PIR vs Mineral Wool Insulation: Performance, Cost and When to Use Each

Summary

Every insulation quote comes down to the same trade-off: thermal performance per millimetre versus cost per square metre versus fire and moisture behaviour. PIR and mineral wool sit at opposite ends of that trade-off, and confusing them — or defaulting to whichever one a merchant has in stock — is one of the most common pricing and compliance mistakes on refurb and extension jobs.

PIR board (sold under names like Celotex, Kingspan Kooltherm/K-series, Recticel Eurothane) is a rigid closed-cell foam insulation with a foil facing. Its low thermal conductivity means a 100mm PIR board does roughly the job of 150-160mm of mineral wool. That makes it the go-to for loft conversions where rafter depth is fixed, warm flat roofs, and internal wall insulation where losing living space matters. The trade-off is cost (typically 2-3x more per m² than mineral wool of equivalent thickness) and fire performance — most PIR boards are Euroclass D or E, meaning they will burn and need mineral wool fire breaks or fire-rated plasterboard in multi-storey or high-risk applications.

Mineral wool (glass wool or rock/stone wool — Rockwool, Knauf, Isover) is non-combustible (Euroclass A1 for rock wool, A2-s1,d0 for most glass wool ranges), cheaper, and vapour-permeable, which suits breathable constructions and reduces interstitial condensation risk. It's the standard for cavity wall insulation, loft insulation between and over joists, and acoustic partitions where its open-cell structure also absorbs sound. Its lower R-value per mm means thicker cavities or joist depths are needed, which isn't always available in a retrofit.

The common misconception is that one material is universally "better." They're specified for different constraints. A tradesperson quoting a job needs to check depth available, fire strategy, moisture risk, and budget before defaulting to either — and needs to know when Building Control will reject a spec outright (e.g. combustible insulation above 18m in relevant buildings under the amended Building Regulations).

Key Facts

• PIR thermal conductivity (lambda) — typically 0.020-0.023 W/m·K depending on brand and board age (lambda increases slightly over the product's life, "thermal drift")
• Mineral wool thermal conductivity — glass wool typically 0.032-0.044 W/m·K; rock wool typically 0.034-0.038 W/m·K depending on density
• Fire classification — PIR boards are generally Euroclass D-s2,d0 or E; rock wool is Euroclass A1 (non-combustible); glass wool is typically A2-s1,d0
• Combustible cladding ban — Building Regulations (England) restrict combustible insulation and materials on the external walls of "relevant buildings" over 18m (and 11m from December 2026 changes) — PIR is not permitted in these applications; mineral wool (A1/A2) is
• Cost differential — PIR board typically costs 2-3x more per m² than mineral wool slab of equivalent thickness, though the gap narrows once you account for the reduced depth (and therefore reduced structural build-up) PIR allows
• Compressive strength — PIR boards have useful compressive strength (typically 100-150 kPa) making them suitable for below-slab and inverted roof applications; standard mineral wool slab has low compressive strength unless specified as a "floor grade" or "flat roof grade" product
• Vapour permeability — mineral wool is vapour-open (breathable), which suits older solid-wall and timber-frame constructions; PIR is effectively vapour-closed and needs careful vapour control layer (VCL) detailing to avoid interstitial condensation
• Moisture performance — mineral wool loses thermal performance significantly if it gets wet and stays wet (capillary action wicks water through the fibres); PIR's closed-cell structure resists water absorption, useful below DPC or in flood-risk builds
• BBA certification — always check the specific product has a current BBA (British Board of Agrément) certificate for the intended application — thermal performance figures vary by manufacturer and board generation
• U-value targets (Approved Document L, England, 2021 edition) — new dwelling: walls 0.18 W/m²K, floor 0.13 W/m²K, roof 0.11 W/m²K; existing dwelling extensions/conversions ("worst elemental" method): walls 0.28 W/m²K, floor 0.22 W/m²K, roof 0.16 W/m²K — figures vary for Wales (Part L Wales) and Scotland (Section 6) —
• Acoustic performance — mineral wool's open fibre structure gives significantly better sound absorption than PIR's closed-cell foam, making it the default for acoustic partitions and Part E-compliant separating walls/floors
• Sustainability/embodied carbon — mineral wool (especially rock wool) generally has lower embodied carbon and is often manufactured with high recycled content; PIR is petrochemical-derived with higher embodied carbon per unit, though its thinner build-up partially offsets this over the building's operational life
• Fixing method — PIR boards are typically mechanically fixed or adhesive-bonded with foil tape at joints (critical to avoid air gaps that create cold bridging); mineral wool is friction-fit (cavity wall, between joists) or held with insulation retaining pins/wire (external wall systems)
• Board sizes — PIR typically supplied in 2400x1200mm sheets, various thicknesses from 25mm to 150mm+; mineral wool slab typically 1200x600mm or as rolls, thicknesses from 25mm to 200mm+

Quick Reference Table

Detailed Guidance

Loft conversions and rafter insulation

Rafter depth on most UK roofs is 100-150mm, and Part L targets for a warm roof conversion typically require a U-value around 0.16-0.18 W/m²K for existing dwellings. Fitting mineral wool between rafters alone rarely gets there without adding a second layer over the rafters (which eats into headroom you don't have in a loft conversion). PIR between rafters, combined with a thinner over-rafter or under-rafter PIR layer, hits target U-values in far less depth — this is why PIR dominates loft conversion specs. Always leave the mandatory 50mm ventilated air gap above the insulation for a cold/hybrid roof unless you're doing a fully vapour-closed warm roof build-up with VCL below the insulation — see warm roof cold roof for the full detailing.

Cavity walls

New-build and retrofit cavity wall insulation is dominated by mineral wool (blown fibre or batts) and PU/EPS bead, not PIR board, because cavity widths (typically 100-150mm on newer builds, narrower on older stock) and partial-fill/full-fill strategies are designed around wool and bead products. PIR board can be used as partial-fill rigid board with a residual clear cavity for full-fill risk mitigation, but it's a less common spec than blown mineral wool. See cavity wall for partial-fill vs full-fill decision criteria.

Internal wall insulation (IWI) and solid walls

For solid brick or stone walls without a cavity, internal wall insulation is a common retrofit measure. PIR-faced plasterboard laminate (insulated plasterboard) is popular because it combines the insulation and the finish layer in one fix, saving labour — but it creates a vapour-closed buildup that risks interstitial condensation at floor/ceiling junctions and around window reveals unless detailed correctly with a VCL and often a small air gap. Breathable IWI systems (wood fibre, mineral wool with a vapour-open finish) are increasingly specified on period or solid-wall properties to avoid trapping moisture in the original masonry. See internal wall insulation for the full comparison and condensation risk mapping.

Floor insulation

Suspended timber floors typically take mineral wool slab supported on netting or battens between joists — cheap, easy to install, adequate depth usually available. Solid concrete floors (new slab or floating floor over existing slab) favour PIR because of its compressive strength under a screed or floating floor deck, and because floor build-up height is almost always at a premium against door thresholds and stair nosings. See floor insulation for full build-ups including compatibility with wet underfloor heating.

Fire safety and where PIR cannot be used

Since the Grenfell Tower fire, Building Regulations Part B (Approved Document B) and subsequent amendments have progressively restricted combustible materials in external wall systems on relevant buildings — currently 18m and above in England, with government commitments to extend restrictions to 11m+ buildings. This affects external wall insulation (EWI) specification directly: PIR and other combustible foam boards are excluded from these applications; mineral wool (A1 rated rock wool specifically) is mandated. This is a compliance issue, not a preference — using PIR on a restricted-height building's external wall is a Building Regs breach. Always check current height thresholds before quoting EWI on any building over a few storeys — see render insulation systems and external wall insulation.

Mixing materials in one build-up

Many real-world specs combine both: mineral wool fire breaks within PIR external wall insulation systems at every floor level (a requirement, not optional, in EWI systems using combustible insulation below the restricted height threshold), or PIR floor insulation with mineral wool acoustic quilt in a separating floor above it. Don't assume a spec should be "all one material" — read the architect's or SAP assessor's build-up drawing carefully, and price both materials into the same job where specified.

Frequently Asked Questions

Can I substitute PIR for mineral wool (or vice versa) to save cost without telling the client?

No — never substitute insulation type on a spec'd job without agreement, and definitely not on anything requiring Building Control sign-off or a SAP calculation. The U-value calculation, fire strategy, and condensation risk assessment are all based on the specified material's exact lambda value and vapour permeability. Swapping without recalculating can fail Building Control inspection or invalidate the EPC/SAP assessment tied to the property.

Does mineral wool insulation need protecting from rain during construction?

Yes. Mineral wool loses a significant proportion of its thermal performance when wet and can take a long time to fully dry out within a closed structure, sometimes never fully recovering if trapped against a vapour-closed layer. Always keep mineral wool covered and dry until the building is weathertight, and never install it into a cavity or void that could see standing water.

Is PIR insulation safe to use in a loft with downlights or hot flues nearby?

PIR is combustible (Euroclass D or E) and manufacturer instructions typically require clearance from heat sources like recessed downlights, flue pipes, and electrical equipment that generates heat. Fire-rated downlight covers or repositioning is often needed — check the specific PIR manufacturer's technical data sheet for minimum clearances, and never pack PIR tight against an unprotected flue.

Why did my PIR insulation cost quote come in higher than the merchant's per-m² price suggested?

PIR boards are typically cut and taped at every joint with foil tape to prevent air movement through gaps (cold bridging can account for a significant real-world performance loss if joints are left untaped). Factor taping labour and material into every PIR quote — it's not optional if you want the insulation to perform as calculated.

Can I use mineral wool where PIR was specified, if depth allows?

If the SAP/U-value calculation was done for PIR's lambda value, you cannot simply substitute a thicker mineral wool layer without re-running the calculation — the numbers don't scale linearly with other factors in the build-up (bridging from studs/joists, air gaps). Always get a revised U-value calculation from whoever produced the original SAP/Part L compliance documents before substituting.

Regulations & Standards

• Building Regulations Approved Document L (Conservation of fuel and power) — sets U-value targets for new and existing dwellings; England 2021 edition current at time of writing —

• Building Regulations Approved Document B (Fire safety) — governs combustible material restrictions in external wall systems, particularly for buildings above the current height threshold

• BS EN 13165 — specification for factory-made rigid polyurethane foam (PIR/PUR) thermal insulation products

• BS EN 13162 — specification for factory-made mineral wool (MW) thermal insulation products

• BS EN ISO 6946 — method for calculating thermal resistance and transmittance (U-value) of building elements

• BBA (British Board of Agrément) certification — third-party product certification confirming a specific insulation product's stated performance and application suitability

• PAS 2035/2030 — required specification and installation standard for retrofit insulation measures installed under government-funded schemes (ECO4, GBIS)

• Approved Document L: Conservation of fuel and power — UK Government

• Approved Document B: Fire safety — UK Government

• BBA Certificates search — British Board of Agrément

• Kingspan Technical Insulation Data — Manufacturer technical documentation (example, not endorsement)

• Rockwool Technical Insulation Guidance — Manufacturer technical documentation (example, not endorsement)

• rigid insulation boards — direct comparison of PIR, EPS, XPS, and phenolic board lambda values and applications

• insulation materials — full insulation material selector across mineral wool, PIR, EPS, phenolic, sheep wool

• floor insulation — floor-specific insulation build-ups for suspended timber and solid concrete floors

• u value — U-value calculator with worked examples for cavity wall, loft, and ground floor