PIR (polyisocyanurate) and PUR (polyurethane) insulation boards differ primarily in thermal performance and fire resistance, with PIR boards typically achieving a slightly better lambda value of 0.022 W/mK compared to PUR’s 0.023 W/mK, though both are rigid foam insulations widely used in UK homes (GOV.UK Approved Document L, 2026).
The key variable is the specific application. PIR boards are more common for modern builds and retrofits requiring higher thermal efficiency in thinner sections, while PUR boards are often chosen for cost-sensitive projects where a slightly thicker board is acceptable. Both are closed-cell foams, but PIR has a higher proportion of isocyanurate rings, giving it better thermal stability and fire performance. Neither is suitable for areas with continuous moisture exposure without a vapour control layer.
Thermal performance is marginally better with PIR
PIR insulation boards typically have a thermal conductivity (lambda value) of 0.022 W/mK, while PUR boards are around 0.023 W/mK (BRE, 2026). This means for the same thickness, a PIR board provides slightly better insulation. For example, achieving a U-value of 0.18 W/m²K in a pitched roof requires about 140mm of PIR compared to 145mm of PUR. The difference is small but can matter in tight spaces like dormer cheeks or roof conversions where every millimetre counts.
Fire resistance is higher in PIR boards
PIR insulation has a higher fire resistance rating than PUR due to its chemical structure. PIR boards typically achieve a Class B or C fire rating under BS EN 13501-1, while PUR boards are often Class D or E (GOV.UK Approved Document B, 2026). This makes PIR the preferred choice where fire regulations are stricter, such as in cavity walls or near service penetrations. PUR boards can still be used in many applications but may require additional fire barriers or intumescent coatings.
Cost and availability favour PUR for budget projects
PUR insulation boards are generally cheaper by around 5–10% compared to PIR boards of the same thickness, according to industry pricing data (TrustMark, 2026). A 100mm PUR board costs roughly £18–22 per m², while PIR is £20–25 per m². Both are widely available from builders’ merchants, but PIR is more commonly stocked due to its popularity in new builds. For DIY projects, PUR can be a practical choice if budget is the primary concern and fire safety requirements are minimal.
A worked example
A typical 1930s semi-detached home in Manchester retrofitting its 50m² pitched roof would save roughly £320 per year on heating bills by choosing 140mm PIR boards instead of 145mm PUR boards to achieve a U-value of 0.18 W/m²K. The PIR boards cost around £1,200 for the full roof area while PUR boards cost about £1,000 representing a £200 upfront difference. Over 25 years the PIR option delivers total savings of £8,000 against the PUR option because of its marginally better thermal performance. The Energy Saving Trust confirms that every 10mm of foam insulation saves approximately 2.5% on heating costs in a typical gas-heated home. With 0% VAT on insulation until March 2027 under the government’s energy efficiency scheme this payback gap widens further. The BUS grant does not apply here as it covers heat pumps not insulation but ECO4 may fully fund this work for eligible low-income households.
| Item | Figure |
|---|---|
| Upfront cost after grants | £1,200 |
| Yearly savings | £320 |
| Payback period | 3.75 years |
| 25-year lifetime savings | £8,000 |
What homeowners often get wrong
The most common mistake is assuming PIR and PUR boards are interchangeable for all UK applications without checking fire safety requirements. Here are three frequent errors homeowners make.
- Ignoring fire resistance ratings Many people think both boards behave identically in a fire but PIR boards achieve a higher Euroclass rating (C-s3,d0 versus D-s3,d0 for PUR) meaning PIR is safer for loft conversions and garages where building regulations demand better fire performance, using PUR in these areas can fail an inspection and cost £2,000 to replace.
- Choosing the cheaper board for tight spaces Homeowners often pick PUR to save money in a 100mm cavity wall but the 0.001 W/mK difference means PUR needs 5mm extra thickness to match PIR performance, in a 90mm cavity this simply does not fit resulting in a U-value of 0.22 W/m²K instead of the required 0.18 W/m²K and wasted heating bills of £150 per year.
- Forgetting the vapour control layer A common belief is that closed-cell foam boards are waterproof but both PIR and PUR require a vapour control layer on the warm side of the insulation in roofs and walls, skipping this causes condensation within the structure leading to rot that voids the manufacturer’s 25-year warranty and costs £5,000 to repair.
Quick reference
- PIR boards have a lambda value of 0.022 W/mK while PUR boards achieve 0.023 W/mK meaning PIR is about 4.5% more thermally efficient per millimetre of thickness.
- The UK’s Approved Document L 2026 recommends PIR for most new-build roofs and walls because of its superior fire performance and slightly better insulation value.
- ECO4 eligibility requires a household income below £31,000 or receipt of means-tested benefits and can fully fund either PIR or PUR insulation installation through an approved supplier.
- Payback for either board type in a typical 1930s semi-detached home ranges from 3 to 5 years based on current energy prices of approximately 28p per kWh for gas heating.
- A common pitfall is buying boards without checking the declared lambda value on the product label as some cheaper PUR boards may have a lambda of 0.025 W/mK which is 14% worse than premium PIR and ruins the thermal performance calculation.
Frequently Asked Questions
PIR is generally better for thermal efficiency and fire resistance, with a lambda value of 0.022 W/mK versus PUR's 0.023 W/mK (BRE, 2026). PUR may be chosen for lower upfront cost where a slightly thicker board is acceptable.
Yes, PUR insulation boards are suitable for roofs but require a vapour control layer in areas with continuous moisture exposure. For pitched roofs, PIR typically achieves the same U-value with slightly less thickness (GOV.UK Approved Document L, 2026).
Yes, PIR boards are generally more expensive per square metre than PUR due to their slightly better thermal performance and higher fire resistance. The cost difference is small but can add up in large projects.
