PIR Insulation: The Complete Guide for UK Trade Professionals

PIR insulation (polyisocyanurate) is a rigid foam insulation board with a typical thermal conductivity of 0.022 W/mK, making it one of the best-performing insulation materials available for UK construction. 

PIR boards consist of a closed-cell foam core bonded between facings (usually aluminium foil or glass tissue), providing thermal insulation, structural rigidity, and vapour resistance in a single product. Trade professionals use PIR insulation across roofs, walls, and floors where space constraints demand maximum thermal performance per millimetre of thickness.

What is PIR Insulation?

PIR insulation is a thermoset plastic foam created through a chemical reaction between polyisocyanurate and other compounds. The manufacturing process produces a rigid foam with closed-cell structure, meaning each tiny cell in the foam is sealed and contains inert gas that provides excellent thermal insulation properties.

The “PIR” name comes from polyisocyanurate, the primary polymer in the foam structure. This differs from PUR (polyurethane) insulation, though the two materials share similar chemistry and performance characteristics. PIR offers slightly better high-temperature performance and fire resistance compared to PUR, which is why PIR dominates UK construction specifications.

Key characteristics of PIR insulation:

• Thermal conductivity: 0.022-0.023 W/mK
• Closed-cell structure: 95%+ closed cells
• Density: 30-35 kg/m³
• Compressive strength: 100-200 kPa depending on grade
• Operating temperature: -50°C to +80°C
• Fire classification: typically Class E or better
• Moisture resistance: very low water absorption

The closed-cell structure gives PIR insulation several advantages over alternatives. Water and water vapour struggle to penetrate the material. The foam maintains its thickness under load. The thermal performance stays consistent over the building’s lifetime because air can’t circulate through the material.

How PIR insulation compares to other materials:

*Approximate thickness for solid floor construction achieving 0.18 W/m²K

The performance advantage becomes clear when space is limited. A 100mm PIR board provides the same thermal resistance as 150mm of mineral wool or 160mm of EPS. For retrofit projects, tight floor-to-ceiling heights, or cavity walls with limited width, this thickness difference often determines whether a project is viable.

How PIR Insulation Achieves Superior Thermal Performance

PIR insulation’s thermal performance comes from three factors working together: closed-cell structure, low-conductivity gas trapped in cells, and reflective facings.

Closed-Cell Structure

The foam consists of millions of tiny closed cells, each acting as a sealed bubble. This structure prevents air circulation through the material. Air circulation is what kills thermal performance in open-cell materials – warm air moves through the material carrying heat with it.

With closed cells, heat can only transfer through the solid foam structure itself, which is minimal. The closed-cell structure also prevents moisture ingress. Water can’t move through sealed cells, so PIR maintains its thermal performance even in damp conditions where fibrous insulation would deteriorate.

Low-Conductivity Cell Gas

During manufacturing, cells fill with a low-conductivity gas (typically a hydrocarbon blend or HFC). This gas has lower thermal conductivity than air, which improves insulation performance. Over time, some gas diffuses out and air diffuses in, but this happens slowly and is accounted for in the declared lambda value.

The declared lambda value of 0.022 W/mK represents aged performance after gas diffusion has stabilised, not fresh-from-factory performance. This means real-world thermal performance matches calculations rather than degrading over time.

Reflective Facings

Most PIR boards have aluminium foil facings bonded to both sides. These reflective surfaces reduce radiant heat transfer across air gaps adjacent to the board. This matters in cavity constructions or where boards sit against air spaces.

The foil facings also provide vapour resistance, act as an airtightness layer when joints are taped, and give the boards improved surface strength for handling and installation.

Temperature Performance

PIR insulation maintains consistent thermal performance across a wide temperature range. Unlike some materials that lose effectiveness at low temperatures, PIR works equally well at -20°C as at +20°C. This matters for cold roof voids, unheated spaces, and external wall applications exposed to winter temperatures.

The upper temperature limit of +80°C continuous (higher for short periods) means PIR suits most building applications. For situations requiring higher temperature resistance – like directly beneath torch-applied roofing felts – manufacturers produce boards with enhanced high-temperature facings.

PIR Insulation Specifications by Thickness

PIR boards are manufactured in thicknesses from 20mm to 200mm, though the most commonly specified thicknesses for meeting Part L requirements range from 50mm to 150mm.

Standard Thickness Range

*U-values shown for solid floor construction: PIR above concrete slab beneath screed. Actual U-values vary with complete build-up.

Selecting Thickness for Part L Compliance

Part L 2021 sets maximum U-values for building elements:

• Roofs: 0.16 W/m²K
• Walls: 0.26 W/m²K
• Floors: 0.18 W/m²K

The 100mm PIR thickness typically achieves compliance for most applications. For floors, 100mm PIR beneath screed achieves 0.17-0.18 W/m²K. For cavity walls, 100mm PIR in partial fill construction achieves 0.16-0.17 W/m²K. For pitched roofs, 100mm between rafters achieves 0.19-0.21 W/m²K depending on the complete build-up.

Flat roofs often need 120-150mm to comfortably achieve the 0.16 W/m²K target. The exposed nature of flat roofs and weather exposure means specifiers tend to add extra performance margin rather than designing right to the minimum requirement.

Compressive Strength Variations

Not all PIR boards at the same thickness have identical compressive strength. Floor-grade boards achieve 120-150 kPa minimum, adequate for residential and light commercial floor loading. Wall and roof applications might use boards with lower compressive strength (100 kPa) since they don’t carry loading.

Some manufacturers produce enhanced-strength boards achieving 200+ kPa for heavy commercial applications, car parks, or situations with concentrated loads. Check the technical data sheet – don’t assume all 100mm PIR boards have the same load-bearing capacity.

Board Dimensions

Standard board size is 2400mm x 1200mm regardless of thickness. This gives 2.88m² coverage per board. Some manufacturers produce 2400mm x 1250mm boards (3m² coverage) or other sizes for specific applications.

Weight increases with thickness. A 50mm board weighs approximately 4-5kg. A 100mm board weighs 9-10kg. A 150mm board weighs 13-14kg. This matters for manual handling – thicker boards become awkward for one person to handle safely, particularly when working at height.

Applications for PIR Insulation Boards

PIR insulation works across almost every insulated element in a building. Different applications have different installation methods and performance requirements.

Pitched Roofs

Between rafters: PIR boards fit friction-tight between roof rafters, providing thermal insulation while maintaining the roof void as accessible loft space. This is the most common pitched roof insulation method for residential construction. You need 100-150mm thickness depending on rafter depth and target U-value.

Above rafters: For warm pitched roofs, PIR boards sit above the structural rafters beneath the roof covering. This method eliminates thermal bridging through rafters and provides superior airtightness. It’s the preferred approach for high-performance buildings and Passivhaus construction. Typical thickness: 120-180mm depending on specification.

Below rafters: Adding a secondary insulation layer beneath rafters (typically 50-75mm PIR) supplements between-rafter insulation to achieve very low U-values. This creates a warm roof void, which has advantages for services installation and condensation control.

Flat Roofs

PIR boards are the standard insulation choice for warm flat roof construction. The boards sit above the structural deck, keeping the entire roof structure at or near room temperature. This prevents condensation issues that plagued older cold-deck flat roofs.

Typical flat roof build-up:

1. Structural deck (timber, concrete, or metal)
2. Vapour control layer
3. PIR insulation (120-180mm typical)
4. Waterproofing membrane

Some flat roof systems use tapered PIR insulation to create falls for drainage rather than sloping the structural deck. This maintains consistent insulation thickness across the roof while providing adequate water runoff.

Cavity Walls

Partial fill: PIR boards sit against the inner leaf blockwork in a cavity wall, maintaining a minimum 50mm residual cavity between insulation and outer brick leaf. This 50mm cavity provides drainage path for any water penetrating the outer leaf. Typical PIR thickness: 50-100mm depending on target U-value.

Full fill: For walls with wider cavities or when maximum thermal performance is needed, PIR boards can fill the full cavity width. This requires specialist cavity batts or boards designed for full-fill use with appropriate water resistance. Less common than partial fill but used in some applications.

Solid Walls (Internal Insulation)

For retrofit insulation of solid-walled buildings, PIR boards fix to the internal wall surface, then plasterboard finishes over the insulation. This approach works when external wall insulation isn’t possible due to planning restrictions or building appearance preservation.

Typical build-up: 50-100mm PIR fixed to wall with mechanical fixings or adhesive, vapour control layer, service cavity with battens, plasterboard finish. The main drawback is loss of internal floor area.

Solid Walls (External Insulation)

PIR boards can form part of external wall insulation (EWI) systems, though phenolic boards are sometimes preferred for EWI due to thinner required thickness. PIR boards fix to the external wall surface, then render or cladding finishes over the insulation.

Solid Floors

PIR boards sit above the concrete ground-bearing slab and damp-proof membrane, beneath the screed. This is probably the most straightforward PIR application – the boards simply lay loose with joints taped, then screed goes over the top.

The 120 kPa compressive strength of standard floor-grade PIR handles residential floor loading without compression. For underfloor heating, PIR boards sit beneath the heating pipes embedded in screed, directing heat upward into the room rather than down into the ground.

Suspended Timber Floors

PIR boards can install between floor joists (supported on battens) or above floor joists beneath the floor deck. Between-joist installation requires adequate joist depth. Above-joist installation loses some ceiling height but works well for retrofit applications.

PIR Insulation and Building Regulations

Building Regulations Part L sets thermal performance requirements that PIR insulation helps achieve. Understanding how regulations apply to different project types matters for specification.

New Build Requirements (Part L1A – Dwellings)

New dwellings must meet maximum U-values for each element:

• Walls: 0.26 W/m²K
• Roofs: 0.16 W/m²K
• Floors: 0.18 W/m²K
• Windows: 1.6 W/m²K

The building must also meet an overall Target Emission Rate (TER) calculated using SAP. This whole-building approach means you can have some elements slightly above maximum U-values if other elements exceed requirements, though hitting individual element targets is standard practice.

Extensions (Part L1B – Existing Dwellings)

Extensions must meet the same U-value requirements as new builds. This is where PIR’s thickness advantage matters. Extending a house with 2.4m ceiling height means every millimetre of ceiling build-up matters. Using 100mm PIR instead of 150mm mineral wool saves 50mm of ceiling height – often the difference between normal ceiling height and a space feeling cramped.

Renovations (Part L1B – Existing Dwellings)

When renovating existing buildings, requirements depend on the extent of work. If you’re replacing or upgrading more than 50% of an element (like a complete re-roof), you must make reasonable provision to improve thermal performance. The target is typically 0.25 W/m²K for walls and 0.18 W/m²K for roofs and floors where achievable.

“Where achievable” recognises that retrofit insulation faces constraints. If adding insulation creates moisture problems, reduces ceiling height below minimum, or causes other technical issues, you can justify a lower performance standard. This flexibility helps older buildings get partial improvements rather than avoiding improvement entirely due to inability to meet new-build standards.

Scotland Building Standards

Scotland has separate (and generally stricter) building standards than England and Wales. Scottish buildings typically require:

• Walls: 0.19 W/m²K
• Roofs: 0.13 W/m²K
• Floors: 0.15 W/m²K

These tighter requirements usually mean thicker insulation. A pitched roof achieving 0.13 W/m²K typically needs 100mm between rafters plus 50-75mm over rafters. Flat roofs typically need 150-180mm PIR to achieve 0.13 W/m²K comfortably.

Wales Requirements

Wales follows Part L 2021 of the Building Regulations, same as England. Some local authorities have additional requirements through planning policy, but the baseline thermal performance requirements match English requirements.

Thermal Bridging Considerations

Part L compliance calculations must account for thermal bridging through wall ties, lintels, floor edges, and other junctions. These bridging effects can add 0.02-0.05 W/m²K to your calculated U-value depending on detailing quality.

Using low-conductivity wall ties, insulated lintels, and continuous insulation at junctions minimises thermal bridging. PIR’s high performance per millimetre means less area is lost to edge detailing compared to thicker, lower-performance materials.

Comparing PIR Brands: Celotex, Kingspan, Recticel, Ecotherm

The UK PIR insulation market includes several major manufacturers. All produce boards meeting BS EN 13165 with similar core performance, but there are some differences worth understanding.

Celotex

Celotex (owned by Saint-Gobain) is one of the most widely recognised PIR brands in UK construction. The GA4000 series represents general application boards suitable for most uses. Celotex also produces specialist ranges for specific applications.

Product ranges:

• GA4000: General application, most common range
• TB4000: Thin board applications
• CW4000: Cavity wall specific
• XR4000: Roofing applications
• FL4000: Floor applications with enhanced compressive strength

Lambda value: 0.022 W/mK across most products Compressive strength: 120-150 kPa depending on grade Availability: Very high – stocked by most builders merchants

Kingspan

Kingspan produces PIR boards under several brand names and has the widest product range of any manufacturer. Kingspan boards are known for consistent quality and extensive technical support.

Product ranges:

• Kingspan Kooltherm: Premium range with enhanced performance
• Kingspan Therma: Standard PIR range
• Kingspan TP10: Pitched roof specific
• Kingspan TW50: Partial fill cavity walls

Lambda value: 0.018-0.023 W/mK depending on product Compressive strength: 100-200 kPa depending on grade Availability: High – widely stocked

Kingspan’s Kooltherm range uses modified phenolic technology achieving lambda values as low as 0.018 W/mK. These premium boards cost more but need less thickness for the same U-value.

Recticel

Recticel (Eurethane and Powerroof brands) offers good-quality PIR boards, often at competitive pricing compared to Celotex and Kingspan. Technical performance matches other major brands.

Product ranges:

• Eurethane GP: General purpose
• Eurethane AL: Aluminium-faced
• Powerroof: Flat roof specific

Lambda value: 0.022 W/mK Compressive strength: 120-150 kPa depending on grade Availability: Medium – less widely stocked than Celotex/Kingspan

Ecotherm

Ecotherm produces PIR boards focused on lower environmental impact manufacturing while maintaining technical performance. The boards perform identically to other PIR brands in use.

Product ranges:

• Eco-Versal: General application
• Eco-Cavity: Cavity wall specific
• Eco-Deck: Flat roof applications

Lambda value: 0.022 W/mK Compressive strength: 120-150 kPa depending on grade
Availability: Medium – growing distribution network

IKO Enertherm

IKO produces PIR insulation alongside their roofing felt and membrane business. This makes them a natural choice for flat roof projects where you’re sourcing both insulation and waterproofing from one manufacturer.

Lambda value: 0.022 W/mK Compressive strength: 100-150 kPa depending on grade Availability: Medium – strong in roofing merchants

Does Brand Matter?

For most applications, brand choice comes down to availability and pricing rather than performance differences. All major brands meet BS EN 13165, achieve similar lambda values (0.022 W/mK), and perform identically in use.

Where brand might matter:

• Warranty requirements (some waterproofing manufacturers specify compatible insulation brands)
• Technical support (larger manufacturers offer more detailed technical services)
• Product range (some manufacturers offer better options for specialist applications)
• Regional availability (some brands have stronger distribution in certain regions)

For standard applications like floors, cavity walls, or pitched roofs, any reputable PIR brand at the right thickness and compressive strength will perform well. Don’t overthink brand selection unless you have specific warranty or technical support requirements.

Selecting the Right PIR Board for Your Project

Choosing the right PIR specification depends on application, performance requirements, and practical constraints.

Thickness Selection

Start with your target U-value and work backwards to required thickness. Part L sets maximum U-values, but you might target lower values for future-proofing or energy performance certificates.

For a solid floor achieving 0.18 W/m²K, 100mm PIR works. For 0.15 W/m²K, you need 120mm. For 0.13 W/m²K, you need approximately 140mm. These figures assume standard floor construction with 65-75mm screed over insulation above a concrete slab.

Build-up depth constraints sometimes dictate thickness. Retrofit floor insulation might be limited to 75mm because you can’t raise floor level more than 100mm total without creating steps at doorways. Cavity width limits cavity wall insulation thickness. Work within your constraints while achieving the best thermal performance practical.

Compressive Strength Selection

For floors, always specify floor-grade PIR with minimum 120 kPa compressive strength. Using wall-grade PIR (100 kPa) beneath floors risks compression over time, causing screed cracking and reduced thermal performance.

For walls and roofs, standard wall-grade PIR (100 kPa) works fine. The extra cost of floor-grade material offers no advantage when boards aren’t carrying loading.

Heavy commercial applications might need enhanced-strength PIR achieving 150-200 kPa. This includes plant rooms, commercial kitchens, car parks, or anywhere with concentrated point loads or vehicle access.

Facer Selection

Most PIR boards come with aluminium foil facings as standard. This suits most applications. Some specific applications benefit from alternative facers:

Glass tissue facings: Required when torch-applying felt or other heat-applied membranes. The foil would melt under heat, but glass tissue handles the temperature.

Coated or textured facings: Some manufacturers offer boards with modified surface textures to improve adhesion for liquid-applied membranes or single-ply systems.

Paper facings: Occasionally used for internal applications where vapour permeability is needed.

For most projects, standard foil-faced PIR boards are the right choice. Only specify alternative facers when system manufacturers specifically require them.

Fire Performance

Standard PIR boards achieve Class E fire classification. This meets requirements for most residential and commercial applications when used as part of a complete construction.

Buildings over 18m height (approximately 6 storeys) face additional scrutiny on combustible materials in external walls following post-Grenfell regulations. For these buildings, consider whether PIR insulation in external wall build-ups meets current guidance, or specify non-combustible alternatives like mineral wool.

Check current Building Regulations guidance and your specific project fire strategy rather than assuming PIR automatically suits all applications.

Working with PIR Insulation: Trade Considerations

Getting good performance from PIR insulation depends on proper handling and installation.

Storage and Handling

Store PIR boards flat in their packaging until needed. Stack maximum 10 boards high to prevent bottom boards compressing. Keep boards dry and protected from direct sunlight – UV exposure degrades foil facings over weeks of exposure.

Handle boards carefully to prevent edge damage. Crushed edges create gaps when boards fit together, allowing air infiltration that reduces thermal performance. Boards dropped on corners often get damaged – take care when unloading and moving materials.

Cutting Techniques

Cut PIR boards using a fine-toothed saw or sharp knife. For knife cutting, score deeply through the facing and into the foam, then snap the board over a straight edge. A deeply scored line gives a clean break.

Some installers prefer saws for cleaner cuts, particularly for complicated shapes. A standard wood saw works, though the foam can clog teeth. Clean teeth regularly during cutting.

For friction-fit applications between rafters or joists, cut boards 5-10mm oversized. The compression creates a tight fit preventing air gaps and board movement. For applications where boards butt together without compression, cut accurately to size for tight joints.

Joint Treatment

Tape all board joints with aluminium foil tape to prevent air infiltration. This is critical for achieving designed thermal performance. Air movement through untaped joints can reduce effective R-value by 20-30%.

Apply tape to clean, dry board surfaces. Press firmly for good adhesion. Overlap tape ends by 50mm minimum. For wet or dusty conditions, clean board edges before taping – tape won’t stick to dirty or wet surfaces.

Some installers skip taping, particularly on floors where screed will cover boards. This is poor practice. The investment in tape is minimal compared to the thermal performance benefit.

Fixing Methods

Fixing varies by application:

Floors: Loose-laid with screed weight holding boards in place. No mechanical fixing needed.

Roofs above deck: Mechanical fixings through boards into deck. Follow manufacturer fixing patterns and density for wind uplift resistance. More fixings than necessary creates thermal bridging, but too few risks wind damage.

Walls: Mechanical fixings, adhesive dabs, or cavity wall ties depending on system. For internal wall insulation with adhesive, apply dabs to board backs following system guidance on dab size and spacing.

Between rafters: Friction fit with no additional fixing needed. The compression holds boards in place.

Vapour Control

PIR boards with foil facings provide some vapour resistance (approximately 200 MNs/gm), but this often isn’t adequate as the sole vapour control layer. For warm roofs, walls containing moisture-sensitive materials, and floors over unheated spaces, install a separate vapour control layer on the warm side of insulation.

The vapour control layer (VCL) prevents warm, moisture-laden air from inside the building reaching cold surfaces where condensation would occur. Position matters – VCL always goes on the warm side (inside face) of insulation.

Working in Poor Weather

PIR boards can install in most weather conditions, but wet conditions complicate tape application and adhesive bonding. Below -5°C, adhesives and tapes lose effectiveness. Protect boards from rain during installation – wet boards need drying before taping.

Wind affects roof installations more than ground-level work. Secure loose boards before they become wind-borne hazards. Don’t leave partially completed flat roofs without temporary weatherproofing overnight – rain on exposed insulation creates problems even though PIR itself resists water absorption.

Health and Safety

PIR board installation presents few health hazards. The material doesn’t cause skin irritation like fibrous insulation. Dust from cutting is minimal. The main risks are standard construction hazards – working at height, manual handling, and using cutting tools.

Wear gloves when handling boards – the edges can be sharp. For overhead installations between rafters, use proper access equipment and support boards adequately during installation. A 2400x1200mm board catching wind overhead can pull you off balance.

Waste Management

PIR offcuts can’t be recycled in most areas. Minimise waste by planning cutting patterns before starting. Measure twice, cut once actually matters when waste goes to landfill.

Small offcuts work for filling gaps, blocking penetrations, or other detail work. Save them rather than binning immediately. Larger offcuts might work for another area of the project – if you’re insulating multiple rooms, coordinate cutting to use offcuts where possible.Looking for reliable PIR insulation for your next project? Online Insulation stocks all major PIR brands across the full range of insulation thicknesses, with fast UK delivery for trade professionals.