PIR insulation thickness directly determines thermal performance, with U-values improving as thickness increases due to the material’s consistent 0.022 W/mK thermal conductivity. For Part L compliance, 100mm PIR achieves 0.17-0.18 W/m²K in solid floor constructions, while 120-150mm achieves the 0.13-0.16 W/m²K required for roofs and Scotland Building Standards. Selecting appropriate thickness involves balancing regulatory requirements, space constraints, cost considerations, and target thermal performance across different building applications.

Understanding PIR Thickness and U-Value Relationship

The relationship between PIR insulation thickness and U-value follows a predictable pattern based on the material’s thermal conductivity. Understanding this relationship helps specification without needing detailed calculations for every project.

How Thermal Conductivity Determines Performance

PIR insulation has a thermal conductivity (lambda value) of 0.022 W/mK. This figure represents how readily heat flows through the material. Lower values mean better insulation – heat struggles to pass through.

The thermal resistance (R-value) of insulation equals thickness divided by thermal conductivity:

R-value = Thickness (metres) / Lambda (W/mK)

For 100mm PIR: R = 0.100 / 0.022 = 4.55 m²K/W

The U-value represents the overall heat loss through a complete construction (not just insulation). It’s calculated as 1 divided by the total R-value of all layers plus surface resistances.

Why Doubling Thickness Doesn’t Halve U-Value

You might expect doubling insulation thickness to halve the U-value. It doesn’t work that way because U-value depends on the complete build-up, not just insulation:

*Assumes solid floor: concrete slab, DPM, PIR, 65mm screed

Doubling thickness from 50mm to 100mm improves U-value from 0.32 to 0.18 (not 0.16). The concrete slab, screed, and surface resistances contribute to overall thermal resistance. They don’t change when insulation thickness changes, limiting the improvement from thicker insulation.

This diminishing returns effect means initial thickness increases give larger U-value improvements than later increases. Going from 50mm to 100mm makes a big difference. Going from 100mm to 150mm makes a smaller difference.

Practical Implications

For Part L compliance, you need specific U-value targets:

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

Working backwards from target U-values to required thickness is the practical approach. You know you need 0.18 W/m²K for a floor, so you calculate or look up which thickness achieves that.

The sections below cover each common thickness, showing what U-values they achieve and where they’re appropriate.

25mm PIR Insulation: Specifications and Applications

25mm represents the thinnest commonly-specified PIR thickness. This thin profile suits space-constrained applications where achieving Part L compliance isn’t possible with standard thicknesses.

Technical Specifications

• Thickness: 25mm
• Thermal resistance: 1.14 m²K/W (based on 0.022 W/mK)
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 2-2.5kg
• Compressive strength: 100-120 kPa depending on product grade
• Coverage: 2.88m² per board

U-Value Performance

25mm PIR alone doesn’t achieve Part L compliance for any application. The thermal resistance is insufficient for current Building Regulations requirements.

Typical U-values with 25mm PIR:

Where 25mm PIR Gets Used

Despite not meeting Part L as sole insulation, 25mm PIR has specific applications:

Supplementary insulation layer: Used alongside thicker insulation to fine-tune overall thickness. For example, 75mm + 25mm = 100mm total where 100mm boards don’t fit in available space.

Internal wall insulation (severely space-constrained): Historic buildings or properties where room size limitations prevent thicker insulation. You get partial thermal improvement even though full Part L compliance isn’t achievable.

Door reveals and window reveals: When insulating reveals internally around new windows, 25mm provides some thermal improvement without excessive reveal depth.

Service boxing and bulkheads: Where services require boxing in and the box depth is limited, 25mm insulation provides some thermal break.

Retrofit additions: Adding 25mm over existing inadequate insulation improves overall performance. For example, a property with 50mm existing insulation adding 25mm achieves equivalent to 75mm new insulation.

Cost Considerations

25mm PIR costs approximately £4-6 per m² depending on brand and quantity. While cheaper per board than thicker options, the cost per R-value is actually higher – you pay more to achieve the same thermal resistance using multiple thin boards versus fewer thick boards.

Only specify 25mm when space constraints force it. Where space allows, thicker boards provide better value.

50mm PIR Insulation: Specifications and Applications

50mm PIR provides moderate thermal performance suitable for some applications but still falls short of current Part L requirements in most situations.

Technical Specifications

• Thickness: 50mm
• Thermal resistance: 2.27 m²K/W
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 4-5kg
• Compressive strength: 100-120 kPa depending on grade
• Coverage: 2.88m² per board

U-Value Performance

50mm PIR achieves Part L compliance for cavity walls but not floors or roofs in most constructions:

Where 50mm PIR Works Best

Cavity walls (partial fill): 50mm PIR in a cavity wall with 50mm residual cavity achieves approximately 0.22 W/m²K, comfortably meeting the 0.26 W/m²K wall requirement. This represents one of the most common applications for 50mm thickness.

For cavity walls, the build-up typically is:

• 100mm inner leaf blockwork
• 50mm PIR insulation
• 50mm residual cavity (for drainage)
• 102mm outer brick leaf

This achieves good thermal performance while maintaining adequate cavity for water drainage. Wider cavities (125mm total) can accommodate 75mm PIR with 50mm residual cavity for better performance.

Internal wall insulation (space-constrained): Where room size constraints limit insulation thickness but some thermal improvement is better than none, 50mm provides meaningful benefit. Combined with plasterboard and battens, the total build-up adds approximately 75-80mm to wall thickness.

Below-rafter insulation: As a supplementary layer beneath rafters in combination with between-rafter insulation. For example, 100mm between rafters plus 50mm below rafters achieves good overall U-values while maintaining ceiling height.

Retrofit applications: Upgrading properties where space limitations prevent thicker insulation. 50mm provides partial improvement toward Part L targets even when full compliance isn’t achievable.

Limitations of 50mm Thickness

50mm doesn’t meet current Part L requirements for:

• Ground floors (needs approximately 100mm for 0.18 W/m²K)
• Roofs (needs 120-140mm for 0.16 W/m²K)
• Internal wall insulation in most situations (needs 75-100mm for 0.26 W/m²K)

Where regulations permit lower standards (renovations where achieving Part L creates disproportionate difficulty), 50mm might be acceptable. Always discuss with Building Control before proceeding with thicknesses that don’t meet standard requirements.

75mm PIR Insulation: Specifications and Applications

75mm PIR provides a middle-ground thickness suitable for walls and some roof applications, though it still falls short for floors under current Part L requirements.

Technical Specifications

• Thickness: 75mm
• Thermal resistance: 3.41 m²K/W
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 6-7kg
• Compressive strength: 100-120 kPa depending on grade
• Coverage: 2.88m² per board

U-Value Performance

75mm achieves Part L compliance for walls and approaches requirements for some roof constructions:

Where 75mm PIR Works Best

Cavity walls with good margin: 75mm in partial fill cavity construction achieves 0.17 W/m²K, well below the 0.26 W/m²K requirement. This margin helps where thermal bridging through wall ties or other junctions increases the actual U-value above calculations.

Internal wall insulation: 75mm comes very close to the 0.26 W/m²K requirement for walls. In some constructions with favourable surface resistances, it achieves compliance. For retrofit internal wall insulation where space constraints matter, 75mm represents a reasonable compromise between performance and internal space loss.

Supplementary roof insulation: Used as a below-rafter or over-rafter layer in combination with thicker between-rafter insulation. For example:

• 100mm between 150mm rafters
• 75mm over rafters beneath counter battens
• Combined U-value: approximately 0.14 W/m²K

This combination achieves excellent thermal performance while managing rafter depth and ceiling height constraints.

Retrofit floors (where full compliance isn’t achievable): In retrofit situations where raising floor level by more than 100mm total creates unacceptable steps, 75mm PIR provides meaningful thermal improvement even though it doesn’t achieve 0.18 W/m²K. This might be acceptable under renovation provisions where achieving full compliance creates disproportionate cost or technical problems.

When 75mm Isn’t Enough

75mm falls short for:

• New build ground floors (needs 100mm minimum)
• Flat roofs (needs 120-140mm for 0.16 W/m²K)
• Pitched roofs as sole insulation (needs 100-150mm)
• Scotland Building Standards applications (generally needs thicker insulation)

For new builds where space isn’t constrained, specify 100mm minimum rather than 75mm. The additional cost is modest compared to the thermal performance benefit and compliance certainty.

100mm PIR Insulation: Specifications and Applications

100mm represents the workhorse thickness for Part L compliance across multiple applications. This thickness achieves requirements for floors, approaches requirements for roofs, and exceeds requirements for walls.

Technical Specifications

• Thickness: 100mm
• Thermal resistance: 4.55 m²K/W
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 9-10kg
• Compressive strength: 100-120 kPa (wall/roof grade) or 120-150 kPa (floor grade)
• Coverage: 2.88m² per board

U-Value Performance

100mm PIR achieves Part L compliance for most applications:

Where 100mm PIR Is the Standard Choice

Ground floors: 100mm achieves 0.17-0.18 W/m²K in typical solid floor construction (concrete slab, DPM, PIR, screed). This comfortably meets the 0.18 W/m²K requirement for England and Wales. The thickness has become the default specification for floor insulation in residential construction.

Cavity walls: 100mm in partial fill cavity construction achieves 0.16 W/m²K with significant margin below the 0.26 W/m²K requirement. This margin accommodates thermal bridging through wall ties, lintels, and other junctions without compromising compliance.

Pitched roofs (between rafters): 100mm between rafters achieves 0.19-0.21 W/m²K depending on complete build-up. This approaches the 0.16 W/m²K requirement. Many specifications use 100mm as sole insulation where calculations show compliance, or combine 100mm between rafters with additional insulation over rafters for lower U-values.

Internal wall insulation: 100mm provides good thermal performance for internal wall insulation with U-values well below the 0.26 W/m²K requirement. The main constraint becomes internal space loss (approximately 120-130mm total build-up including plasterboard and battens).

Flat roofs (marginal): 100mm achieves approximately 0.20 W/m²K in typical flat roof warm deck construction. This doesn’t quite meet the 0.16 W/m²K requirement, but is close enough that some projects use 100mm where calculations including favourable factors show compliance.

Why 100mm Dominates Specifications

100mm represents the minimum thickness achieving Part L floor requirements and approaches requirements for roofs. The thickness is readily available, competitively priced (lower per-m² cost than thinner boards due to lower handling/cutting labour), and suits most standard building constructions without requiring oversized framing or excessive build-up depths.

For specifiers, 100mm provides a reliable default. Unless specific calculations show thinner boards achieve compliance or constraints force thinner boards, 100mm works across most applications.

When to Specify Thicker Than 100mm

Specify 120mm or 150mm instead of 100mm when:

• Targeting Scotland Building Standards (stricter requirements)
• Achieving 0.16 W/m²K for flat roofs comfortably
• Future-proofing against anticipated regulation tightening
• Pursuing low-energy building standards (Passivhaus, etc.)
• EPC rating improvements matter for property value

The additional cost for 120mm versus 100mm is modest. For new builds, the marginal cost increase is typically 10-15% on insulation (small proportion of total project cost) while providing 15-20% better thermal performance.

120mm and 150mm PIR Insulation: High-Performance Applications

Thicker PIR boards suit applications requiring U-values below standard Part L minimums, Scotland Building Standards compliance, or future-proofing against anticipated regulation changes.

120mm PIR Technical Specifications

• Thickness: 120mm
• Thermal resistance: 5.45 m²K/W
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 11-12kg
• Compressive strength: 100-120 kPa (wall/roof grade) or 120-150 kPa (floor grade)
• Coverage: 2.88m² per board

120mm U-Value Performance

150mm PIR Technical Specifications

• Thickness: 150mm
• Thermal resistance: 6.82 m²K/W
• Board size: 2400mm x 1200mm (2.88m² per board)
• Weight per board: approximately 13-14kg
• Compressive strength: varies by product
• Coverage: 2.88m² per board

150mm U-Value Performance

When to Specify 120mm

Scotland Building Standards: Scotland requires stricter U-values than England. For floors, 120mm achieves the 0.15 W/m²K requirement. For roofs, 120mm gets close to the 0.13 W/m²K requirement though 150mm provides more comfortable margin.

Flat roofs (England/Wales): 120mm achieves approximately 0.16 W/m²K, meeting requirements with minimal margin. Many specifiers prefer 120mm for flat roofs rather than specifying 100mm and hoping calculations work out.

Future-proofing new builds: Building Regulations will continue tightening. Specifying 120mm now costs relatively little extra but provides 15-20% better thermal performance than 100mm. When regulations change requiring 0.15 W/m²K for floors or 0.13 W/m²K for roofs, buildings with 120mm already comply.

High-performance targets: Projects pursuing lower U-values for EPC rating improvements, reduced running costs, or marketing purposes (“high-efficiency home”) use 120mm as a cost-effective upgrade from 100mm.

When to Specify 150mm

Scotland roofs: Achieving 0.13 W/m²K for Scottish roofs requires 150mm in most constructions. Using 120mm for floors and 150mm for roofs represents the common approach in Scotland.

Passivhaus and low-energy buildings: Buildings targeting very low energy consumption need excellent thermal performance. 150mm PIR helps achieve these targets, though such projects often use even thicker insulation (200mm+) or combine PIR with other insulation types.

Cold climate applications: Buildings in exposed locations or areas with harsh winters benefit from extra insulation thickness beyond regulatory minimums. The additional upfront cost pays back through reduced heating requirements.

Commercial buildings with long lifespan: Commercial buildings might remain in use for 50+ years. Specifying 150mm provides performance margin that will remain relevant as standards evolve and energy costs increase.

Cost vs Performance for Thick Insulation

*Approximate trade prices, varies by supplier and quantity

The cost per R-value remains relatively consistent across thicknesses. Thicker boards cost more in absolute terms but don’t cost proportionally more per unit of thermal performance.

The main cost impact comes from increased build-up depth potentially requiring:

• Deeper floor joists (suspended floors)
• Modified door/window reveals
• Additional wall cavity width
• Raised floor levels with associated threshold details

Where these complications don’t apply (ground floors, warm roofs), specifying thicker insulation adds minimal cost while providing meaningful performance benefit.

U-Value Calculations for Each Thickness

Understanding how each thickness performs in typical constructions helps specification without running detailed calculations for every project.

Solid Floor U-Values by Thickness

Standard build-up: 150mm concrete slab, DPM, PIR insulation (varying), 65mm screed

*Includes edge effects for typical 50m² floor

Cavity Wall U-Values by Thickness

Standard build-up: 100mm blockwork inner leaf, PIR insulation (varying), 50mm residual cavity, 102mm brick outer leaf

Pitched Roof U-Values by Thickness (Between Rafters)

Standard build-up: 12.5mm plasterboard ceiling, PIR between rafters (varying), breathable membrane, battens, tiles

These values account for thermal bridging through rafters. Actual U-values vary with rafter spacing, rafter material, and complete build-up details.

Flat Roof U-Values by Thickness (Warm Roof)

Standard build-up: Structural deck, vapour control layer, PIR insulation (varying), waterproofing membrane

Calculation Variables That Affect These Numbers

The U-values shown above assume typical constructions with standard materials. Your actual project U-values may differ based on:

Deck/slab thickness: Thicker concrete slabs or timber decks add thermal resistance, slightly improving U-values.

Screed thickness: Thicker screeds add thermal resistance to floor constructions.

Surface resistances: Indoor surface finish (carpet vs tiles) and outdoor conditions affect surface resistance values.

Air gaps and cavities: Cavities and air gaps contribute thermal resistance, improving overall U-values.

Thermal bridging: Wall ties, rafters, fixings, and edge details increase heat loss above centre-of-panel calculations.

For Building Regulations submissions, use approved calculation software or manufacturer U-value calculators that account for these variables properly. The tables above provide guidance for initial specification, not final compliance calculations.

Selecting the Right Thickness for Part L Compliance

Choosing appropriate PIR thickness involves working backwards from target U-values while considering practical constraints.

Decision Framework

Step 1: Identify target U-value

Part L 2021 requirements (England and Wales):

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

Scotland Building Standards:

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

Step 2: Determine minimum thickness for compliance

From the tables above:

For England/Wales floors: 100mm minimum For England/Wales walls: 50-75mm (depending on construction) For England/Wales roofs: 120-140mm typical

For Scotland floors: 120mm minimum For Scotland walls: 75-100mm typical For Scotland roofs: 150mm typical

Step 3: Check practical constraints

Space constraints: Available depth for floor build-up, cavity width, ceiling height all limit maximum practical thickness.

Weight constraints: Thicker insulation weighs more. Check structural adequacy for increased loading, particularly for roofs.

Door/window reveals: Internal wall insulation thickness affects reveal depths and window sill relationships.

Floor level: Ground floor insulation raises finished floor level, affecting door clearances and DPC relationships.

Step 4: Consider future-proofing

Regulations will continue tightening. Specifying one thickness increment above minimum (110mm instead of 100mm, or 130mm instead of 120mm) adds minimal cost while extending compliance lifetime and improving property value/EPC rating.

Step 5: Verify with calculations

Use manufacturer U-value calculators or approved software to verify selected thickness achieves target U-value for your specific construction. Don’t rely on rule-of-thumb thickness selection for final specifications.

Common Specification Approaches

Residential new build (England/Wales):

• Floors: 100mm PIR
• Cavity walls: 75-100mm PIR
• Pitched roofs: 100mm between rafters, consider 50-75mm additional layer
• Flat roofs: 120-140mm PIR

Residential new build (Scotland):

• Floors: 120mm PIR
• Cavity walls: 100mm PIR
• Pitched roofs: 150mm total (various configurations)
• Flat roofs: 150-180mm PIR

Extensions (matching new build): Use same thicknesses as new build. Extensions must meet new build U-value requirements.

Renovations (where achievable): Target 0.25 W/m²K for walls/roofs, 0.18 W/m²K for floors where reasonably practical. Thicknesses vary depending on constraints – document why full compliance isn’t achievable if you specify less than new build standards.

Commercial buildings: Often specify thicker than Part L minimums for:

• Better EPC ratings (affects rental values)
• Lower running costs (commercial energy prices typically higher than domestic)
• Longer building lifespan justifying better performance

PIR Thickness Comparison Table

Quick reference comparing all common thicknesses across key performance and practical parameters:

*Floor U-value: solid floor construction with concrete slab, DPM, PIR, screed **Wall U-value: partial fill cavity wall with 50mm residual cavity

Cost Comparison

Approximate trade prices (per m² coverage):

Prices vary by supplier, brand, quantity, and region. These figures represent typical trade prices for standard products.

Performance per Pound

While thicker boards cost more per board, the cost per unit of thermal performance (cost per R-value) remains relatively consistent. This means specifying 120mm instead of 100mm gives you proportionally more thermal performance for the additional cost.

The main cost impact isn’t the insulation itself but associated works:

• Thicker floor insulation requires deeper screed or raises floor level
• Thicker wall insulation requires wider cavities or loses more internal space
• Thicker roof insulation may require deeper rafters or modified details

For new builds where you specify these elements from scratch, thicker insulation adds minimal cost. For retrofits where existing construction constrains options, thicker insulation might require expensive modifications making it disproportionately costly.

Availability by Thickness

All thicknesses from 50mm to 150mm are readily available from most suppliers. Very thin (25mm) or very thick (180mm+) boards may require special order with longer lead times.

For mainstream projects, stick to commonly-stocked thicknesses (75mm, 100mm, 120mm, 150mm) for best pricing and availability. Unusual thicknesses often cost more and have longer delivery times even when they’re technically available.

Need PIR insulation in the right thickness for your project? Online Insulation stocks the complete range from 25mm to 200mm across all major brands, with fast UK delivery for trade professionals.

Article 7:

URL Slug: /part-l-2021-pir-insulation-u-value-requirements/

Title: Meeting Part L 2021 with PIR Insulation: U-Value Requirements Guide

Meta Description: Complete Part L 2021 compliance guide for trade professionals. Covers U-value requirements, PIR insulation specifications for roofs, walls, floors, and Scotland Building Standards differences.

Meeting Part L 2021 with PIR Insulation: U-Value Requirements for Trade Professionals

Part L 2021 of the Building Regulations sets maximum U-value targets of 0.16 W/m²K for roofs, 0.26 W/m²K for walls, and 0.18 W/m²K for floors in new dwellings and extensions across England and Wales. PIR insulation achieves these requirements with 120-140mm for roofs, 75-100mm for walls, and 100mm for floors in typical constructions, making it the standard choice for Part L compliance in UK residential construction. Understanding specific requirements for new builds, extensions, and renovations helps trade professionals specify appropriate insulation thicknesses and avoid Building Control queries during construction.

Overview of Part L 2021 Requirements

Part L of the Building Regulations addresses conservation of fuel and power in buildings. The current version, Part L 2021 (implemented June 2022), tightened thermal performance standards compared to previous editions.

What Part L Covers

Part L splits into four documents:

Part L1A: Conservation of fuel and power in new dwellings Part L1B: Conservation of fuel and power in existing dwellings (extensions, renovations, material alterations) Part L2A: Conservation of fuel and power in new buildings other than dwellings (commercial) Part L2B: Conservation of fuel and power in existing buildings other than dwellings

For residential construction, L1A covers new houses and L1B covers work on existing houses. These documents set requirements that affect insulation specification.

Two Compliance Routes

Part L compliance works through two complementary requirements:

1. Fabric performance (U-values): Each building element must meet maximum U-value limits. Roofs can’t exceed 0.16 W/m²K, walls can’t exceed 0.26 W/m²K, floors can’t exceed 0.18 W/m²K.

2. Whole building performance (Target Emission Rate): The complete building must achieve carbon emissions below a calculated target. This accounts for heating system efficiency, hot water system, ventilation, thermal bridging, and fabric U-values together.

In practice, most buildings achieve compliance by:

• Meeting individual element U-value limits
• Installing reasonably efficient heating (condensing boiler or heat pump)
• Addressing thermal bridging adequately
• Providing adequate ventilation

The whole building calculation (using SAP software) confirms compliance but rarely drives specification decisions if individual elements meet their U-value targets.

Key Changes in Part L 2021

Part L 2021 tightened requirements compared to Part L 2013:

Wall U-values: Changed from 0.28 to 0.26 W/m²K (7% improvement required) Floor U-values: Changed from 0.22 to 0.18 W/m²K (18% improvement required) Roof U-values: Changed from 0.18 to 0.16 W/m²K (11% improvement required)

These changes mean insulation specified under Part L 2013 (the previous version) doesn’t automatically comply with current requirements. Specifications need updating to achieve the tighter U-values.

Who Must Comply

Part L requirements apply to:

• All new dwellings (houses, bungalows, flats)
• Extensions to existing dwellings
• Loft conversions creating additional habitable space
• Material alterations (significant changes to building fabric)
• Replacement of thermal elements (new roofs, replacement windows)
• Renovations where over 50% of a building element is replaced

The specific requirements vary by project type. New builds face the strictest requirements. Renovations have flexibility where achieving full compliance creates disproportionate cost or technical difficulty.

U-Value Targets by Building Element

Part L sets maximum U-values for each building element. These represent worst-case limits – actual designs can achieve better performance.

Maximum U-Values for New Dwellings (Part L1A 2021)

*Typical PIR insulation thickness required in standard constructions

Maximum U-Values for Extensions (Part L1B 2021)

Extensions must meet the same U-value limits as new builds. There’s no relaxation for extensions – if you’re building new fabric, it must meet current new build standards regardless of whether it’s a complete house or an extension to an existing property.

Renovation U-Value Targets (Part L1B 2021)

When renovating existing buildings, requirements depend on work extent:

Replacing over 50% of an element: Must make “reasonable provision” to improve thermal performance. Target U-values:

• Walls: 0.25 W/m²K where achievable
• Roofs: 0.18 W/m²K where achievable
• Floors: 0.18 W/m²K where achievable

The “where achievable” caveat recognises retrofit constraints. If achieving target U-values creates disproportionate cost, reduces room sizes below acceptable minimums, creates moisture problems, or causes other technical issues, lower performance standards are acceptable with proper justification to Building Control.

Replacing under 50% of an element: No thermal performance requirements beyond ensuring work meets Building Regulations in other respects (structural adequacy, fire safety, etc.).

Thermal Bridging Requirements

Part L requires limiting thermal bridging where building elements meet. The default approach uses published thermal bridging values (psi-values) from accredited construction details.

Higher thermal bridging adds to heat loss, potentially making an otherwise-compliant building fail overall Target Emission Rate calculations. Good practice details include:

• Low-conductivity wall ties in cavity walls
• Insulated lintels over openings
• Continuous insulation at floor edges and wall junctions
• Proper lapping of insulation layers where roof meets wall

Roof Insulation Requirements Under Part L

Roofs face the tightest U-value requirement at 0.16 W/m²K due to heat rising and roofs experiencing the greatest temperature difference between inside and outside.

Pitched Roof Insulation Approaches

Between Rafters: Install PIR insulation between roof rafters. This maintains the roof void as accessible loft space.

U-value achievement:

• 100mm PIR between 150mm rafters: 0.19-0.21 W/m²K (doesn’t meet 0.16 requirement)
• 120mm PIR between 175mm rafters: 0.17-0.19 W/m²K (borderline or doesn’t meet requirement)
• 150mm PIR between 200mm rafters: 0.15-0.17 W/m²K (meets requirement with margin)

Between-rafter insulation alone often struggles to achieve 0.16 W/m²K unless very thick insulation fits between deep rafters. Rafter thermal bridging reduces overall performance compared to centre-of-panel calculations.

Between Rafters Plus Over Rafters: Combining insulation between rafters with additional insulation over rafters eliminates rafter thermal bridging and achieves excellent U-values.

Common configurations:

• 100mm between rafters + 50mm over rafters: 0.14-0.16 W/m²K
• 100mm between rafters + 75mm over rafters: 0.12-0.14 W/m²K
• 75mm between rafters + 75mm over rafters: 0.13-0.15 W/m²K

This approach has become standard for new builds pursuing reliable 0.16 W/m²K achievement without excessive single-layer thickness.

Above Rafters Only (Warm Roof): Insulation entirely above rafters creates a warm roof void. The entire structure remains at near room temperature.

U-value achievement:

• 140mm PIR above rafters: 0.15 W/m²K
• 150mm PIR above rafters: 0.14 W/m²K
• 180mm PIR above rafters: 0.12 W/m²K

This method suits new builds with specified rafter depths or re-roofing projects where existing rafters remain but insulation can be added above.

Flat Roof Requirements

Flat roofs also need 0.16 W/m²K maximum. Warm roof construction places insulation above the structural deck beneath waterproofing.

U-value achievement:

• 120mm PIR warm roof: 0.16 W/m²K (meets requirement exactly)
• 140mm PIR warm roof: 0.14 W/m²K (meets requirement with margin)
• 150mm PIR warm roof: 0.13 W/m²K (significant margin)

Most specifications use 120-140mm PIR for flat roofs to achieve 0.16 W/m²K reliably. The exact thickness depends on deck type, waterproofing system, and whether the construction includes additional layers contributing thermal resistance.

Loft Conversions

Creating habitable space in existing loft voids requires insulation meeting new build standards. The roof becomes part of the thermal envelope and must achieve 0.16 W/m²K.

Common approach:

• Insulate between rafters to maximum practical depth (often 100-120mm limited by existing rafter depth)
• Add insulation beneath rafters (50-75mm) to achieve required total thickness
• Accept some ceiling height loss in exchange for compliance

Loft conversions face constraints from existing rafter depths, ceiling height limitations, and head room requirements. These constraints sometimes mean achieving exactly 0.16 W/m²K is difficult. Document constraints and discuss acceptable performance with Building Control before starting work.

Wall Insulation Requirements Under Part L

Walls face less stringent requirements than roofs at 0.26 W/m²K maximum. This allows relatively thin insulation while achieving compliance.

Cavity Wall Insulation

Standard cavity wall construction with partial fill insulation achieves Part L requirements comfortably.

Typical build-up:

• 100mm inner leaf blockwork
• 75-100mm PIR insulation
• 50mm residual cavity (for drainage)
• 102mm outer brick leaf

U-value achievement:

• 50mm PIR partial fill: 0.22 W/m²K (meets requirement with margin)
• 75mm PIR partial fill: 0.17 W/m²K (significant margin)
• 100mm PIR partial fill: 0.16 W/m²K (exceeds requirement substantially)

Most cavity wall specifications use 75-100mm PIR. The choice depends on:

• Available cavity width (wider cavities allow thicker insulation)
• Desired performance margin (extra thickness provides buffer against thermal bridging effects)
• Cost considerations (thicker insulation costs more but provides better performance)

Full Fill Cavity Walls

Full fill insulation (filling the complete cavity width without residual air gap) achieves very low U-values but requires specialist products designed for full-fill use.

Full fill applications use cavity batts (partial width boards) rather than full boards, with products specifically approved for full-fill use. This isn’t common practice for standard residential construction – partial fill remains the standard approach.

Internal Wall Insulation (IWI)

Insulating solid walls from inside (common for retrofit on properties that can’t have external wall insulation) requires careful specification.

Typical build-up:

• Existing solid wall
• 75-100mm PIR boards mechanically fixed or adhesive-fixed
• Service cavity with battens (optional, improves installation)
• 12.5mm plasterboard finish

U-value achievement:

• 75mm PIR on solid wall: 0.26 W/m²K (meets requirement exactly, tight margin)
• 100mm PIR on solid wall: 0.20 W/m²K (comfortable margin)

IWI loses internal floor area. A 100mm insulation system takes approximately 120-130mm of room width when plasterboard and battens are included. This matters for room sizes, door/window reveals, and overall space perception.

The 0.26 W/m²K requirement is achievable with 75mm PIR in most solid wall constructions. Where space constraints force thinner insulation, calculate actual U-values and discuss acceptable performance with Building Control for renovation projects.

External Wall Insulation (EWI)

Insulating solid walls from outside avoids losing internal space. EWI systems typically use PIR or phenolic boards beneath render or through-coloured finish.

Typical thickness: 90-120mm PIR beneath render system

U-value achievement similar to IWI – 90-100mm achieves 0.20-0.22 W/m²K comfortably meeting requirements.

EWI requires planning permission in many situations (changes external appearance, may affect building lines, conservation area restrictions). Always check planning requirements before specifying EWI.

Floor Insulation Requirements Under Part L

Ground floors require 0.18 W/m²K maximum, a significant tightening from the previous 0.22 W/m²K under Part L 2013.

Solid Concrete Ground Floors

Standard solid floor construction places insulation above the concrete slab beneath screed.

Typical build-up:

• 150mm concrete ground-bearing slab
• Damp-proof membrane
• 100mm PIR insulation
• Perimeter edge strip
• 65mm screed minimum
• Floor finish

U-value achievement:

• 90mm PIR: 0.19 W/m²K (doesn’t meet requirement)
• 100mm PIR: 0.17-0.18 W/m²K (meets requirement)
• 110mm PIR: 0.16 W/m²K (comfortable margin)
• 120mm PIR: 0.15 W/m²K (significant margin)

100mm PIR is the standard specification for Part L compliance in solid floors. This thickness reliably achieves 0.18 W/m²K across various room sizes and constructions.

Edge effects increase floor U-values above centre-of-panel calculations. Small rooms (under 25m²) have worse edge effects than large rooms. For very small rooms (WCs, utility rooms), 110mm PIR provides margin ensuring compliance despite edge losses.

Suspended Timber Ground Floors

Suspended floors allow three insulation positions: between joists, above joists, or below joists.

Between joists (most common for new builds):

• 100mm PIR between 150mm joists: 0.19-0.21 W/m²K (doesn’t meet requirement due to joist thermal bridging)
• 150mm PIR between 200mm joists: 0.15-0.17 W/m²K (meets requirement)

Above joists (eliminates thermal bridging):

• 100mm PIR over joists: 0.17-0.19 W/m²K (meets requirement in most constructions)
• 120mm PIR over joists: 0.15 W/m²K (comfortable margin)

Above-joist insulation achieves better U-values than between-joist because it eliminates thermal bridging through timber joists. The trade-off is losing ceiling height by the insulation thickness plus floor deck thickness.

Below joists (retrofit option):

• 100mm PIR beneath joists: 0.20-0.22 W/m²K (marginal, often doesn’t meet requirement)

Below-joist insulation is the least effective option. Joists become cold (they’re outside the insulation layer), and achieving 0.18 W/m²K requires thicker insulation than other methods.

Beam and Block Floors

Beam and block construction requires insulation above the blocks beneath screed, similar to solid concrete floors.

Typical specification: 100mm PIR above blocks, beneath screed achieves 0.17-0.18 W/m²K meeting Part L requirements.

Achieving Part L with PIR Insulation

PIR insulation’s thermal performance (0.022 W/mK lambda value) makes Part L compliance straightforward with readily-available board thicknesses.

Why PIR Suits Part L Compliance

Performance per millimetre: PIR achieves better thermal resistance per millimetre than alternatives like mineral wool (0.035-0.044 W/mK) or EPS (0.031-0.038 W/mK). This matters where space constraints limit insulation thickness.

Consistent performance: The declared lambda value of 0.022 W/mK remains stable over time. Some insulation types degrade, but PIR maintains performance across the building’s lifetime.

Compressive strength: PIR boards carry floor loading without compression, maintaining designed thickness and performance. Fibrous insulation would compress under floor loading, reducing effectiveness.

Moisture resistance: Closed-cell structure prevents moisture affecting thermal performance. PIR maintains its lambda value even in damp conditions where fibrous insulation would deteriorate.

Workability: PIR boards cut cleanly, install quickly, and handle easily. Fast installation reduces labour costs, offsetting higher material cost compared to cheaper alternatives.

Standard PIR Thicknesses for Part L 2021

Quick reference for achieving Part L with PIR in typical constructions:

These thicknesses assume standard constructions. Complex or unusual build-ups require specific calculations.

Combining PIR with Other Insulation Types

Some projects combine PIR with other insulation materials to optimize cost, performance, or specific characteristics:

PIR + Mineral wool: Between-rafter PIR combined with below-rafter mineral wool for acoustic performance. The mineral wool adds sound insulation while PIR provides primary thermal performance.

PIR + Natural fiber insulation: Similar approach for projects pursuing natural material content while maintaining good thermal performance. PIR provides thermal efficiency, natural fibers add acoustic and moisture buffering properties.

PIR + Multifoil: Multifoil products combined with PIR thickness. This is controversial – multifoil thermal performance is disputed, and Building Control often won’t accept multifoil contribution to U-value calculations without proper testing evidence.

For straightforward Part L compliance, PIR alone is the simplest approach. Combining materials adds complexity and potential for Building Control queries.

New Build vs Extension Requirements

Part L applies differently depending on project type.

New Build Dwellings (Part L1A)

New houses, bungalows, and flats must meet:

Individual element U-values: Maximum 0.16 W/m²K roofs, 0.26 W/m²K walls, 0.18 W/m²K floors, 1.6 W/m²K windows/doors

Target Emission Rate (TER): Overall building carbon emissions below calculated target based on building size, geometry, and default specifications

Air permeability: Maximum 8 m³/h/m² at 50 Pa (often designed to 5 m³/h/m² for margin)

Thermal bridging: Thermal bridging limited using approved construction details or calculated psi-values

Fixed services: Heating, hot water, ventilation, and lighting meet efficiency standards

Compliance demonstration requires SAP calculation by qualified assessor. The SAP report proves the building meets all Part L requirements and provides target fabric specifications for construction.

Extensions (Part L1B)

Extensions to existing dwellings must meet the same U-value limits as new builds. There’s no relaxation – extension fabric must achieve 0.16 W/m²K for roofs, 0.26 W/m²K for walls, 0.18 W/m²K for floors.

Extensions don’t need whole-building SAP calculations. Demonstrating individual element U-values meet requirements is sufficient.

Where extensions differ from new builds:

No whole-building carbon target: Only fabric U-values matter, not overall building emissions

Existing building unaffected: The original house doesn’t need upgrading to current standards when adding an extension

Junction details: Where new extension meets existing building, thermal bridging must be addressed but achieving perfect continuity isn’t always practical

Loft Conversions

Converting existing loft space to habitable rooms counts as creating new dwelling space. The new rooms must meet new build thermal standards (0.16 W/m²K for roof elements).

The rest of the house remains unchanged. You’re not required to upgrade existing floors or walls when converting a loft.

Material Alterations and Renovations

Replacing over 50% of an element: Must upgrade thermal performance to “reasonably achievable” standards. Targets are 0.18 W/m²K for roofs, 0.25 W/m²K for walls, 0.18 W/m²K for floors where achievable.

The “where achievable” caveat matters. Building Control will accept lower performance if you can demonstrate achieving targets creates:

• Disproportionate cost (upgrading costs more than 15% of building value)
• Technical problems (thickness conflicts with existing structural elements, creates moisture risk, etc.)
• Unacceptable impacts (room sizes below minimum standards, head room inadequate, etc.)

Document why full compliance isn’t achievable and discuss acceptable performance with Building Control before starting work.

Replacing under 50% of an element: No thermal upgrade requirements. Work must meet other Building Regulations (structural, fire, ventilation) but thermal performance improvements aren’t mandatory.

Scotland Building Standards Differences

Scotland has separate building standards that are generally stricter than England and Wales Part L requirements.

Scotland U-Value Requirements (Section 6 Energy)

Maximum U-values for new dwellings:

PIR Thickness Implications for Scotland

Scotland’s tighter U-values require thicker insulation:

Ground floors: 120mm PIR instead of 100mm (Scotland vs England/Wales)

Cavity walls: 100mm PIR for comfortable compliance instead of 75mm

Pitched roofs: Typically 100mm between rafters plus 75-100mm over rafters (or 150-180mm single layer if space permits)

Flat roofs: 150-180mm PIR instead of 120-140mm

These increased thicknesses add material cost and may affect build-up depths. For projects in Scotland, factor stricter requirements into early planning and cost estimates.

Scotland Extensions and Conversions

Extensions in Scotland must meet new build U-value requirements (0.13 W/m²K roofs, 0.19 W/m²K walls, 0.15 W/m²K floors). Same as England/Wales approach – extensions match new build standards.

Renovations have similar flexibility to England/Wales where achieving full compliance creates disproportionate difficulty. The term used is “not technically feasible or would not be cost effective” rather than England’s “reasonable provision.”

Cross-Border Projects

Buildings straddling the England-Scotland border follow the regulations of the country where the majority of floor area sits. In practice, cross-border residential buildings are rare.

For businesses working across both countries, maintain awareness that a specification achieving Part L compliance in England might not achieve Scottish compliance. Always verify requirements for the specific location.

PIR Product Selection for Part L Compliance

Selecting appropriate PIR products for Part L compliance requires matching product specifications to application requirements.

Product Selection Checklist

Step 1: Determine target U-value Identify which Part L requirement applies (0.16 W/m²K roof, 0.26 W/m²K wall, or 0.18 W/m²K floor for England/Wales).

Step 2: Calculate required thickness Use manufacturer U-value calculators or approved software to determine thickness needed for your specific construction achieving target U-value.

Step 3: Select appropriate product grade

Floors: Specify floor-grade PIR with minimum 120 kPa compressive strength. Products like Celotex GA4000 or Kingspan TF70 meet this requirement.

Walls: Standard wall-grade PIR (100-120 kPa) is adequate. Any general application PIR works.

Roofs: Standard roof-grade PIR works for most applications. For torch-applied waterproofing, specify glass tissue faced boards (Celotex Crown-Fix, Kingspan TR26, or equivalent).

Step 4: Verify lambda value Confirm the product declares 0.022 W/mK (or better) lambda value. Most major brand PIR boards meet this. Check the product Declaration of Performance (DoP) or technical datasheet.

Step 5: Check availability Verify your merchant stocks the specified thickness. Uncommon thicknesses (like 110mm or 130mm) might require special order with longer lead times. Standard thicknesses (75mm, 100mm, 120mm, 150mm) stock readily.

Documentation for Building Control

Building Control submissions require evidence that specified insulation achieves required U-values. Provide:

Product technical datasheets: Showing declared lambda value and product specifications

U-value calculations: Using approved calculation software or manufacturer calculators, demonstrating complete constructions achieve Part L targets

Construction details: Drawings showing installation method, layer sequence, and junction details

Declaration of Performance: For CE marked products, the DoP confirms product characteristics

Installation specifications: Confirming installation will follow manufacturer guidance (fixing density, joint treatment, vapour control layer positioning)

Complete documentation prevents Building Control queries and streamlines approval.

Common Specification Mistakes

Using wall-grade PIR for floors: Wall-grade boards (100 kPa compressive strength) compress under floor loading. Always specify floor-grade (120 kPa minimum) for beneath-screed applications.

Assuming old specifications still comply: Specifications written under Part L 2013 often don’t achieve Part L 2021 requirements. Review and update specifications for current standards.

Not accounting for thermal bridging: Centre-of-panel U-value calculations don’t account for thermal bridging through fixings, rafters, wall ties. Add margin to ensure real-world performance meets targets.

Specifying thickness without calculation: Don’t assume 100mm achieves Part L compliance for all applications. Run proper calculations for your specific construction.

Ignoring Scotland differences: Scotland requires different U-values. A specification complying with England/Wales Part L doesn’t automatically comply with Scottish standards.Need PIR insulation that meets Part L 2021 requirements? Online Insulation stocks the complete range of PIR thicknesses from all major manufacturers, with fast UK delivery and expert technical support for trade professionals.