Insulation Board Types: PIR, EPS, Mineral Wool

Summary

Choosing an insulation board is a four-variable decision: thermal target (W/m²K U-value, dictating thickness), fire requirement (Euroclass A1/A2/B/C/D/E/F driven by Building Regulations Part B), moisture exposure (above or below DPC, internal or external wall, ventilated cavity or not), and budget. The wrong choice can fail Building Regs, void manufacturer warranties, or massively over-spend on materials.

The most common mistake is treating all rigid boards as interchangeable on the basis of price. PIR is half the thickness of EPS for the same U-value — but PIR is Euroclass F (combustible), and certain applications require Euroclass A2 or better. Mineral wool is non-combustible but needs twice the thickness for the same U-value. Phenolic foam is the best thermally but costs 60–100% more than PIR.

This guide covers each board family's thermal performance, fire reaction, density and compressive strength, typical applications, installation considerations, and selection criteria for the most common UK installations. For matched-application work see cavity wall insulation types, floor insulation, and loft insulation types.

Key Facts

• PIR (polyisocyanurate) — λ = 0.020–0.023 W/m·K, Euroclass F, density 30–35 kg/m³
• PIR with foil facing — additional reflective surface boosts in-cavity R-value
• Phenolic foam (Kingspan Kooltherm, Recticel Phenolic) — λ = 0.018–0.021 W/m·K, Euroclass C-D, density 33–38 kg/m³
• EPS (expanded polystyrene, Type EPS70/100/150/200) — λ = 0.030–0.038 W/m·K, Euroclass E, density 15–25 kg/m³
• XPS (extruded polystyrene) — λ = 0.029–0.033 W/m·K, Euroclass E, denser/heavier than EPS, water-resistant
• Mineral wool (rock wool / slag wool) — λ = 0.032–0.044 W/m·K, Euroclass A1/A2, density 30–150 kg/m³
• Glass mineral wool (Knauf Mineral Wool) — λ = 0.030–0.044 W/m·K, Euroclass A1/A2
• Wood fibre board (natural) — λ = 0.038–0.048 W/m·K, Euroclass C-D, density 110–270 kg/m³
• Vacuum insulation panel (VIP, premium) — λ = 0.005–0.008 W/m·K, Euroclass A2, very expensive (£60–£120/m²)
• PIR 75mm board (1.2m × 2.4m, 2.88m²) — £45–£70 per board (£15–£24 per m²)
• PIR 100mm board — £55–£85 per board (£19–£30 per m²)
• EPS70 75mm board — £18–£30 per board (£6–£10 per m²)
• EPS100 100mm board — £25–£40 per board (£8–£14 per m²)
• Mineral wool batt (100mm, 6.7m² pack) — £30–£60 per pack (£4.50–£9/m²)
• Mineral wool slab (50mm rigid) — £35–£60 per pack (£5–£8/m²)
• Phenolic 75mm board — £80–£120 per board (£28–£42/m²)
• Wood fibre 100mm board — £45–£80 per board (£16–£28/m²)
• U-value target for new-build wall (Part L 2022) — 0.18 W/m²K (Approved Document L Volume 1)
• U-value target for new-build floor (Part L 2022) — 0.13 W/m²K
• U-value target for new-build roof (Part L 2022) — 0.11 W/m²K
• Required thickness PIR for U=0.18 W/m²K wall — 100mm in cavity
• Required thickness EPS for U=0.18 W/m²K wall — 150–180mm in cavity
• Required thickness mineral wool for U=0.18 W/m²K wall — 160–200mm
• Compressive strength PIR — 100–150 kPa
• Compressive strength XPS — 200–500 kPa (Type IV)
• Compressive strength EPS100 — 70 kPa
• VAT — 20% standard rate

Quick Reference Table

Detailed Guidance

PIR (Polyisocyanurate) — The Default Premium

PIR is the dominant rigid insulation board in UK domestic construction. Brand names: Celotex, Kingspan, Recticel. Key characteristics:

• Lowest λ of common boards (0.020–0.023 W/m·K) — thinnest for a given U-value
• Foil-faced (both sides) reflects radiant heat
• Closed-cell rigid foam, low water absorption
• Euroclass F (combustible) — the trade-off for thermal performance

Use cases:

• Cavity wall (full-fill or partial fill) — 100mm gives U=0.18 W/m²K with brick + 100mm + block + plaster
• Pitched roof insulation (between rafters, with ventilated cavity OR over rafters / "sarking insulation")
• Internal wall insulation (where Euroclass F is acceptable)
• Floor insulation (with VCL above, screed below)

Avoid in:

• Direct exposure to fire (escape routes, in cavities of certain residential buildings post-Grenfell)
• Wet locations without protection (foil only modest water resistance)
• Where Building Regulations Part B requires Euroclass A2 (relevant for some high-rise refurbs)

Phenolic Foam — Best Thermal, Premium Price

Phenolic foam (Kingspan Kooltherm K range) achieves the lowest λ of mainstream insulation:

• λ 0.018–0.021 W/m·K
• Euroclass C–D (improved fire reaction over PIR)
• Higher cost: 50–100% more than equivalent PIR

Use cases:

• New build with strict floor-area budget (thinner insulation = more habitable space)
• Internal Wall Insulation (IWI) on solid walls where wall thickness matters
• High-spec retrofit where the customer is willing to pay for thinness

The "you save 25mm wall thickness" pitch is the typical sale. On a 4m × 3m room, 25mm saved is ~6 m² floor area difference over walls, perimeter — meaningful in expensive areas (London Zone 1–3) where £8,000+ per m² is normal.

EPS — Cheap, Low Performance, Versatile

EPS (Styrofoam, Jablite, Tilan) is the polystyrene "white bead" board. Two density classes:

• EPS70 — light, cheap, modest performance. For under-slab where compressive strength isn't critical.
• EPS100/150/200 — denser, higher compressive strength, slightly better λ. For floors and pavements.

Use cases:

• Floor insulation (especially below screed, where compressive strength matters but not extreme)
• Cold bridging at junctions (low-cost solution)
• Below-DPC insulation (water resistant enough; EPS expands when wet, regains form)
• Raft foundation (in some lightweight construction)

Avoid in:

• Cavity walls of premium specs (EPS thicker than PIR for same U-value)
• Above ground roof applications where thickness is at a premium

XPS — Water-Resistant Premium EPS

XPS (Styrofoam IB or II, Foamglas alternative) is extruded polystyrene:

• λ 0.029–0.033 W/m·K (slightly better than EPS)
• High compressive strength (Type IV: 200–500 kPa)
• Water-resistant (much better than EPS in saturated conditions)
• More expensive than EPS

Use cases:

• Below DPC walls and floors
• Below-slab insulation
• Plinth detail at floor-wall junction
• Inverted (warm) roof insulation
• Below ground retaining walls
• Wet-room sub-floor

Mineral Wool — Non-Combustible Alternative

Mineral wool (rock wool / glass wool) is the non-combustible alternative to rigid foam. Brand names: Rockwool, Knauf, Isover, Earthwool.

• Euroclass A1 (best fire reaction)
• λ 0.032–0.044 W/m·K (thicker than PIR for same U-value)
• Acoustic properties: high sound reduction
• Vapour-permeable (breathable)
• Lower cost than rigid foam

Use cases:

• Loft insulation (the traditional choice)
• Internal stud partition walls (acoustic + fire)
• External Wall Insulation systems (EWI) where mineral wool slabs are preferred for fire
• Cavity walls where post-Grenfell fire regulation requires non-combustible
• Soffit insulation (where vapour movement matters)

Mineral wool batts (loose flexible) and slabs (rigid) serve different roles:

• Batts: stud cavities, loft floors
• Slabs: EWI, rainscreen, below-membrane flat roof, fire-rated partitions

Wood Fibre — Natural / Breathable

Wood fibre rigid boards (Pavatex, Steico, Isovlas) are natural-fibre insulation:

• λ 0.038–0.048 W/m·K (denser, lower than PIR but better than EPS in same thickness)
• Euroclass C–D
• Vapour-permeable (essential for some lime-finished period properties)
• Hygroscopic (absorbs and releases moisture without performance loss)
• Premium price

Use cases:

• Internal wall insulation on solid-walled period buildings (where breathability is critical)
• Eco-spec new build
• Renovation of timber-framed buildings with breathable membrane

The Application-Driven Choice

Cavity wall (full-fill or partial fill): PIR is the standard; mineral wool slab for fire-critical buildings.

Pitched roof: PIR or phenolic between rafters; mineral wool if budget-limited or fire-rated detail needed.

Flat roof (warm): PIR or mineral wool slab; XPS for inverted (warm) roof.

Floor: EPS100 or PIR; XPS below DPC; mineral wool only for suspended timber floors with batts between joists.

Loft (between joists): mineral wool batts (cheapest, easy).

Loft (between rafters for room-in-roof): PIR or phenolic between rafters; mineral wool over for ventilated arrangement.

Internal walls (IWI on solid masonry): PIR + plasterboard for performance; phenolic for thinnest profile; wood fibre for period/breathable detail.

External walls (EWI): mineral wool slab (fire); EPS for budget; PIR for thinnest non-fire-critical.

Compressive Strength — When It Matters

Compressive strength matters where the insulation supports load:

• Below screed: needs to support screed weight + future floor finishes
• Below paving: needs to support pedestrian / occasional vehicle loads
• Below industrial flooring: high load

For domestic floors below screed, EPS100 (70 kPa) or PIR (100 kPa) both work. For pavement/external use, XPS Type IV (200–500 kPa) is needed.

Vapour Control and Cold Bridging

All rigid insulation needs vapour control:

• PIR foil-faced: foil acts as VCL on warm side
• EPS / mineral wool: separate VCL membrane required on warm side

Cold bridging at junctions (floor-wall, wall-window) reduces effective U-value. Specify insulation continuity at junctions; brick/blockwork projecting through insulation is a thermal bridge.

Frequently Asked Questions

Is PIR safe to use after the Grenfell fire?

Yes, in domestic dwellings below 18m. PIR remains in widespread use in cavity walls, lofts and roofs. Post-Grenfell regulation (Building Safety Act 2022 and updated Building Regulations) restricts combustible cladding on buildings ≥18m. Domestic single-family dwellings, and most low-rise residential, can still use PIR. The choice is project-specific.

What's the difference between EPS70 and EPS100?

The number indicates compressive strength: EPS70 = 70 kPa, EPS100 = 100 kPa. Higher numbers = denser, more compressive strength, slightly better λ. EPS200 = 200 kPa for heavy load applications. Choose by load: EPS70 for non-load floor below DPC; EPS100 for typical floor below screed.

Can I use foil-faced PIR in a cavity wall?

Yes — partial-fill cavity wall is the standard application. Maintain a ventilated air gap on the cold side of the insulation (typically 25mm minimum) to ensure rainwater that penetrates the outer leaf can drain without saturating the insulation. Some systems are designed for full-fill (no air gap) — check manufacturer guidance.

Why is wood fibre considered "breathable"?

Wood fibre is vapour-permeable — it allows water vapour to pass through. In a period property with lime mortar walls (also vapour-permeable), wood fibre insulation maintains the wall's ability to dry out, preventing trapped moisture and interstitial condensation. Non-permeable insulation (PIR, EPS) on a lime wall traps moisture against the wall, causing failure. The "breathable" specification matters in heritage refurbishment.

Which board is best for a budget loft conversion?

For loft conversion between rafters: PIR is the standard choice (60mm + 30mm thermal-cushion if needed for 0.18 W/m²K target). Mineral wool batts between rafters is cheaper but needs more thickness, reducing usable head height. The £150–£300 saving on mineral wool vs PIR is usually overshadowed by the head-height loss.

Regulations & Standards

• Building Regulations Part B (England) — Approved Document B Volume 1 (Dwellings) and Volume 2 — fire safety, including external wall combustibility

• Building Regulations Part L (England) — Approved Document L Volume 1 (Dwellings), 2021+2022 — U-value requirements

• Building Safety Act 2022 — restricts combustible cladding on buildings ≥18m

• BS EN 13501-1:2018 — Fire classification of construction products and building elements (Euroclass)

• BS EN 13162 — Mineral wool products

• BS EN 13163 — EPS products

• BS EN 13164 — XPS products

• BS EN 13165 — PUR (Polyurethane / PIR) products

• BS EN 13166 — Phenolic foam products

• BS EN 13986 — Wood-based panels for construction

• PAS 2030:2023 — Specification for installation of energy efficiency measures

• Kingspan Insulation: Technical specifications

• Celotex Insulation: Product data sheets

• Rockwool: Specifications and U-value calculators

• Knauf Insulation: Technical resources

• Approved Document L — Conservation of fuel and power

• Approved Document B — Fire safety

• cavity wall insulation types — cavity wall spec

• loft insulation types — loft applications

• floor insulation — floor build-ups

• external wall insulation — EWI spec

• internal wall insulation — IWI spec