled conditions, so small differences against declared levels should be judged against measurement uncertainty.
When comparing measured means against declared CS(10) levels, the acceptance rules of the applicable product standard apply. In general, the mean of the tested specimens must reach or exceed the declared level, and any significant shortfall should trigger a review of specimen direction, edge quality, conditioning, and production consistency rather than an immediate rejection of the material.
Applications
Compressive strength results directly govern where an insulation product may be used in building and industrial construction:
- Floor and foundation insulation: under-slab and floor heating applications typically require EPS grades of at least 150 kPa or XPS grades of 300 kPa and above, with load-distribution layers accounting for point loads.
- Roof systems: inverted roofs, green roofs, and ballasted roofs rely on high-strength XPS or dense EPS so that the insulation layer is not crushed by gravel, soil, or maintenance traffic.
- Cold storage and industrial floors: high-load XPS or cellular glass products with compressive strength of 400 to 500 kPa and above are specified, combined with long-term creep verification.
- Facades and curtain walls: rock wool boards with declared compressive stress and point-load levels are used where panels must resist fixing pressures and wind-induced movement.
- Civil engineering: road and railway subgrade insulation uses XPS grades validated through both short-term strength and multi-decade compressive creep testing.
For any sustained load, short-term compressive strength must be supplemented by compressive creep data obtained under constant load, since the permissible long-term stress of foamed materials is only a fraction of the short-term value.
FAQ
Why is compressive strength reported at 10% deformation?
Most cellular insulation materials do not fracture under compression; they deform progressively as cells buckle and collapse. A fixed reference strain of 10% provides an objective, reproducible basis for comparing products, which is why standards such as ISO 844, EN 826, and GB/T 8813 define the reported value at this deformation unless yield occurs earlier.
What is the difference between compressive strength and compressive stress at 10% deformation?
Compressive strength refers to the stress at the yield point, the maximum force before the curve drops or plateaus. If no yield occurs before 10% strain, the stress at exactly 10% deformation is reported instead, often labeled CS(10). The test report must state which criterion was applied, because the two values describe different points on the same curve.
Does the loading direction affect the result?
Yes. PUR/PIR foams and XPS are anisotropic, with elongated cells along the rise or extrusion direction, and mineral wool strength depends heavily on fiber orientation. Results measured parallel versus perpendicular to the board faces can differ significantly, so the tested direction must always match the specification and be reported.
Why might measured values fall below the declared level?
Common causes include testing in the wrong orientation, damaged or crushed specimen edges, insufficient conditioning or aging of fresh PUR/PIR foam, deviation from the specified loading rate, naturally low production density, or ordinary measurement variability. A structured review of these factors should precede any conclusion of nonconformity.
Can results from ISO 844, ASTM D1621, and EN 826 be compared directly?
The test principles are equivalent, but the standards differ in preferred specimen geometry, conditioning, and some procedural details. Results are usually broadly comparable, yet for contractual or certification purposes the standard named in the product specification should be used, and cross-standard comparisons should be treated as indicative rather than exact.
Is short-term compressive strength sufficient for structural design?
No. Insulation under permanent loads deforms over time through compressive creep. Design decisions for floors, roofs, and subgrade insulation must combine the short-term compressive result with creep data obtained under sustained load, together with appropriate safety factors defined in the applicable design codes.