U-Value Calculator 2026 | Part L Building Regulations UK
U-Value Calculator 2026
Calculate Thermal Transmittance for Part L Compliance
UK Building Regulations Energy Efficiency Standards
U-Values measure thermal transmittance through building elements and are critical for Building Regulations Approved Document Part L (Conservation of Fuel and Power) compliance. Our U-Value calculator helps determine heat loss through walls, roofs, floors, windows, and doors, ensuring your 2026 construction project meets mandatory energy efficiency standards.
Proper U-Value calculations reduce heating costs, improve comfort, and meet SAP assessment requirements. Part L 2021 (updated for 2026) requires maximum U-Values ranging from 0.11 to 0.18 W/m²K for walls, 0.11 to 0.15 W/m²K for roofs, and 0.13 to 0.18 W/m²K for floors. This calculator incorporates BRE standards and current thermal conductivity values.
🌡️📊 U-Value Calculator
Calculate thermal transmittance of building elements
Building Element Details
Layer Construction (Inside to Outside)
Calculated U-Value
0.00 W/m²K
-
Thermal Performance
Total R-Value
0.00 m²K/W
Part L Target
0.00 W/m²K
Heat Loss (20°C Δ)
0 W/m²
Annual Cost/m² (£0.24/kWh)
£0.00
Layer-by-Layer Breakdown
Internal Surface Resistance0.13 m²K/W
Layer 1 R-Value0.00 m²K/W
Layer 2 R-Value0.00 m²K/W
Layer 3 R-Value0.00 m²K/W
Layer 4 R-Value0.00 m²K/W
External Surface Resistance0.04 m²K/W
Recommendations
Compliance Status-
Improvement Needed-
Additional Insulation-
Part L 2021 U-Value Requirements 2026
Building Regulations Approved Document L (Conservation of Fuel and Power) sets maximum U-Values for new builds and renovations. These standards, updated in 2021 and applicable throughout 2026, significantly tighten thermal performance requirements to reduce carbon emissions and energy consumption.
Maximum U-Values for New Dwellings 2026
Building Element
Max U-Value (New Build)
Max U-Value (Extensions)
Notional Building
External Walls
0.18 W/m²K
0.28 W/m²K
0.18 W/m²K
Party Walls
0.20 W/m²K
0.20 W/m²K
0.00 W/m²K
Ground Floors
0.13 W/m²K
0.22 W/m²K
0.13 W/m²K
Exposed Floors
0.13 W/m²K
0.22 W/m²K
0.13 W/m²K
Pitched Roof - Insulation at Ceiling
0.11 W/m²K
0.16 W/m²K
0.11 W/m²K
Pitched Roof - Insulation at Rafter
0.15 W/m²K
0.18 W/m²K
0.15 W/m²K
Flat Roof / Roof Terrace
0.11 W/m²K
0.18 W/m²K
0.11 W/m²K
Windows & Glazed Doors
1.40 W/m²K
1.60 W/m²K
1.20 W/m²K
Roof Windows (< 70° Pitch)
1.40 W/m²K
1.60 W/m²K
1.20 W/m²K
Solid Doors (Opaque)
1.00 W/m²K
1.40 W/m²K
1.00 W/m²K
External Walls
New Build Max0.18 W/m²K
Extensions Max0.28 W/m²K
Notional0.18 W/m²K
Pitched Roof (at Ceiling)
New Build Max0.11 W/m²K
Extensions Max0.16 W/m²K
Notional0.11 W/m²K
Ground Floors
New Build Max0.13 W/m²K
Extensions Max0.22 W/m²K
Notional0.13 W/m²K
Windows & Glazed Doors
New Build Max1.40 W/m²K
Extensions Max1.60 W/m²K
Notional1.20 W/m²K
Understanding U-Value Calculations
U-Value (thermal transmittance) measures heat flow through a building element in watts per square meter per degree Kelvin (W/m²K). Lower U-Values indicate better insulation. The calculation involves summing thermal resistances (R-Values) of all layers including surface resistances, then taking the reciprocal: U = 1 / (Rsi + R1 + R2 + R3... + Rse).
Thermal Resistance (R-Value)
Formula: R = Thickness (m) / λ (W/mK)
Example: 100mm Rockwool (λ=0.035)
Calculation: R = 0.1 / 0.035 = 2.86 m²K/W
Higher R-Values = Better insulation
Thermal Conductivity (λ Lambda)
Definition: Heat flow through 1m thickness of material
Units: W/mK (Watts per meter-Kelvin)
Low λ Materials: PIR (0.022), Rockwool (0.035)
High λ Materials: Concrete (1.13), Brick (0.77)
Surface Resistances
Internal (Rsi): 0.13 m²K/W (walls/roofs/floors)
External (Rse): 0.04 m²K/W (walls/roofs)
Ground Contact: Use 0.00 for Rse (floor)
These account for convection and radiation at surfaces
Selecting appropriate insulation materials is critical for achieving Part L compliance. Material choice depends on space constraints, budget, fire performance, and environmental considerations. Modern insulation products offer superior thermal performance in minimal thickness.
Insulation Material
Lambda (λ) W/mK
Thickness for R=2.85
Cost per m² (100mm)
Phenolic Foam (Kingspan K5)
0.020
57mm
£18-25
PIR/Polyisocyanurate (Celotex)
0.022
63mm
£15-22
XPS Extruded Polystyrene
0.034
97mm
£12-18
Mineral Wool / Rockwool
0.035
100mm
£6-10
EPS Expanded Polystyrene
0.038
108mm
£8-12
Cellulose Fibre (Warmcel)
0.038
108mm
£10-15
Glass Wool (Knauf)
0.040
114mm
£5-9
Sheep's Wool (Natural)
0.038
108mm
£18-28
Wood Fibre Board
0.038
108mm
£20-30
Multifoil (Reflects only)
Variable
Approx R=1.5 typical
£12-20
Phenolic Foam (Best)
Lambda (λ)0.020 W/mK
Thickness for R=2.8557mm
Cost (100mm)£18-25/m²
PIR (Celotex/Kingspan)
Lambda (λ)0.022 W/mK
Thickness for R=2.8563mm
Cost (100mm)£15-22/m²
Mineral Wool (Popular)
Lambda (λ)0.035 W/mK
Thickness for R=2.85100mm
Cost (100mm)£6-10/m²
Achieving Part L Compliance
Meeting Part L requires either elemental approach (each element meets maximum U-Values) or whole-building SAP calculation showing overall carbon compliance. Most projects use elemental method for simplicity. SAP assessments allow trade-offs between elements.
Wall Insulation - Achieving 0.18 W/m²K
Cavity Wall (50mm): 100-120mm PIR or 150mm mineral wool in cavity + partial fill
Solid Wall External: 120-150mm PIR external insulation system
Solid Wall Internal: 100mm PIR + plasterboard (room size loss)
Timber Frame: 140mm mineral wool studs + 50mm PIR sheathing
Roof Insulation - Achieving 0.11 W/m²K
Ceiling Level: 270-300mm mineral wool quilt (cheap, easy)
Between Rafters: 150mm PIR or 200mm mineral wool
Over Rafters: 140mm PIR boarding (cold roof)
Flat Roof: 180-200mm PIR above deck (warm roof preferred)
Floor Insulation - Achieving 0.13 W/m²K
Ground Bearing Slab: 150mm PIR or 200mm EPS under slab
Beam & Block: 100-120mm PIR between beams + 50mm screed
Suspended Timber: 100mm mineral wool between joists + 50mm PIR under
Edge Insulation: Vertical 75mm to 1m depth (perimeter heat loss)
Ventilated Cavities: Air-brick ventilated cavities have R=0.00, not 0.18 (breaks thermal barrier)
Air Gap Limits: Cavities > 50mm still only count R=0.18 maximum
Multifoil Exaggeration: Multifoil R-Values often overstated - require air gaps both sides to work
SAP Calculations and Energy Performance
Standard Assessment Procedure (SAP) is the UK government methodology for assessing dwelling energy performance. SAP calculations consider U-Values, air tightness, thermal bridging, heating systems, renewables, and ventilation to produce an Energy Performance Certificate (EPC) rating. All new builds require SAP assessment.
🏠📊 SAP Assessment Requirements 2026:
New Dwellings: Design Stage SAP (planning) + As-Built SAP (completion) mandatory
Target Fabric Energy Efficiency (TFEE): Maximum 52 kWh/m²/year for compliance
Primary Energy Rate (PER): Must not exceed Target Primary Energy Rate (TER)
Carbon Emissions: 31% reduction vs 2013 standards (75-80% lower than 2006)
Air Tightness: Test required showing < 8 m³/h/m² @ 50Pa (typically achieve 5-6)
Thermal Bridging (Psi-Values): Accredited construction details or calculated corrections
EPC Rating: Minimum Band B (81-91) typically achieved for Part L compliance
SAP Assessor: Must be on Elmhurst/Stroma register (£400-800 per dwelling)
Thermal Bridging and Y-Values
U-Values calculated for main building elements don't account for heat loss through junctions, lintels, balconies, and structural penetrations. Thermal bridges can increase whole-building heat loss by 10-30%. Y-Value (linear thermal transmittance) corrections account for these effects in SAP assessments.
Common Thermal Bridges
Wall-to-Roof Junction: Psi = 0.05-0.15 W/mK
Wall-to-Floor Junction: Psi = 0.10-0.40 W/mK
Window Reveals: Psi = 0.03-0.08 W/mK per meter
Party Wall Bypass: Psi = 0.05-0.20 W/mK
Reducing Thermal Bridging
Continuous Insulation: Wrap entire building envelope without gaps
Insulated Lintels: Use proprietary insulated lintels above openings
Service Penetrations: Seal all holes with expanding foam
When renovating or extending existing buildings, Part L requires controlled elements (walls, roofs, floors, windows, doors) to be upgraded if technically, functionally, and economically feasible. Extension U-Values are less stringent than new build (e.g. 0.28 vs 0.18 for walls) but must still demonstrate improvement.
✅ Renovation U-Value Targets:
External Wall Insulation (EWI): Achieve 0.30 W/m²K with 100mm EPS or 75mm PIR
Internal Wall Insulation (IWI): Achieve 0.30 W/m²K with 50-75mm PIR + plasterboard
Loft Insulation Top-Up: Achieve 0.16 W/m²K with 270mm total mineral wool (£300-500)
Exemptions: Listed buildings, technical impossibility, payback > 15 years
U-Value Calculator FAQs
What is a U-Value and why is it important?
U-Value measures how well a building element conducts heat, expressed in W/m²K (watts per square meter per Kelvin). Lower U-Values mean better insulation. A wall with U=0.18 W/m²K loses 0.18 watts of heat per square meter for every 1°C temperature difference. Part L Building Regulations mandate maximum U-Values to reduce energy consumption, heating bills, and carbon emissions. Typical savings: upgrading walls from 0.50 to 0.18 W/m²K saves £200-400/year on heating for average semi-detached house.
How do I calculate U-Value from R-Values?
U-Value is the reciprocal of total thermal resistance: U = 1 / R-total. Calculate R for each layer (R = thickness in meters / lambda), add surface resistances (Rsi = 0.13, Rse = 0.04), sum all R-Values, then take 1 divided by total. Example: Total R = 4.50 m²K/W gives U = 1 / 4.50 = 0.22 W/m²K. Higher total R = lower U = better insulation.
What insulation thickness do I need to meet Part L?
For 0.18 W/m²K external wall (cavity): 100mm PIR (λ=0.022) or 140mm mineral wool (λ=0.035). For 0.11 W/m²K pitched roof at ceiling: 270-300mm mineral wool quilt. For 0.13 W/m²K ground floor: 150mm PIR under slab. Actual thickness depends on existing construction layers and their thermal properties. Use our calculator with your specific materials to determine exact requirements.
Do I need a SAP calculation or can I just meet U-Values?
New build dwellings require full SAP calculation by registered assessor showing compliance with Fabric Energy Efficiency and Primary Energy targets. Extensions > 50m² or > 25% existing floor area also need SAP. Smaller extensions can use elemental method (simply meet maximum U-Values for each element). SAP allows trade-offs - worse U-Value compensated by better heating system or renewables. Cost: £400-800 per dwelling for SAP assessment.
What is the difference between U-Value and R-Value?
R-Value measures thermal resistance (how well material resists heat flow) in m²K/W. U-Value measures thermal transmittance (how easily heat flows through) in W/m²K. They are reciprocals: U = 1 / R. Higher R is better (more resistance), lower U is better (less heat flow). R-Values add up through layers, U-Values don't. Always calculate total R first, then convert to U. Example: R=5.0 m²K/W = U=0.20 W/m²K.
Can I use multifoil insulation instead of thick insulation?
Multifoil reflective insulation is controversial. Manufacturers claim high R-Values (R=3-4) but these only apply with specific air gaps both sides and proper installation. BBA certificates show actual performance often R=1.5-2.0, equivalent to 50-70mm PIR. Part L compliance requires independent test data. Most Building Control officers skeptical - safer to use proven materials (PIR/mineral wool) with established lambda values. Multifoil works best combined with other insulation, not as sole insulator.
What happens if my U-Values don't meet Part L requirements?
Building Control will refuse to pass your project. You must either: increase insulation thickness to achieve compliant U-Values, use higher-performance materials (PIR instead of mineral wool), or commission SAP calculation to demonstrate alternative compliance pathway (e.g. compensate with highly efficient heating, solar panels, reduced glazing area). Non-compliance risks: cannot get completion certificate, house unsaleable, mortgage problems, enforcement notice requiring remedial work costing £5,000-20,000+.
How do I account for timber studs in my U-Value calculation?
Timber studs create thermal bridging - they conduct more heat than insulation between them. For timber frame walls, calculate weighted average: typically 85% insulation + 15% timber. Use bridging calculation: U-total = (0.85 × U-insulated) + (0.15 × U-stud). Alternatively use specialist software (THERM, BuildDesk). Approximation: timber frame with 140mm mineral wool between studs + 50mm PIR sheathing achieves approximately 0.18 W/m²K accounting for bridging. Always verify with SAP assessor for new builds.
What U-Value do double glazed windows typically achieve?
Standard double glazing (two panes, 16mm air gap): U=2.8-3.0 W/m²K (old, non-compliant). Low-E coated double glazing (argon filled, warm edge spacers): U=1.4-1.6 W/m²K (meets Part L extensions). High-performance double glazing (soft-coat low-E, argon/krypton): U=1.2-1.4 W/m²K (meets new build). Triple glazing: U=0.8-1.2 W/m²K (exceeds requirements, £150-250 extra per m²). Whole window U-Value includes frame - timber/uPVC better than aluminium.
How much does poor insulation cost me in heating bills?
Heat loss (watts) = U-Value × Area × Temperature difference. Example: 100m² wall, U=0.50 W/m²K (poor), 20°C difference = 1000W continuous heat loss. Over heating season (Oct-Apr, 180 days × 8hrs/day avg): 1000W × 1440hrs = 1,440 kWh. At £0.24/kWh gas = £346/year. Same wall upgraded to U=0.18 saves £218/year (63% reduction). Whole-house upgrade saves £400-800/year typically. Payback: 5-15 years depending on insulation costs.
