Extruded Polystyrene Thickness, R-value, U-value & Cost Estimator – Updated 2026 Prices
Enter your application and area details to calculate the required XPS insulation thickness to achieve a target U-value or R-value, the total boards needed, and an estimated cost. XPS (extruded polystyrene) is specified for below-ground, inverted roof, and ground-bearing floor applications where moisture resistance is critical.
XPS (extruded polystyrene) is a rigid closed-cell foam insulation board manufactured by extruding polystyrene polymer through a die to produce a homogeneous, fine-celled foam structure with an integral skin. The closed-cell structure — in which individual foam cells are completely sealed from each other — gives XPS its key distinguishing properties: very low water vapour permeability, negligible water absorption, high compressive strength, and dimensional stability that are maintained even after long-term exposure to moisture. These properties make XPS the insulation of choice for applications where the insulation is exposed to water, ground moisture, or high compressive loading — specifically below-ground floors, inverted roofs, basement walls, cold stores, and road sub-bases.
XPS is manufactured in the UK and Europe by several major brands including Dow Styrofoam, Jackon Jackodur, Ravago Ravatherm, Fibran, and Ursa XPS. All XPS products are manufactured to BS EN 13164:2012+A1:2015 (factory-made extruded polystyrene foam products — specification). The thermal conductivity (λ-value) of standard XPS products typically ranges from 0.028–0.036 W/mK depending on density, cell structure, and whether graphite enhancement is used. Graphite-enhanced XPS (sometimes marketed as grey or silver XPS) achieves λ values of 0.028–0.032 W/mK — significantly better than standard white XPS — through the incorporation of graphite particles that reflect infrared radiation within the foam cells.
The key advantage of XPS over EPS (expanded polystyrene) and mineral wool in moisture-exposed applications is its long-term thermal performance stability. EPS absorbs some moisture over time (typical long-term water absorption 2–5% by volume), which degrades its thermal performance. Mineral wool, if saturated, loses most of its thermal resistance. XPS, by contrast, has a long-term water absorption of typically less than 0.7% by volume (to BS EN 12091), maintaining over 95% of its declared λ value even after prolonged ground contact or inversion under water on a roof. This stability is the fundamental reason XPS is specified for below-ground and inverted roof applications.
The three main rigid insulation board types used in UK construction in 2026 each have distinct properties and appropriate applications:
XPS insulation resources UK 2026: Dow Styrofoam | Jackon Jackodur | Planning Portal | GOV.UK Part L Approved Document | NHBC Standards
The table below shows the required XPS insulation thickness to achieve common Building Regulations U-value targets, assuming the XPS is the primary insulation layer and the existing construction provides the baseline R-value shown. Calculations use the simplified method: R_xps = thickness/λ; U = 1/(R_existing + R_xps + R_surfaces).
| Application | Target U-value (W/m²K) | Existing R (m²K/W) | XPS R Required (m²K/W) | XPS Thickness (mm) – λ 0.034 | XPS Thickness (mm) – λ 0.030 | Standard Board to Specify |
|---|---|---|---|---|---|---|
| New dwelling floor (Part L 2021) | 0.13 | 0.45 | 7.24 | 246mm | 217mm | 2 × 120mm or 250mm (special) |
| New dwelling floor (Part L 2021) | 0.15 | 0.45 | 6.22 | 212mm | 187mm | 2 × 110mm |
| Extension / renovation floor threshold | 0.25 | 0.45 | 3.57 | 121mm | 107mm | 120mm or 2 × 60mm |
| Extension / renovation floor threshold | 0.25 | 0.60 | 3.23 | 110mm | 97mm | 110mm or 2 × 60mm |
| New dwelling roof (Part L 2021) | 0.11 | 0.85 | 8.24 | 280mm | 247mm | 2 × 140mm or 3 × 100mm |
| Inverted flat roof (renovation) | 0.18 | 0.85 | 4.71 | 160mm | 141mm | 160mm or 2 × 80mm |
| Flat roof renovation threshold | 0.28 | 0.70 | 2.87 | 98mm | 86mm | 100mm |
| Basement floor (new dwelling) | 0.13 | 0.30 | 7.39 | 251mm | 222mm | 2 × 130mm |
| Basement wall external (renovation) | 0.30 | 0.20 | 3.13 | 106mm | 94mm | 110mm |
| Cold store floor (medium) | 0.10 | 0.30 | 9.53 | 324mm | 286mm | 2 × 160mm or 3 × 110mm |
Surface resistances used: floor (downward heat flow): Rsi = 0.17 m²K/W + Rse = 0.04 m²K/W. Roof (upward heat flow): Rsi = 0.10 m²K/W + Rse = 0.04 m²K/W. Note that for inverted roofs, a correction factor of approximately 5–10% must be applied to account for rainwater cooling of the XPS boards — this is specified in BS EN ISO 6946 and typically requires an additional 5–10mm of XPS to compensate.
| Standard Thickness | Common Board Size | Area per Board (m²) | Compressive Strength Grade | Typical Application | λ Typical (W/mK) |
|---|---|---|---|---|---|
| 20mm | 600 × 1,250mm | 0.75 | 200–300 kPa | Perimeter edge insulation, cavity wall | 0.034–0.036 |
| 30mm | 600 × 1,250mm | 0.75 | 200–300 kPa | Perimeter edge, under screed top-up | 0.034–0.036 |
| 40mm | 600 × 1,250mm | 0.75 | 200–300 kPa | Under screed, flat roof | 0.034–0.036 |
| 50mm | 600 × 1,250mm | 0.75 | 200–700 kPa | General floor, inverted roof | 0.032–0.036 |
| 60mm | 600 × 1,250mm | 0.75 | 200–700 kPa | General floor, inverted roof, basement | 0.032–0.036 |
| 70mm | 600 × 1,250mm | 0.75 | 200–500 kPa | Floor slab, inverted roof | 0.032–0.036 |
| 80mm | 600 × 1,250mm | 0.75 | 200–700 kPa | Ground-bearing floor, inverted roof – most common | 0.032–0.036 |
| 100mm | 600 × 1,250mm | 0.75 | 200–700 kPa | Ground floor, basement floor, inverted roof | 0.030–0.036 |
| 120mm | 600 × 1,250mm | 0.75 | 200–500 kPa | High-performance floor, basement | 0.030–0.034 |
| 150mm | 600 × 1,250mm | 0.75 | 200–500 kPa | High-performance floor, cold store | 0.030–0.034 |
| 200mm | 600 × 1,250mm | 0.75 | 200–300 kPa | Cold store, sub-base insulation, Passivhaus floor | 0.030–0.034 |
| Thickness | Grade | Price per Board (600×1,250mm) | Price per m² | Price per m³ | Notes |
|---|---|---|---|---|---|
| 50mm | Standard (λ 0.034) | £8–£13 | £11–£18 | £220–£350 | — |
| 60mm | Standard (λ 0.034) | £10–£15 | £13–£20 | £220–£340 | — |
| 80mm | Standard (λ 0.034) | £13–£20 | £17–£27 | £215–£335 | Most common size for UK floors |
| 100mm | Standard (λ 0.034) | £16–£25 | £21–£33 | £210–£330 | — |
| 100mm | High-perf. (λ 0.030) | £20–£32 | £27–£43 | £265–£425 | Graphite-enhanced |
| 120mm | Standard (λ 0.034) | £19–£30 | £25–£40 | £208–£333 | — |
| 150mm | Standard (λ 0.034) | £24–£38 | £32–£50 | £213–£335 | — |
| 200mm | Standard (λ 0.034) | £32–£50 | £43–£67 | £213–£333 | Special order in some grades |
| Various | High compressive (≥500 kPa) | +20–40% | +20–40% | — | Premium for heavy-duty grades |
All prices are indicative 2026 South East England supply-only estimates excluding VAT and delivery. Deduct 8–15% for North England, Scotland, and Northern Ireland. Add 10–18% for Greater London. Prices vary significantly between suppliers — obtain at least two quotes. Volume discounts typically apply for orders above 100m².
Both XPS and EPS are polystyrene-based rigid foam insulants, but their manufacturing processes differ fundamentally. EPS is made by expanding polystyrene beads with steam in a mould — producing a granular structure with visible bead boundaries. XPS is made by continuously extruding molten polystyrene through a die — producing a homogeneous, fine-celled, closed-cell structure with an integral skin. The XPS structure has significantly lower water vapour permeability and water absorption, making it suitable for below-ground and inverted roof applications where EPS would degrade over time.
XPS is available in multiple compressive strength grades for different loading conditions. Standard residential floor grades (200–300 kPa at 2% deformation) are suitable for domestic floor slabs. Commercial floor grades (400–500 kPa) suit light industrial and retail loading. Heavy-duty grades (700 kPa+) are used for cold stores with forklift traffic and heavily loaded commercial floors. Always confirm the compressive strength requirement with a structural engineer before specifying XPS grade.
For inverted (upside-down) flat roofs, XPS boards above the waterproofing membrane are cooled by rainfall, which reduces the effective thermal resistance. BS EN ISO 6946 Annex D requires a correction factor (typically adding 5–15% to the calculated XPS thickness) to account for this effect. Most XPS manufacturers provide inverted roof design software and datasheets that include the correction factor — always use these rather than the simple R = d/λ calculation for inverted roof specifications.
In radon-affected areas (much of Devon, Cornwall, Northamptonshire, and parts of Scotland and Northern Ireland), the DPM/radon barrier must be correctly positioned relative to XPS insulation. For floors with XPS below the slab, the radon barrier is typically placed on top of the hardcore and below the XPS. For XPS above the slab (below screed), the radon barrier goes between the slab and XPS. The XPS itself does not act as a radon barrier. Always consult BRE Report BR 211 and the relevant Building Regulations guidance for radon-affected areas.
One of the most significant thermal bridges in UK ground floor construction is the heat path around the perimeter of the floor slab — through the external wall foundations and slab edge. XPS perimeter edge insulation (vertical XPS board at the slab perimeter, extending from DPC level down to the underside of the floor insulation) addresses this thermal bridge. Part L and NHBC Standards require perimeter insulation for new dwellings. Typically 50–100mm of vertical XPS at the perimeter, corresponding to a linear thermal transmittance (Ψ-value) reduction of 0.05–0.10 W/mK.
XPS insulation has historically used HFC blowing agents with high global warming potential (GWP). From 2023–2024, major UK XPS manufacturers have transitioned to CO₂ and HFO blowing agents with significantly lower GWP (GWP100 below 150 vs historical values of 1,300–1,400). XPS typically contains less recycled content than EPS (which can incorporate up to 15–20% recycled polystyrene). The Environmental Product Declaration (EPD) for each XPS product should be consulted when assessing embodied carbon for Part Z (Future Homes Standard) compliance.
XPS is a combustible material — it falls into Euroclass E (easily flammable) or F (no performance determined) unless treated with a flame retardant, which typically achieves Euroclass E. Flame-retardant XPS achieves Euroclass B–D depending on the product and test configuration. Building Regulations require that insulation used in certain positions (e.g. external walls above 18m) must meet specific Euroclass requirements. For most below-ground and floor applications, the fire performance of XPS is not a regulatory issue as the material is embedded in or covered by non-combustible materials. Always check Building Regulations fire requirements for your specific application.
XPS boards are installed with staggered butt joints to avoid continuous gaps that create cold bridges. For two-layer installations, the second layer should be offset by half a board length and half a board width from the first, ensuring all joints are covered. Most XPS boards have shiplap or rebated edges for interlocking — check the product specification. Boards laid below concrete slabs do not require adhesive; boards on vertical basement walls are typically mechanically fixed or adhered with a compatible adhesive. Never use solvent-based adhesives on polystyrene — use water-based or polyurethane foam adhesives.
The declared λ value of XPS must reflect its long-term aged thermal conductivity, accounting for any blowing agent diffusion from the cells over the design life of the building. BS EN 13164 requires manufacturers to declare an aged λ value (typically aged for 10 years or extrapolated to 25 years). The aged λ value is what must be used in Building Regulations calculations — not the initial test value. For modern XPS with CO₂/HFO blowing agents, the aged λ value is typically 5–10% higher than the initial test value.
Meeting the Part L 2021 new dwelling floor target of U ≤ 0.13 W/m²K with XPS (λ = 0.034) alone requires approximately 240–260mm of XPS depending on the existing construction R-value. In practice, two layers of 120–130mm XPS with staggered joints are typically used. For extensions (U ≤ 0.25 W/m²K), a single layer of 100–120mm XPS is typically sufficient. The Part L limiting U-value of 0.25 W/m²K for extension floors is a minimum threshold — the overall SAP calculation for the extension may allow slightly worse or require slightly better values depending on the total energy performance.
Passivhaus floor U-value targets are typically U ≤ 0.10–0.15 W/m²K, requiring approximately 200–300mm of XPS for ground-bearing floor applications. This is typically achieved with two or three layers of XPS with carefully staggered joints and XPS perimeter insulation. The Passivhaus Planning Package (PHPP) software should be used to calculate the effective U-value of the floor element, accounting for ground coupling thermal mass effects to BS EN ISO 13370.
U-value (W/m²K) and R-value (m²K/W) are reciprocals: U = 1/R_total where R_total includes all material layers plus surface resistances. The XPS R-value contribution is simply: R_xps = thickness (m) / λ (W/mK). For example, 100mm of XPS at λ = 0.034: R_xps = 0.1/0.034 = 2.94 m²K/W. A higher R-value means better insulation; a lower U-value means better insulation. The UK Part L standard specifies U-values; North American specifications often use R-values (in Imperial units of h·ft²·°F/BTU — always check units when comparing products).
XPS (extruded polystyrene) insulation is used primarily in applications where the insulation will be exposed to moisture, high compressive loads, or both. In UK construction in 2026, the main applications are: ground-bearing floor slabs (below or above the concrete slab, where ground moisture would degrade less resistant insulants); inverted flat roofs (above the waterproofing membrane, where the insulation must resist rain and ponding); basement floors and walls (where below-ground moisture levels are permanently high); perimeter / edge insulation at the slab perimeter; and cold store floors where very high compressive loads from forklift traffic must be carried on an insulated floor slab. XPS is not typically used in pitched roof spaces, stud walls, or cavity walls — these applications are better served by mineral wool or EPS which are less expensive and provide comparable thermal performance in dry conditions.
The required XPS thickness for a ground floor depends on the target U-value and the thermal properties of the rest of the floor construction. For a new dwelling floor in England (Part L 2021 target U ≤ 0.13 W/m²K), approximately 240–260mm of XPS at λ = 0.034 W/mK is required for a typical concrete slab and screed construction — typically specified as two layers of 120–130mm with staggered joints. For an extension floor (Part L threshold U ≤ 0.25 W/m²K), approximately 100–120mm of XPS is typically sufficient. For a renovation where only the floor is being upgraded (existing U-value must not worsen, and achieve U ≤ 0.25 W/m²K where technically and economically feasible), 80–100mm is often used. Use the XPS calculator above to calculate the exact thickness for your specific floor construction and target U-value.
Both XPS and EPS are polystyrene rigid foam boards, but with important differences in their manufacturing process, structure, and performance. XPS is continuously extruded through a die, producing a homogeneous closed-cell structure with an integral skin — this gives XPS very low water absorption (typically ≤ 0.7% by volume long-term) and high compressive strength. EPS is made from expanded polystyrene beads, producing a granular structure — EPS is more permeable to water vapour and absorbs more moisture over time (typically 2–5% long-term), which reduces its thermal performance in wet conditions. XPS is therefore specified for below-ground and inverted roof applications; EPS is suitable for cavity walls, EWI, and protected floor/roof applications. XPS is typically more expensive than EPS of the same thickness by 20–40%.
The λ (lambda) value to use in Building Regulations U-value calculations must be the declared (aged) thermal conductivity from the manufacturer's product datasheet or BBA certificate, not the initial test value. For standard XPS products in the UK in 2026, declared λ values are typically: standard white XPS: 0.034–0.036 W/mK; high-performance XPS: 0.032–0.034 W/mK; graphite-enhanced XPS: 0.028–0.032 W/mK. The aged value is typically 5–10% higher than the initial test value to account for blowing agent diffusion over the building's design life. Always use the manufacturer's declared aged λ value — using a lower (better) initial λ value will overestimate the thermal performance of the insulation in Building Regulations calculations.
Yes — XPS is the standard specification for inverted (upside-down) flat roofs and is specifically designed for this application. In an inverted roof, the XPS boards are placed above the waterproofing membrane (rather than below it as in a conventional warm flat roof). The waterproofing membrane is protected from UV, thermal cycling, and mechanical damage by the XPS and ballast above. The XPS must have very low water absorption to remain effective when exposed to rain and any ponding water — XPS with ≤0.7% long-term water absorption is suitable. An important note: in an inverted roof, rainfall cools the XPS boards, reducing their effective thermal resistance. BS EN ISO 6946 Annex D requires a correction factor to be applied to the XPS thickness to account for this — typically requiring an additional 5–15% of XPS thickness compared with a conventional warm flat roof calculation. Use manufacturer-specific inverted roof design tools for accurate calculations.
The thermal resistance (R-value) of any insulation layer is calculated by the formula: R = d / λ, where d is the thickness in metres and λ is the thermal conductivity in W/mK. For XPS at λ = 0.034 W/mK: 50mm R = 0.050/0.034 = 1.47 m²K/W; 80mm R = 0.080/0.034 = 2.35 m²K/W; 100mm R = 0.100/0.034 = 2.94 m²K/W; 120mm R = 0.120/0.034 = 3.53 m²K/W; 150mm R = 0.150/0.034 = 4.41 m²K/W. The total thermal resistance of the construction element (RT) is the sum of all layer R-values plus the internal surface resistance (Rsi) and external surface resistance (Rse). The U-value is then U = 1/RT.
XPS insulation prices in the UK in 2026 vary by thickness and grade. Standard XPS (λ = 0.034, compressive strength 200–300 kPa) costs approximately: 80mm: £17–£27/m²; 100mm: £21–£33/m²; 120mm: £25–£40/m²; 150mm: £32–£50/m² (all supply only, excluding VAT and delivery). High-performance graphite XPS (λ = 0.030) costs approximately 25–35% more per m² than standard grade for equivalent thickness. XPS is typically 20–40% more expensive than EPS of the same thickness, and 10–30% less expensive than PIR of the same R-value. Installed costs (including labour to lay boards, but excluding screed or waterproofing) add approximately £8–£18/m² depending on the application. Always obtain at least two supplier quotes — XPS prices vary significantly between merchants.
For ground-bearing floors, XPS itself has very low water vapour permeability (SD value typically 80–300m depending on thickness and grade) — so in many floor construction details, the XPS acts as a combined insulation and vapour check layer, particularly when boards are tightly butted or have rebated/shiplap edges. However, a separate DPM (damp-proof membrane — typically 250-micron polythene or equivalent) is still required as the primary damp-proof course, typically positioned between the hardcore and the XPS for below-slab installations. For inverted roofs, a vapour control layer is placed above the deck and below the waterproofing membrane — the XPS sits above the waterproofing, so no separate VCL is required above the XPS. For basement walls, the XPS is typically on the external face of the waterproofing system and does not form part of the vapour control strategy. Always confirm the correct build-up and vapour control strategy with the insulation and waterproofing system manufacturers.
The required compressive strength of XPS for a floor application depends on the loading from the floor construction above and any imposed loads. The compressive strength of XPS is typically specified at 2% or 10% deformation — for floor insulation, the deformation under long-term loading must be limited to avoid screed or floor finish cracking. For domestic residential floors: standard grade XPS at 200–300 kPa (at 2% deformation) is typically adequate, as domestic floor loading rarely exceeds 50–70 kPa. For commercial floors with racking or equipment: 300–500 kPa. For cold stores with forklift traffic: 500–700 kPa. For highway and railway sub-base: specialist grades may be required. The XPS manufacturer's technical team can confirm the appropriate grade for your specific loading — always provide the expected imposed loads, slab thickness, and any point loads from equipment or machinery when seeking technical advice.
XPS is technically suitable for external wall insulation (EWI) systems and is used in some EWI products in Europe, but it is less common for this application in the UK than EPS or mineral wool. The main reasons are: cost (XPS is more expensive than EPS for equivalent thermal performance); fire performance (XPS requires a non-combustible render system to meet Building Regulations fire requirements for EWI above certain heights); and the fact that wall insulation in an EWI system is typically dry and well-protected, meaning the moisture resistance advantage of XPS over EPS is less important than for below-ground applications. Where XPS is used in EWI, it is typically in applications where the wall is subject to significant splash or ground moisture — base of wall / plinth zone applications, or coastal buildings. EPS-based EWI systems are significantly more cost-effective for typical residential retrofit applications.
XPS insulation itself does not directly comply or fail to comply with Building Regulations Part L — Part L sets U-value targets for building elements, and XPS is a material used to achieve those targets. The key requirements are: the U-value of the completed element (floor, roof, or wall) must meet the relevant Part L limit; the XPS λ value used in calculations must be the manufacturer's declared aged value; the installation must be in accordance with the manufacturer's guidance; and the overall building energy performance must meet the SAP target primary energy rate (new dwellings) or the notional building performance (extensions). XPS with the appropriate thickness and correctly installed achieves Part L compliance for floors, roofs, and walls. Always use an accredited SAP assessor and BBA-certificated or BS EN 13164-compliant XPS products for Building Regulations submissions.
Additional XPS insulation resources UK 2026: NHBC Standards | GOV.UK Approved Document L | Planning Portal | BBA – British Board of Agrément | ATTMA – Airtightness Testing | Checkatrade – Find an Insulation Installer