XPS or EPS Under a Floor Slab?

6 August 2026

For insulation placed directly under a floor slab or on the ground, XPS is the technically superior choice: its closed-cell structure gives it negligible long-term water absorption (WL(T) ≤ 0.7 Vol.-%), and grades such as XPS 300 deliver ≥ 300 kPa compressive strength (CS(10\Y)300), ensuring the insulation layer does not creep or compress under structural loads. EPS can perform adequately in dry, lightly loaded conditions, but it absorbs significantly more moisture and offers lower compressive resistance, making it less reliable in the sub-slab environment typical of Central and Eastern European construction.

The Direct Answer

XPS is the recommended insulation material for use under a floor slab or on the ground. PNP XPS grades rated CS(10\Y)300 (≥ 300 kPa) and above are specifically engineered for foundations, floor slabs, and other load-bearing horizontal assemblies. Their homogeneous closed-cell structure resists moisture, holds its thickness under sustained load, and maintains declared thermal conductivity (λD 0.034–0.035 W/(m·K)) throughout the service life of the building.

This does not mean EPS is never used under slabs — this alone does not prove EPS is always unsuitable — but XPS addresses the two main risks in sub-slab position (moisture and load) more reliably, and a qualified structural engineer or designer should confirm the appropriate product and thickness for your specific project.

Why the Sub-Slab Environment Is Demanding

Insulation placed between the bearing layer (compacted granular fill or lean concrete) and the structural slab faces conditions that are more severe than those on an above-ground wall:

Sustained compressive load. The full dead and live load of the building acts on the insulation continuously. Creep — slow compression over years — reduces effective thickness and thermal resistance.
Ground moisture. Even with a vapour barrier, capillary moisture, seasonal groundwater fluctuation, and construction water can keep the insulation damp for extended periods.
Limited access for repair. Once the slab is cast, replacing degraded insulation is prohibitively costly.

These conditions set the baseline requirements: high compressive strength, low long-term water absorption, and dimensional stability.

How XPS Meets Those Requirements

PNP XPS boards intended for sub-slab use are tested to EN 13164:2012+A1:2015. The key declared properties of XPS 300 (the most commonly specified grade for floor slabs and foundations) are:

Property	Declared value (XPS 300, >30 mm)
Compressive strength	CS(10\Y)300 — ≥ 300 kPa
Compressive creep	CC(2/1,5/50)130
Long-term water absorption (immersion)	WL(T)0,7 — ≤ 0.7 Vol.-%
Water absorption by diffusion	WD(V)3 — ≤ 3.0 Vol.-%
Freeze–thaw resistance	FTCD1 — ≤ 1 Vol.-%
Dimensional stability	DS(70,90)
Declared thermal conductivity (λD)	0.034 W/(m·K) for 20–100 mm; 0.035 W/(m·K) for 120–150 mm
Tensile strength perpendicular to faces	TR 400
Reaction to fire	Euroclass E

The creep class CC(2/1,5/50)130 means that under a sustained compressive stress of 130 kPa, the long-term deformation remains within the tested limits. For projects with higher sustained loads — industrial floors, cold stores, parking decks — XPS 500 (≥ 500 kPa) or XPS 700 (≥ 700 kPa) are available. A qualified structural designer should confirm the required compressive class for a specific project.

Choosing the Right XPS Grade

PNP XPS offers a range of compressive strength classes. The table below summarises the grades relevant to floor and foundation applications:

Product	Compressive strength	Typical sub-slab use case
XPS 300	≥ 300 kPa (>30 mm)	Residential and commercial floor slabs, shallow foundations
XPS 400	≥ 400 kPa	Car parks, cold stores, heavily loaded commercial floors
XPS 500	≥ 500 kPa	Industrial floors, road construction, high-load slabs
XPS 700	≥ 700 kPa	Extreme civil engineering loads, heavy infrastructure

Where the finished floor surface will be bonded directly to the insulation, or a screed is adhesive-bonded, the waffled-surface variants (XPS 300 W) provide improved mechanical key for the bonding layer. See also XPS 300 vs XPS 300 W — Smooth or Waffled Surface? and Which PNP XPS Should I Choose? Application Guide.

Typical Assembly: Slab-on-Grade Floor

The PNP solutions catalogue lists the following layer sequence for a slab-on-grade floor (from bottom to top):

Undisturbed soil
Geotextile separation layer
Sand and gravel coat
PNP XPS thermal insulation
Polyethylene film (vapour control)
Cement-sand screed
Floor covering

Exact layer thicknesses, vapour barrier specification, edge-insulation detailing, and waterproofing continuity must be determined by a qualified designer based on the local thermal regulations, groundwater conditions, and structural loads of the project. Consult the current TDS and DoP for the selected product before specifying.

Sources

XPS 300 technical data sheet — TDS pnp_xps300-300TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 300 Declaration of Performance — DoP PNP XPS 300 EMI_en (U).pdf (updated 2025-10-14)
XPS 300 W technical data sheet — TDS pnp_xps300W-300WTB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 300 W Declaration of Performance — DoP PNP XPS 300 W EMI_en (U).pdf (updated 2025-10-13)
XPS 500 technical data sheet — TDS pnp_xps500-500TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 500 Declaration of Performance — DoP PNP XPS 500_en (U).pdf (updated 2025-07-22)
XPS 700 technical data sheet — TDS pnp_xps700_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
XPS 700 Declaration of Performance — DoP PNP XPS 700_eng.pdf (updated 2025-03-17)
XPS 400 technical data sheet — TDS pnp_xps400-400TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
XPS 400 Declaration of Performance — DoP PNP XPS 400_en (U).pdf (updated 2026-04-10)
EN 13164:2012+A1:2015 — Thermal insulation products for buildings: factory-made extruded polystyrene foam (XPS) products, specification
PNP XPS solutions catalogue — Slab-on-grade floor assembly
PNP XPS solutions catalogue — Foundations and buried structures

Frequently asked questions

Can EPS be used under a floor slab at all?

EPS can be used under slabs in certain conditions, but this alone does not prove it is equally reliable to XPS in sub-slab position. XPS has a closed-cell structure with declared long-term water absorption of WL(T) ≤ 0.7 Vol.-% and compressive strengths up to 700 kPa; EPS is an open-cell material with higher moisture susceptibility and lower compressive resistance. The appropriateness of either material for a specific project must be confirmed by a qualified designer.

What compressive strength class do I need for a residential ground floor?

XPS 300, with a compressive strength of ≥ 300 kPa (CS(10\Y)300 for boards above 30 mm), is widely specified for residential and light commercial floor slabs. However, the required class depends on the actual sustained load, slab thickness, and local standards — a structural engineer should confirm the specification for your project. See the current TDS for XPS 300 for the full declared property set.

Does XPS under a slab need a separate vapour barrier?

Yes — the typical slab-on-grade assembly listed in the PNP solutions catalogue includes a polyethylene film vapour control layer placed above the XPS boards, between the insulation and the screed. XPS's low water absorption protects the insulation itself, but vapour control above the boards is a separate building-physics measure that must be specified by the project designer based on local climate and usage conditions.

What thickness of XPS is required under a floor slab?

Required thickness depends on the thermal transmittance (U-value) target set by national energy regulations, the subsoil temperature, and the floor assembly. PNP XPS 300 is available in thicknesses from 20 mm to 150 mm in the standard range; thicker boards are available on request. A thermal calculation by a qualified designer is required — see the current TDS for declared λD values.

Is XPS 300 W (waffled surface) needed under a slab, or is XPS 300 (smooth) sufficient?

For a conventional floating screed or a polyethylene-film-separated assembly, the smooth XPS 300 surface is sufficient — the film slides freely and no adhesion to the insulation is required. XPS 300 W is the appropriate choice when a bonding layer (adhesive mortar, render, or similar) is applied directly to the insulation surface. For a detailed comparison, see the knowledge article <a href="/knowledge/xps-300-vs-xps-300-w/">XPS 300 vs XPS 300 W — Smooth or Waffled Surface?</a>.

Where can I get the Declaration of Performance and technical data sheet for XPS 300?

The current DoP (DoP PNP XPS 300 EMI_en, updated 2025-10-14) and TDS (updated 2026-07-22) are available from the documentation section of the PNP XPS website at /documentation/, or contact the sales team at sales@pnpinsulation.com.