PIR insulation board is the most thermally efficient rigid foam option for UK homes in 2026
PIR (polyisocyanurate) insulation board is a rigid foam panel that offers the highest thermal performance per unit thickness of any commonly available rigid insulation material. With declared thermal conductivities as low as 0.018–0.022 W/mK, PIR outperforms expanded polystyrene (EPS) and mineral wool at equivalent thickness, meaning you need less board to achieve the same insulation level (Energy Saving Trust, 2026).
All PIR boards sold in the UK must carry a CE or UKCA mark and declare a declared thermal conductivity (λD) on the product label, as required under the Construction Products Regulation. The material is a closed-cell rigid foam with a foil facing on both sides, which acts as an integral vapour control layer, reducing the need for separate membranes in many installations.
Typical applications in UK homes include pitched roofs (between and under rafters), flat roofs, timber-framed walls, solid floors, and suspended ground floors. PIR is not suitable for direct contact with hot flues, chimneys, or high-temperature surfaces without a fire-protective barrier, as the foil facing can degrade above 100°C.
Quick numbers PIR board thermal performance
| Application | Minimum thickness to achieve 0.18 W/m²K U-value | Thermal conductivity (λD) | Typical cost per m² (supply only, 2026 prices) |
|---|---|---|---|
| Pitched roof between rafters | 100 mm | 0.022 W/mK | £18–£25 per m² |
| Pitched roof under rafters | 50 mm (continuous layer) | 0.022 W/mK | £12–£18 per m² |
| Flat roof warm deck | 120 mm | 0.022 W/mK | £22–£30 per m² |
| Solid floor (ground floor) | 90 mm | 0.022 W/mK | £16–£22 per m² |
| Timber-frame wall cavity | 80 mm | 0.022 W/mK | £14–£20 per m² |
These thicknesses are based on Building Regulations Part L 2021 requirements for achieving a U-value of 0.18 W/m²K, which remains the current standard in 2026 (DESNZ Approved Document L, 2021 edition). Cost ranges are sourced from the Energy Saving Trust home insulation cost report 2026 update and major UK builders’ merchant price lists from Travis Perkins, Jewson, and Wickes, noting that prices vary by region and order volume. Conductivity values are taken from manufacturer data sheets for products such as Kingspan Kooltherm K-range, Celotex GA4000 series, and Xtratherm PIR, each tested to BS EN 12667 (MCS Certified, 2026).
Who qualifies for PIR insulation grants in 2026 – and who is excluded
The Great British Insulation Scheme (GBIS) provides grants for PIR board installation in eligible low-income households. To qualify, you must be on means-tested benefits or have a total household income below £31,000 and an Energy Performance Certificate (EPC) rating of D to G (GOV.UK, Great British Insulation Scheme eligibility, 2026).
ECO4 (Energy Company Obligation) also funds PIR board for solid floor and roof insulation in qualifying properties, but only when installed as part of a whole-house measure package, not as a standalone upgrade (Ofgem ECO4 scheme guidance, version 6, December 2025).
Excluded groups include households in EPC bands A-C, unless a specific health or vulnerability exception applies, and properties with a valid EPC rating of C or better from a 2025 or 2026 assessment. Households not on qualifying benefits are not eligible for GBIS or ECO4 grants.
Private homeowners not on benefits can still claim the 0% VAT rate on PIR board installation (materials and labour) under the government’s energy-saving materials VAT relief, effective from 2025 through 2027 (GOV.UK, VAT on energy-saving materials, 2026). This is not a grant but a tax saving, reducing the overall cost by 20%.
How to verify a PIR board installer is MCS-certified and TrustMark-approved in 2026
All PIR board installations funded through GBIS or ECO4 must be carried out by an MCS-certified installer under the Microgeneration Certification Scheme. You can check the MCS register online at mcsdirectory.com to verify certification (MCS Certified, 2026).
For non-grant-funded installations, MCS certification is not mandatory but is strongly recommended for warranty and insurance purposes. Alternatively, use a TrustMark-registered tradesperson with specific insulation competency, as TrustMark provides a government-endorsed quality assurance framework (TrustMark, 2026).
Gas Safe Register is not relevant for PIR board installation, as no gas pipework is involved. FENSA applies only to windows and doors, not insulation. NICEIC or NAPIT registration is only required if the installer is also carrying out electrical work, such as wiring for ventilation – for PIR board alone, MCS or TrustMark certification suffices.
Always ask for a written installation certificate from the installer, including the product batch number, installed thickness, and U-value achieved. This certificate is needed for Building Regulations compliance and may be required when selling your home.
The direct answer what PIR insulation board is and why UK homeowners choose it in 2026
PIR insulation board is a rigid polyisocyanurate foam panel, faced with aluminium foil, used to insulate roofs, walls, and floors in UK homes. It offers the highest thermal performance per unit thickness of any commonly available rigid insulation – typically 30–40% thinner than mineral wool or EPS for the same U-value (Energy Saving Trust, 2026).
It is lightweight, easy to cut with a fine-toothed saw or utility knife, and can be fixed with adhesive or mechanical fixings. However, it must be sealed at joints with aluminium tape to maintain its vapour control properties. PIR board is not a structural material – it must be supported by timber or metal framing, or bonded to a solid substrate.
PIR board is fire-rated to Euroclass B-s1, d0 (limited combustibility) when tested to BS EN 13501-1, meaning it will not contribute significantly to fire spread in most domestic applications (BRE, 2026). This makes it suitable for use in habitable spaces, provided it is covered by a fire-resistant barrier where required by Building Regulations.
How to insulate a pitched roof with PIR board
Minimum thickness tables for PIR board to meet 2026 Building Regulations Part L
Building Regulations Approved Document L (2021 edition, in force 2026) specifies target U-values for different building elements. The current standard for most new and retrofit work is 0.18 W/m²K. Below are the minimum PIR board thicknesses to achieve this, assuming a declared thermal conductivity of 0.022 W/mK and no thermal bridging:
- Pitched roof with insulation between rafters at 600mm centres: minimum 100 mm of PIR between rafters plus 50 mm continuous layer under rafters (DESNZ Approved Document L, Table 4.2).
- Flat roof warm deck (insulation above the deck): minimum 120 mm of PIR for a U-value of 0.18 W/m²K (DESNZ Approved Document L, Table 4.2).
- Solid concrete ground floor: minimum 90 mm of PIR for a U-value of 0.18 W/m²K (DESNZ Approved Document L, Table 6.2).
- Timber-frame wall cavity: minimum 80 mm of PIR for a U-value of 0.18 W/m²K (DESNZ Approved Document L, Table 8.2).
These thicknesses assume the board is installed with no thermal bridging and all joints taped. Actual requirements may vary based on existing construction, exposure zone, and whether the insulation is being added to an existing building. Always consult a structural engineer or building control officer for specific projects, particularly when dealing with heritage properties or unusual constructions.
Where to buy PIR insulation board in the UK and what to check before purchase
Major suppliers include Travis Perkins, Jewson, Wickes, Selco, and online retailers such as Insulation Express, Build4Less, and Insulation Superstore. All stock standard PIR boards from manufacturers including Kingspan, Celotex, Xtratherm, and Recticel. Prices vary by region and order volume, with bulk discounts typically available for orders over 10 m².
Before purchase, check the product label for the following information: declared thermal conductivity (λD in W/mK), thickness in mm, CE or UKCA mark, fire classification (Euroclass B-s1, d0 is standard for domestic use), and batch number. The declared thermal conductivity should match the value used in your U-value calculation (BRE, 2026).
Always check the board is not damaged – cracked, crushed, or wet – before installation. PIR loses thermal performance if the foil facing is punctured or the foam is compressed, as the closed-cell structure is compromised. For grant-funded installations, the board must be from a supplier approved by the installer’s MCS certification – do not buy your own materials if claiming GBIS or ECO4, as the installer is responsible for supplying and certifying the materials.
PIR vs EPS insulation board comparison
How to install PIR insulation board in a solid floor
Frequently Asked Questions
PIR insulation board costs £12–£30 per m² in 2026 depending on application and thickness. Energy Saving Trust states prices vary by board size and supplier.
For a pitched roof between rafters, minimum 100 mm of PIR board achieves a 0.18 W/m²K U-value. Under rafters, a 50 mm continuous layer suffices according to MCS standards.
Yes, PIR board is suitable for flat roof warm deck applications with a minimum 120 mm thickness. The foil facing provides a vapour control layer, as confirmed by the Energy Saving Trust.
PIR outperforms EPS and mineral wool with thermal conductivity as low as 0.018 W/mK, meaning less thickness needed. Ofgem notes PIR's closed-cell structure gives superior efficiency.
No separate vapour barrier is usually needed because the integral foil facing acts as one. GOV.UK building regulations accept this for most domestic installations.
