PIR insulation boards offer the best thermal performance per millimetre of any commonly available insulation material, making them the go-to choice for meeting Part L 2021 requirements. With typical lambda values of 0.022 W/mK, polyisocyanurate (PIR) boards outperform mineral wool and expanded polystyrene by a significant margin, allowing contractors to hit U-value targets with thinner build-ups.
This guide covers the current Part L requirements, why PIR remains the preferred specification for most projects, and how to avoid common installation mistakes that compromise thermal performance.
What Is Part L of the Building Regulations?
Part L of the UK Building Regulations governs the conservation of fuel and power in buildings. The 2021 edition, which came into force in June 2022, introduced stricter thermal performance requirements for new builds, extensions, and renovations. These changes represent the first step toward the Future Homes Standard, with even tighter requirements expected in coming years.
The regulations set maximum U-values for different building elements. A U-value measures how much heat passes through a material. Lower numbers indicate better insulation. Meeting these targets requires careful material selection and proper installation.
Part L 2021 U-Value Requirements
The current Part L requirements set the following U-value limits for new dwellings in England:
- Roofs: 0.16 W/m²K (limiting) / 0.11 W/m²K (notional dwelling specification)
- Walls: 0.26 W/m²K (limiting) / 0.18 W/m²K (notional dwelling specification)
- Floors: 0.18 W/m²K (limiting) / 0.13 W/m²K (notional dwelling specification)
The “limiting” values represent the absolute maximum allowed. The “notional dwelling” figures indicate what a typical compliant new home should achieve. In practice, most specifications target somewhere between these two figures.
For extensions and renovations, the requirements differ slightly. New elements in existing dwellings typically need to achieve 0.18 W/m²K for walls, 0.15 W/m²K for roofs, and 0.18 W/m²K for floors.
Why PIR Insulation Outperforms Alternatives
PIR insulation boards achieve thermal conductivity values around 0.022 W/mK. Compare that to mineral wool at 0.035-0.044 W/mK or EPS at 0.031-0.038 W/mK, and the thickness advantage becomes clear.
To put real numbers on it: achieving a 0.18 W/m²K wall U-value with mineral wool might require 150-170mm of material. The same performance can be reached with approximately 110-120mm of PIR. That difference matters when cavity width is limited or when maximising internal floor area affects project viability.
The thickness advantage becomes even more significant in retrofit situations. Older properties often have narrow cavities or limited space for internal insulation. PIR allows contractors to meet modern thermal standards without losing excessive internal space or requiring major structural modifications.
Practical Advantages of PIR on Site
Beyond raw thermal performance, PIR boards offer workability benefits that translate to faster installation and less waste.
Compressive strength typically ranges from 100-150 kPa. This means PIR boards can be used in floor applications without additional protection boards in many situations, simplifying build-ups and reducing material costs.
Moisture resistance comes from the closed-cell structure of the foam. PIR boards resist moisture ingress, making them suitable for below-ground applications and situations where temporary weather exposure during construction is unavoidable.
Clean cutting with standard tools means less mess and less waste compared to friable materials like mineral wool. Boards maintain their structural integrity during handling and installation, and offcuts can often be used in other locations.
Choosing Between PIR Brands
The UK market includes several PIR manufacturers: Celotex, Kingspan, Recticel, Ecotherm, and IKO Enertherm among others. All conform to BS EN 13165 and achieve similar lambda values around 0.022 W/mK.
For general applications like cavity walls, pitched roofs, and solid floors, the performance differences between major brands are minimal. The choice often comes down to availability, pricing, and existing supplier relationships.
That said, specific applications may benefit from particular product ranges. Some manufacturers offer boards with factory-applied facings that simplify certain build-ups. Dedicated flat roof products often feature enhanced surface finishes that improve adhesion for waterproofing membranes.
Suppliers like Online Insulation stock multiple brands including Celotex, Kingspan, Rockwool, Knauf, and Recticel. Having access to several manufacturers through one supplier means you can source whatever is available and best priced for your specific project.
Common Specification and Installation Mistakes
Even quality materials fail to deliver designed performance when specification or installation goes wrong. Watch out for these issues.
Thermal bridging at wall ties, lintels, and junctions can increase actual U-values by 0.02-0.04 W/m²K beyond calculated figures. Always specify low-conductivity wall ties and insulated lintels. Pay attention to junctions between walls, floors, and roofs where thermal bridges commonly occur.
Air gaps from poorly fitted boards dramatically reduce performance. Research suggests a 5mm air gap can reduce effective R-value by 20-30%. Tight-fitting installation matters. Use appropriate adhesives where boards meet, and ensure continuous coverage without gaps between boards or around services.
Compression reduces insulation thickness and compromises thermal performance. Avoid over-compressing flexible insulation or installing rigid boards under excessive load. Follow manufacturer guidance on loading limits and support requirements.
Wrong product selection for the application creates problems. Cavity wall products differ from floor products and flat roof products. Each is formulated and faced appropriately for its intended use. Using general-purpose boards in specialist applications may compromise performance or durability.
Planning for Future Regulations
The Future Homes Standard requires even lower U-values than Part L 2021. Indicative specifications suggest targets around 0.11 W/m²K for floors, 0.15 W/m²K for walls, and 0.11 W/m²K for roofs.
Specifying beyond current minimum compliance makes sense for several reasons. It provides a buffer against future regulation changes, reducing the risk that a project approved under current rules becomes non-compliant during construction. It also improves long-term building performance and running costs for occupants.
For example, targeting 0.14 W/m²K for roofs rather than the 0.16 W/m²K limiting value requires only modest additional thickness but provides meaningful headroom.
Getting the Right Materials at the Right Price
PIR insulation remains the practical choice for meeting Part L requirements efficiently. The combination of superior thermal performance, workability, and structural properties makes it suitable for most residential and commercial applications.
The key is matching products to specific project requirements and ensuring proper installation. Working with a supplier that stocks multiple brands and thicknesses, offers competitive pricing, and can provide informed advice simplifies the specification process. Online Insulation carries Celotex, Kingspan, Recticel, and other leading brands with over 100 distribution points across the UK for timely delivery.
Frequently Asked Questions
What is the difference between limiting U-values and notional dwelling U-values in Part L?
Limiting U-values are the absolute maximum allowed for any building element. Notional dwelling values represent what a typical compliant new home should achieve and are used in SAP calculations. Most projects should aim for values between these two figures or better.
How thick does PIR insulation need to be to meet Part L 2021?
Thickness depends on the specific application and target U-value. For a typical wall targeting 0.18 W/m²K, approximately 110-120mm of PIR is usually required. For roofs targeting 0.11 W/m²K, around 130-150mm may be needed. Always run calculations for your specific build-up.
Can I use the same PIR board for walls, floors, and roofs?
Multi-purpose boards like Celotex GA4000 or Kingspan TP10 can be used across applications. However, some situations benefit from application-specific products. Cavity wall boards have different facings than floor boards, and flat roof products often feature surfaces designed for membrane adhesion.
Does PIR insulation require a vapour barrier?
PIR boards typically include low-emissivity foil facings that provide some vapour resistance. However, requirements vary by application. Warm roof constructions and internal wall insulation typically need careful vapour control. Check manufacturer guidance and consider condensation risk analysis for your specific build-up.
What happens if I do not meet Part L requirements?
Building control will not sign off the project until thermal requirements are met. This can delay completion, require remedial work, and impact project costs. Getting specifications right from the start avoids these problems.
Are there any applications where PIR is not suitable?
PIR has temperature limitations. It should not be exposed to temperatures above around 90°C, which limits its use directly adjacent to heat sources like boilers or flues. In these situations, mineral wool or other high-temperature materials are more appropriate.