Calculated U-Value
0.00 W/m²K
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Building Regulations Energy Efficiency Calculator
U-Value Compliance Assessment Tool
Building Regulations Approved Document L (Conservation of Fuel and Power) sets minimum energy efficiency standards for buildings in England. Our Part L Insulation Calculator helps assess U-value compliance for walls, roofs, floors, windows, and doors to meet 2026 thermal performance requirements and reduce heating costs.
The 2021 edition (applying through 2026) introduced significantly upgraded standards with approximately 30% carbon emission reductions. All new builds and extensions must achieve target SAP ratings and meet elemental U-values. This calculator provides preliminary compliance assessment for residential and commercial projects following BRE standards.
Calculate U-values and insulation requirements
U-values measure heat loss through building elements (lower is better). Part L 2021/2026 sets maximum U-values for different elements. All new builds must meet these standards plus achieve overall SAP or SBEM compliance demonstrating whole-building energy performance.
✅ Part L 2026 Maximum U-Values (W/m²K):
| Building Element | New Build Target | Extension Target | Typical Insulation |
|---|---|---|---|
| Cavity Wall | 0.18 W/m²K | 0.30 W/m²K | 100mm PIR or 150mm Mineral Wool |
| Solid Wall (External) | 0.18 W/m²K | 0.30 W/m²K | 120-150mm EWI (External Wall Insulation) |
| Timber Frame Wall | 0.18 W/m²K | 0.30 W/m²K | 140mm between studs + sheathing |
| Pitched Roof (Rafter) | 0.15 W/m²K | 0.18 W/m²K | 170mm PIR or 270mm Mineral Wool |
| Pitched Roof (Ceiling) | 0.15 W/m²K | 0.16 W/m²K | 270-300mm loft insulation |
| Flat Roof (Warm Deck) | 0.15 W/m²K | 0.18 W/m²K | 180mm PIR above deck |
| Solid Ground Floor | 0.15 W/m²K | 0.22 W/m²K | 100-120mm under slab insulation |
| Suspended Timber Floor | 0.15 W/m²K | 0.22 W/m²K | 100-150mm between/under joists |
Different insulation materials have varying thermal conductivity (lambda values). Lower lambda means better insulation per thickness. Material choice affects required thickness, cost, and buildability. Select appropriate materials for each application following BRE Green Guide ratings.
Lambda: 0.022 W/mK (Excellent)
Thickness for 0.18 U-value: 100mm cavity wall
Cost: £8-15/m² (100mm)
Applications: Cavity walls, roofs, floors
Pros: Best performance, thin profile
Cons: Higher cost, combustibility concerns
Lambda: 0.020 W/mK (Best)
Thickness for 0.18 U-value: 90mm cavity wall
Cost: £12-20/m² (100mm)
Applications: Space-constrained areas
Pros: Thinnest option available
Cons: Most expensive, limited availability
Lambda: 0.044 W/mK (Standard)
Thickness for 0.18 U-value: 150mm cavity wall
Cost: £4-8/m² (100mm)
Applications: All elements, lofts, frames
Pros: Economical, fire-resistant, acoustic benefits
Cons: Thicker than PIR, moisture sensitive
Lambda: 0.038 W/mK (Good)
Thickness for 0.18 U-value: 130mm cavity wall
Cost: £3-6/m² (100mm)
Applications: Floors, external wall insulation
Pros: Budget-friendly, moisture resistant
Cons: Less efficient than PIR
Lambda: 0.040 W/mK (Good)
Thickness for 0.18 U-value: 140mm
Cost: £5-9/m² (100mm blown)
Applications: Cavity fill, loft top-up, retrofit
Pros: Eco-friendly, good air-tightness
Cons: Professional installation needed
Lambda: 0.038 W/mK (Good)
Thickness for 0.18 U-value: 130mm
Cost: £10-18/m² (100mm)
Applications: Breathable construction, eco-builds
Pros: Natural, carbon-storing, breathable
Cons: Premium cost, thicker than PIR
U-values calculate total heat transfer through building elements, combining all material layers. Lower U-values indicate better insulation. Calculations follow BS EN ISO 6946:2017 methodology accounting for material thickness, thermal conductivity, surface resistances, and air gaps.
📐 U-Value Calculation Formula:
U-Value = 1 ÷ Total R-Value
Where R-Value = Rsi + R₁ + R₂ + R₃... + Rso
External walls represent the largest heat loss area in buildings. Part L requires 0.18 W/m²K for new builds (approximately 30% better than previous 2013 standards). Cavity walls, solid walls, and timber frame each require different insulation strategies.
| Insulation Material | Target 0.18 W/m²K | Target 0.30 W/m²K | Cost per m² |
|---|---|---|---|
| PIR Board (0.022 λ) | 100mm full fill | 65mm partial fill | £12-18 |
| Phenolic (0.020 λ) | 90mm full fill | 60mm partial fill | £18-25 |
| Mineral Wool (0.044 λ) | Not achievable in 100mm cavity | 125mm full fill | £6-10 |
| EPS (0.038 λ) | Requires 120mm+ (wide cavity) | 100mm full fill | £5-8 |
| Blown Beads (0.040 λ) | Not achievable standard cavity | 100mm cavity fill | £8-12 |
Roof insulation location significantly affects required thickness. Insulation at ceiling level (cold loft) is most economical. Insulation at rafter level (warm loft/room in roof) requires more material but provides usable loft space. Part L requires 0.15 W/m²K for both in new builds.
Target U-Value: 0.15 W/m²K
Mineral Wool: 270-300mm depth
Blown Cellulose: 300-330mm depth
Cost: £3-6/m² material + £15-25/m² installed
Best for: Unused loft spaces
Target U-Value: 0.15 W/m²K
PIR Board: 170mm between/under rafters
Mineral Wool: 270mm (140+140 cross-layers)
Cost: £18-30/m² material + £30-50/m² installed
Best for: Loft conversions, room in roof
Target U-Value: 0.15 W/m²K
PIR Above Deck: 180-200mm
Tapered Insulation: 200mm average to falls
Cost: £20-35/m² insulation + £60-100/m² complete
Best for: New flat roofs, extensions
Target U-Value: 0.15 W/m²K
XPS Above Membrane: 160mm (moisture resistant)
Ballast/Paving: Over insulation
Cost: £25-40/m² insulation + £80-120/m² complete
Best for: Accessible roofs, retrofit
Ground floors require insulation to prevent heat loss to earth. Solid floors use rigid insulation under screed. Suspended timber floors insulate between or under joists. Part L sets 0.15 W/m²K for new builds, 0.22 W/m²K for extensions.
⚠️ Floor Insulation Considerations:
Glazing represents significant heat loss. Modern BFRC-rated windows achieve Part L compliance. Triple glazing offers best performance but costs 40-60% more than double glazing. Door U-values depend on construction and glazing percentage.
| Glazing Type | Typical U-Value | Part L Compliant? | Cost Premium |
|---|---|---|---|
| Double Glazed (Air filled) | 2.0-2.8 W/m²K | ❌ No | Baseline |
| Double Glazed (Argon filled) | 1.4-1.6 W/m²K | ✅ Yes (Standard) | +0% |
| Double Glazed (Low-E, Argon) | 1.2-1.4 W/m²K | ✅ Yes (Good) | +10-15% |
| Triple Glazed (Argon, Low-E) | 0.8-1.0 W/m²K | ✅ Yes (Excellent) | +40-60% |
| Triple Glazed (Krypton, Low-E) | 0.5-0.7 W/m²K | ✅ Yes (Passive House) | +80-100% |
Standard Assessment Procedure (SAP) is the UK methodology for calculating dwelling energy performance. New builds require SAP calculation demonstrating compliance with Part L target emissions rate (TER). An Energy Performance Certificate (EPC) rates buildings A-G.
Required For: All new dwellings and conversions
Designer SAP: Pre-construction assessment
As-Built SAP: Post-construction verification
Cost: £400-800 per dwelling
Assessor: Must be registered SAP assessor
2021/2026 Standard: 31% lower than 2013
Fabric First: Good insulation reduces heating demand
Technologies: Heat pumps, solar PV contribute
Trade-offs: Better fabric = less renewable needed
New Builds 2026: Typically achieve EPC B (81-91)
Extensions: Don't worsen whole-house EPC significantly
Rental Properties: Minimum EPC E required
Future 2025+: New rental minimum EPC C proposed
Option 1: Meet all elemental U-values + SAP
Option 2: Trade-off between elements (within limits)
Option 3: Backstop U-values (maximum allowed)
Recommended: Use Option 1 for simplicity