The EN 13164 designation CS(10\Y)300 declares a minimum compressive stress of 300 kPa, equivalent to 0.30 N/mm² or roughly 3.0 tonnes-force per 0.1 m², measured on the board face at whichever limit comes first: 10% deformation of original thickness, or material yield, under EN 826 [S1][S2].
It is a harmonised product class under EN 13164:2012+A1:2015, the European standard for factory-made extruded polystyrene foam, and the value is a declared minimum carried on the Declaration of Performance (DoP) under Regulation (EU) No 305/2011, not an average test result [S2][S3].
Decoding the CS(10\Y)300 Notation
The class code follows a fixed syntax defined in EN 13164, Section 4.3.4, which addresses "compressive stress at 10% deformation, σ10, or the compressive strength, σm" [S3]. "CS" denotes compressive stress, "(10\Y)" is the mandatory suffix indicating that the governing limit is the 10% deformation strain or the yield point, whichever is reached first, and "300" is the declared class value in kPa [S1][S2].
A 300 kPa class means the board resists at least 300 N over every 1,000 mm² (10 cm × 10 cm) of face area when the load is applied perpendicular to the board face, the same direction as dead loads on a floor or roof [S1]. EN 13164 also requires that the result be expressed as a declared value with a tolerance band, not as a single test point, which is why thinner PNP XPS 300 boards (20–30 mm) drop to 200 kPa while thicker stock holds the full 300 kPa [S1][S2].
Test Method: EN 826 and the 10% Yield Rule
EN 826, the referenced compression test, loads a specimen at a controlled rate between two parallel plates and records the stress at 10% compression or at the earlier yield point [S1][S5]. When yield occurs before the 10% strain is reached, the yield stress governs the declared class, hence the "Y" in the suffix, an edge case for some high-density formulations but routinely verified for production lots [S1].
European Assessment Document EAD 040650-00-1201 confirms that the same 10% deformation or compressive strength metric, "determined according to EN 826 in accordance with EN 13164", feeds the load-bearing and inverted-roof ETA pathways for XPS [S4]. Most PNP XPS 300-family boards sit between 30–150 mm thickness; 20 mm variants drop to 200 kPa because the extrusion process gives thinner boards less cellular structure to bear load, an important data-sheet caveat for slab-edge detailing [S1][S2].
Where CS(10\Y)300 Is the Right Spec, and Where It Falls Short
CS(10\Y)300 is widely specified for foundation slabs and perimeter insulation under building loads, residential and light commercial floor insulation under cement-sand screed, accessible roofs and podium decks, and paving or landscaping details [S1]. It covers typical superimposed dead loads in housing, generally 1.5–3.0 kPa, with a comfortable margin.
It is not adequate for industrial floors with forklift traffic, parking decks on structural slabs, or road construction, where the standard guidance is to step up to CS(10\Y)400, CS(10\Y)500 or CS(10\Y)700 [S1][S6]. The compressive class on its own is not a design approval; a qualified structural engineer must size the board to actual design loads using the declared value together with compressive creep data such as CC(2/1.5/50)130, which gives the long-term 50-year creep strain limit at 130 kPa, well below the 300 kPa short-term class [S1][S2].
Reading the Wider EN 13164 Property Set on the Same DoP
CS(10\Y)300 is one line on a multi-property DoP; the same board typically carries declared thermal conductivity (λD) around 0.034 W/(m·K) for 20–100 mm stock, long-term water absorption by total immersion WL(T)0.7 (≤ 0.7 Vol.-%), water absorption by diffusion WD(V)3 (≤ 3.0 Vol.-%), dimensional stability DS(70,90), freeze–thaw resistance FTCD1 (≤ 1 Vol.-%) and, for inverted-roof and EAD 040650-00-1201 pathways, compressive creep CC(2/1.5/50)130 [S2][S4].
For the load-bearing layer and thermal-insulation-outside-waterproofing use covered by EAD 040650-00-1201, the assessment also asks for the characteristic value of compressive stress, compressive modulus of elasticity, behaviour under shear load on large-sized specimens, and creep under combined compressive and shear load, none of which are visible from CS(10\Y) alone [S4]. Specifiers should always pull the current DoP, not just the data sheet headline, and check that the declared λD, thickness tolerance class (commonly T1) and Euroclass reaction-to-fire rating (E for most PNP XPS 300 products) match the project's regulatory submission.
Comparison of Common EN 13164 Compressive Classes
Across the published PNP XPS range, declared compressive strength scales with nominal density and thickness: CS(10\Y)150 at ≥ 150 kPa for the 150 W X-Grip product, CS(10\Y)300 at ≥ 300 kPa for the 300 / 300 W / 300 SLOPE family above 30 mm, CS(10\Y)400 for XPS 400 (40–100 mm), CS(10\Y)500 for XPS 500 (40–120 mm) and CS(10\Y)700 for XPS 700 (20–150 mm) [S1]. The published Buy Insulation Online density guide treats 100 kPa as the practical floor for any load-bearing XPS use, with higher classes needed as point loads, traffic or long-term creep become governing [S6].
Selection therefore reduces to four decision criteria: declared CS(10\Y) class, available thickness range (because thinner boards lose class), declared λD for the chosen thickness (which sets U-value), and whether the project needs the EAD 040650-00-1201 load-bearing ETA package including creep and shear data [S1][S2][S4][S6].
What Specifiers Often Miss on the Label
Two recurring errors show up on real projects. First, quoting the headline class without checking the thickness band: a CS(10\Y)300 board at 20 mm may only carry 200 kPa, which can drop a slab-edge detail below the required margin [S1]. Second, treating CS(10\Y)300 as the design value rather than a short-term declared minimum: the long-term allowable stress is governed by CC(2/1.5/50), typically one-third to one-half of the short-term class, so the working stress envelope for a 300 kPa board is closer to 130 kPa over 50 years [S1][S2][S4]. For related construction-specification reading, see our cement mix design notes and the Portland cement service-temperature piece, both of which tie back to how insulation interacts with structural toppings.
For inverted-roof and below-foundation uses, cross-reference the current EAD 040650-00-1201 ETA document, not the EN 13164 DoP alone, and confirm the declared freeze–thaw class (FTCD) and dimensional-stability class (DS(70,90)) match the project's hygrothermal exposure. The next signal to track is whether CEN opens a revision of EN 13164 beyond the 2012+A1:2015 amendment currently in force, which would reset the CS(10\Y) level-tolerance bands and the declared-value procedures.
For component-level specifications, see y strainer, pressure transmitter, and flow meter.