Which Rigid Insulation Board Should I Use? PIR vs EPS vs XPS vs Phenolic

Summary

Rigid insulation boards are the workhorses of UK thermal construction. They are used in roofs, walls, and floors in a vast range of applications, from new-build cavity walls and warm flat roofs to retrofit dry-lining and floor insulation. The choice between product types is driven by lambda value (thermal performance), moisture resistance, compressive strength, fire performance, and cost.

The four main categories — PIR, EPS, XPS, and phenolic — are all synthetic, petroleum-derived products with good thermal performance compared to natural alternatives. They have broadly similar applications but differ significantly in their performance characteristics. This article explains the key properties of each type and when to specify them.

A critical point that is often misunderstood on site: the declared lambda value (the figure quoted in product literature) is not always achievable in practice. Lambda values for foil-faced products assume the foil faces are uncontaminated and facing air gaps; actual performance in a composite construction may be lower. Where a U-value calculation is being used for Building Regulations compliance, use the design lambda value rather than the manufacturer's quoted declared value, and check whether the BBA certificate specifies an aged or degraded value.

Key Facts

• Lambda (λ) value — thermal conductivity in W/m·K; lower is better; the primary comparison metric for insulants
• PIR (polyisocyanurate) — λ ≈ 0.022–0.023 W/m·K; foil-faced; used in flat roofs, pitched roof sarking, walls, and floors; compressive strength moderate (typically 100–150 kPa); absorbs limited moisture if cut ends are exposed
• Phenolic foam — λ ≈ 0.018–0.022 W/m·K; the highest-performance synthetic board; foil-faced; similar applications to PIR; more expensive; brittle; dust from cutting is irritant; BS EN 13166 [verify]
• EPS (expanded polystyrene) — λ ≈ 0.030–0.038 W/m·K (grey/graphite EPS ~0.031, white EPS ~0.038); no foil face; open-cell structure in standard grades; good compressive strength in high-density grades (HD300 = 150 kPa); excellent moisture resistance in closed-cell grades; used below ground, in cavity walls, in EWI systems, and in passive haus constructions
• XPS (extruded polystyrene) — λ ≈ 0.033–0.036 W/m·K; smooth skin surface; closed-cell; excellent compressive strength (up to 700 kPa in heavy-duty grades); very low moisture absorption; used in inverted warm roofs, below-slab applications, and car park decks; Styrofoam, Ursa XPS
• Compressive strength — critical for floor and inverted roof applications; EPS HD300 (150 kPa), XPS typical 300 kPa minimum; PIR floor grades 100–150 kPa; phenolic floor grades 100 kPa; check requirements against point loads
• Fire performance — all synthetic rigid insulation boards are combustible; PIR and phenolic typically Class B or E (euroclass) depending on thickness and manufacturer; EPS and XPS Class E; for walls over 18m or in fire-rated constructions, mineral wool or calcium silicate is required
• BBA certificate — all boards used in Building Regulations-compliant constructions should hold a BBA or European Technical Assessment (ETA) certificate; the certificate gives declared and design lambda values and approved applications

Quick Reference Table

Detailed Guidance

PIR (Polyisocyanurate)

PIR is the most widely used rigid insulant in UK domestic construction for above-ground applications. Its closed-cell foam matrix gives it a low lambda value; the two foil faces improve the declared value by adding reflective resistances (which only apply at air gaps facing the foil). Products include Celotex GA4000, Kingspan Kooltherm K series, Recticel Powerline, and Quinn Therm.

The foil facing performs a dual function: it contributes to the declared lambda value (when appropriate air gaps exist) and it provides a vapour control function, restricting moisture diffusion through the board. In wall applications where PIR serves as the VCL, all joints between boards must be taped with aluminium foil tape to maintain continuity.

PIR's principal weakness is at cut edges. The foam itself has moderate moisture resistance, but exposed cut edges allow moisture to penetrate the cell structure over time. In below-ground or high-moisture applications (below slab, within a basement wall), EPS or XPS is a safer choice. Where PIR is used below a floor screed, a protective layer (polyethylene sheet or polyethylene-coated foil) over the top surface is recommended to prevent screed water penetrating cut edges.

PIR boards are available in a wide range of thicknesses, typically 25mm to 200mm in 10–25mm increments. Common face sizes are 1200mm × 2400mm (full board) and 1200mm × 600mm (floor/ceiling tiles). Cutting with a fine-tooth hand saw or circular saw generates irritant dust — wear PPE including dust mask and goggles.

Phenolic Foam

Phenolic foam boards offer marginally lower lambda values than PIR (approximately 0.018–0.020 W/m·K for the best products) and are specified where maximum thermal performance in minimum thickness is required — for example, at window reveals in IWI projects, in conservation area EWI where thickness is restricted by planning, or in floor applications where headroom is critical.

Products include Recticel Eurothane GP, Kingspan Kooltherm K8 flat board, and Xtratherm Thin-R. Phenolic foam is distinguished from PIR by its brittleness — it is more easily snapped if flexed, and cut edges are friable. Handle carefully on site. The dust from phenolic cutting is an irritant; wear appropriate PPE.

Phenolic boards tend to be more expensive than equivalent PIR boards. The cost premium is typically justified only where the thickness saving cannot be achieved with PIR.

Aged performance: phenolic foam can experience a gradual increase in lambda value over time (aged performance), particularly at cut edges where moisture ingress degrades the cellular structure. The BBA certificate for each product specifies an aged lambda value which should be used for U-value calculations in Building Regulations submissions rather than the (lower) declared lambda value.

EPS (Expanded Polystyrene)

EPS is the most common insulant in EWI systems and in cavity wall insulation (as loose beads or full-fill boards). Its open-cell structure in standard grades makes it vapour-permeable — moisture vapour can diffuse through it. This is advantageous in breathable wall constructions (EWI on solid masonry) where a vapour-open insulant is preferable, but means a separate VCL is needed in wall applications where vapour control is required.

Graphite or silver-grey EPS (incorporating graphite particles that scatter infrared radiation) achieves a lambda value of approximately 0.031–0.033 W/m·K compared to approximately 0.036–0.038 W/m·K for standard white EPS. The grey product is used where the slightly reduced thickness is worth the modest price premium.

High-density EPS (HD 300, HD 500) has excellent compressive resistance (150–300 kPa) and is used for floor applications and for roofing where point loads need to be resisted. Standard EPS (20–30 kg/m³ density) is not suitable for heavy floor loading.

EPS has better inherent moisture resistance than PIR at cut edges and can be used below the DPM in below-slab floor insulation applications. Long-term immersion performance makes it suitable for inverted warm roof applications (though XPS is more commonly specified there due to its lower moisture absorption).

XPS (Extruded Polystyrene)

XPS is manufactured by extruding polystyrene through a die, producing a smooth-skinned, closed-cell board with a distinctive density and compressive strength superior to EPS. Products include Styrofoam (Dow/Dupont), Ursa XPS, Jackon, and Floormate. Lambda values are typically 0.033–0.036 W/m·K — slightly higher than PIR or phenolic but comparable to EPS.

XPS is specified primarily for applications where compressive strength and moisture resistance are paramount:

• Inverted warm flat roof — the insulation sits above the waterproof membrane, exposed to rainwater; must resist point loads from foot traffic, and must not absorb moisture which would degrade its thermal performance; XPS in appropriate grades (minimum 300 kPa) is the standard
• Below-slab floor — beneath the ground floor slab, where compressive resistance under the structural load is required; XPS HD 300 or HD 700 as appropriate
• Car park and podium deck — very high compressive resistance grades (XPS 700) for loaded deck applications

XPS moisture absorption in long-term immersion is low (typically < 0.5% by volume to EN 12087 — less than EPS) making it the preferred choice for high-moisture or below-ground applications.

Thermal Bridging and Fixing Details

All rigid insulation boards, regardless of type, are subject to thermal bridging at fixings. Mechanical anchors through PIR or phenolic boards in EWI systems conduct heat from the warm to the cold side. The BBA certificate for EWI systems typically accounts for fixing thermal bridging in the U-value calculation; in other applications, the engineer or energy assessor should account for it using the linear thermal transmittance (Ψ-value) of the fixings.

For floor insulation on a suspended timber floor, rigid boards should be supported across their full area (not just at joists) to avoid flexing and cracking. A polythene sheet VCL above the board and below the floor finish is standard for PIR floor boards.

Frequently Asked Questions

Can I mix different board types in the same application?

In principle yes — for example, a PIR board as the primary insulation with a phenolic board as a thin compensating layer to hit a target U-value. In practice, ensure that:

• Both boards are appropriate for the application (moisture resistance, compressive strength)
• The VCL is correctly positioned (warm side) and continuous across both board types
• The fixing pattern is reviewed to account for the combined thickness
• BBA certificates for both products permit the application

Does the thickness of PIR affect the lambda value?

The lambda value (thermal conductivity) of a material is intrinsic and does not depend on thickness. However, some products have a lower declared lambda value at their marketed standard thickness because the foil-face contribution to the effective thermal resistance has been optimised for that thickness. When boards are cut, the foil-face contribution at the cut edge is lost, and the actual lambda at the cut face is the base foam value (slightly higher). For calculation purposes, use the design lambda value from the BBA certificate, not the declared value.

How do I avoid interstitial condensation with PIR in a wall?

Ensure the foil-faced PIR acts as the VCL on the warm side of the insulation. All joints must be taped with aluminium foil tape. Cut edges at floor, ceiling, and reveal junctions must be treated with proprietary edge trim or tape to maintain the vapour barrier continuity. Any services penetrations through the PIR must be sealed. Refer to the interstitial condensation article for the full condensation risk assessment methodology.

Are there any recycled content options in rigid boards?

EPS boards can incorporate recycled EPS content. Some manufacturers (Jablite, Springvale) produce boards with recycled content. PIR and phenolic foam recycling is more limited. Natural alternatives — wood fibre, cork, hemp — are covered in the natural insulation materials article.

Regulations & Standards

• BS EN 13163 — thermal insulation products for buildings; factory-made expanded polystyrene (EPS) products

• BS EN 13164 — factory-made extruded polystyrene foam (XPS) products

• BS EN 13165 — factory-made rigid polyurethane foam (PUR) products

• BS EN 13166 [verify] — factory-made phenolic foam (PF) products

• BBA Certificates — each manufacturer's product certificate; always use the declared lambda value from the BBA/ETA, not the headline marketing figure

• Building Regulations Approved Document L — U-value targets and the need for certified products in compliance submissions

• BBA Certificates Database — searchable database of certified insulation products including declared lambda values

• Kingspan Technical Literature — Kooltherm and Thermawall PIR product data

• Celotex Technical Literature — GA4000 and other PIR product data sheets

• Styrofoam (DuPont) — XPS Product Data — inverted roof and floor XPS specifications

• TIMSA — Rigid Insulation Guidance — industry guidance on rigid board selection and application

• external wall insulation — EWI system specification including EPS board selection

• internal wall insulation — PIR and mineral wool IWI options

• warm roof cold roof — PIR and XPS in warm flat roof constructions

• natural insulation materials — breathable alternatives to synthetic boards

• pipe lagging — pipe insulation materials and thickness requirements