EPS (expanded polystyrene) and XPS (extruded polystyrene) look similar from a distance and share a common base chemistry, but their manufacturing processes produce meaningfully different physical properties. Specifying one where the other is required leads to either over-spend or underperformance — usually the latter in wet or load-bearing conditions.
How They Are Made
EPS is produced by expanding polystyrene beads with steam inside a mould. The result is a white, fused-bead board with an open, inter-connected cellular structure and variable density (15–30 kg/m³ typically). Air-filled cells mean it is cost-effective but water can enter via capillary action over time.
XPS is produced by extruding molten polystyrene through a die, producing a uniform closed-cell structure with a characteristic skin on both faces. Typical density is 28–45 kg/m³. The closed-cell structure dramatically reduces water absorption and gives higher compressive strength for a given thickness.
Both are thermoplastic and combustible (Euroclass E in most formulations), so fire performance requirements under Approved Document B must be assessed before specifying.
Lambda Values and Thermal Performance
The thermal advantage of XPS over EPS is modest and, in some cases, non-existent. At the same density class, the two materials perform similarly when dry:
| Product | λ declared (W/mK) | λ aged/wet (W/mK) | Notes |
|---|---|---|---|
| EPS 70 (15 kg/m³) | 0.038 | 0.040–0.043 | Entry-level; cavity fill and packaging grade |
| EPS 100 (20 kg/m³) | 0.035 | 0.037–0.040 | General insulation boards |
| EPS 150 (25 kg/m³) | 0.032 | 0.034–0.037 | Floor and under-slab applications |
| EPS 200 (30 kg/m³) | 0.030 | 0.032–0.036 | High-load applications |
| XPS (standard) | 0.034 | 0.034–0.037 | Moisture-resistant; lambda maintained wet |
| XPS (high-performance) | 0.032 | 0.032–0.035 | Inverted roof and perimeter specialist grades |
The key difference is moisture ageing: EPS lambda rises measurably in sustained wet conditions; XPS retains most of its declared performance because water cannot penetrate the cell structure. For dry, interior applications this distinction is academic; for ground-bearing or inverted roof applications it is critical to long-term thermal performance.
Compressive Strength
This is where XPS holds its clearest advantage. Compressive strength (resistance at 10% compression, CS) determines which materials can take structural loads without unacceptable deflection under the slab, screed or loading above.
| Product | CS (kPa) | Deflection under slab + screed |
|---|---|---|
| EPS 70 | 70 | Not suitable under screed |
| EPS 100 | 100 | Marginal for domestic screed |
| EPS 150 | 150 | Adequate for domestic floor screed |
| EPS 200 | 200 | Suitable for most domestic applications |
| EPS 300 | 300 | Light commercial |
| XPS 200 | 200–250 | Entry-level XPS for domestic use |
| XPS 300 | 300 | Standard XPS for domestic and commercial screed |
| XPS 500 | 500–700 | Heavy-duty XPS for industrial floors and roads |
Under a 75 mm sand-cement screed in a domestic dwelling, EPS 150 or EPS 200 (CS 150–200 kPa) is the minimum reasonable specification. XPS 300 provides a comfortable margin and moisture resistance for the same scenario.
Moisture Performance
Water absorption (by volume after 28 days immersion to BS EN 12087) is the specification value that matters most for ground-contact applications:
- EPS 100: typically 3–5% by volume
- EPS 200: typically 1–3% by volume
- XPS: typically < 0.7% by volume (closed-cell; most grades test < 0.3%)
For under-slab applications with a proper DPM (damp-proof membrane), EPS performs adequately because the membrane prevents sustained water contact. Where the DPM integrity cannot be guaranteed, or where insulation is placed outside the waterproofing layer (inverted roof, perimeter/foundation insulation), XPS is the correct choice.
Application Guide
| Application | Recommended product | Notes |
|---|---|---|
| Under ground-bearing slab (domestic, DPM above insulation) | EPS 150 or EPS 200 | Cost-effective; DPM protects |
| Under ground-bearing slab (commercial / high load) | XPS 300–500 | Reliable CS and moisture resistance |
| Inverted flat roof (insulation above waterproofing) | XPS 300 | Wet by design; EPS not suitable |
| Cavity wall partial fill | EPS 70 or EPS 100 | Polished face outward; standard product |
| Floor perimeter / foundation / basement wall | XPS 200–300 | Continuous moisture exposure |
| Under screed over existing concrete (retrofit) | EPS 150 or XPS 300 | Depends on screed load and damp risk |
| EWI (external wall insulation) base board | EPS 100 or EPS 150 | Standard EWI system substrate |
Cost Per m² (2026 Supply Prices)
| Product (100 mm unless noted) | Supply cost (£/m²) |
|---|---|
| EPS 70 — 100 mm | £3–£5 |
| EPS 100 — 100 mm | £4–£7 |
| EPS 150 — 100 mm | £5–£9 |
| EPS 200 — 100 mm | £7–£11 |
| XPS 300 — 60 mm | £6–£10 |
| XPS 300 — 100 mm | £9–£15 |
| XPS 300 — 120 mm | £12–£18 |
XPS is consistently 1.5–2.5× the price of equivalent EPS. For applications where EPS is technically adequate, the cost premium for XPS is hard to justify. For ground-contact and inverted roof applications, the performance case for XPS is clear and the premium is warranted.
Environmental Considerations
Both EPS and XPS are manufactured with blowing agents. XPS historically used HFCs or HCFCs with high global warming potential (GWP); most manufacturers have now transitioned to CO₂ or HFO blowing agents, significantly reducing embodied carbon. EPS uses pentane as its blowing agent, with lower direct GWP, though it is an ozone-depleting substance in manufacturing conditions.
Neither material is easily recycled in standard construction waste streams. EPS can be compacted and reprocessed more readily than XPS; check with the manufacturer for take-back schemes.
Relevant Standards
- BS EN 13163 — factory-made EPS thermal insulation products
- BS EN 13164 — factory-made XPS thermal insulation products
- BS EN 826 — compressive behaviour (determines CS grades)
- BS EN 12087 — water absorption by long-term immersion
- Approved Document C — site preparation, resistance to moisture (ground-floor insulation)
- Approved Document L (2021) — U-value targets (new dwelling ground floor: 0.13 W/m²K)
- BS 8217:2005 — reinforced bitumen membranes for flat roofs (references XPS as inverted roof insulation)
When specifying for a ground floor, ensure the declared lambda (λD) in the product datasheet is used for U-value calculations rather than the nominal value — aged lambda is particularly relevant for EPS in damp conditions.
