New Build Wall Insulation Calculator
Calculate U-values for new build external walls to meet Part L 2021 requirements. New dwellings must achieve 0.18 W/m²K - significantly stricter than the 0.28 W/m²K allowed for extensions.
U-Value Calculator
Check Part L 2021 compliance for your insulation project
Tips for New Build Wall Insulation
- Full-fill cavity with aerated blocks achieves 0.18 without thick insulation
- Partial fill + full fill combination for premium performance
- Thermal bridging at lintels, jambs, and sills must be addressed
- SAP calculations required for Building Control approval
Wall Insulation Options Comparison
| Wall Type | U-Value Range | Cost/m² | Notes |
|---|---|---|---|
| Full-fill cavity | 0.22-0.28 | £8-15 | Extension standard |
| Partial fill PIR | 0.18-0.22 | £18-28 | New build achievable |
| Full-fill + aerated | 0.15-0.18 | £25-35 | Part L 2021 compliant |
| External wall insulation | 0.12-0.18 | £100-150 | Premium/Passivhaus |
Costs are insulation material only. Full wall build costs 150-250/m².
Common Mistakes to Avoid
Ignoring thermal bridging
Problem: Actual U-value 20-40% worse than calculated
Solution: Use accredited thermal bridge details or approved products
Using dense blocks with cavity insulation only
Problem: Cannot achieve 0.18 target
Solution: Specify aerated blocks (Thermalite, H+H) or thicker insulation
Cavity trays blocking insulation
Problem: Cold bridges above windows and at DPC
Solution: Use insulated cavity trays and closers
Wall tie thermal bridging
Problem: Cold spots visible in thermal imaging
Solution: Use basalt or stainless steel ties, not galvanised
Relying on basic U-value only
Problem: SAP assessment fails due to thermal bridging
Solution: Commission SAP calculation early in design
Understanding New Build Wall Insulation
Part L 2021 significantly tightened wall insulation requirements for new dwellings. The 0.18 W/m²K target means most traditional build methods need upgrading with aerated blocks, improved insulation, and careful attention to thermal bridging details.
For retrofit work on existing buildings, see our cavity wall calculator (extensions to 0.28) or external wall insulation for solid walls.
Example: Typical New Build 3-Bed House
External wall area: ~120m²
Construction: 100mm aerated block, 100mm partial-fill PIR, 100mm brick
U-value achieved: 0.17 W/m²K
Insulation cost: ~£3600 (materials)
Meets Part L 2021 with margin
Frequently Asked Questions
Related Calculators
Part L 2021 Key Targets
- • Walls: 0.18 W/m²K
- • Roofs: 0.15 W/m²K
- • Floors: 0.18 W/m²K
- • Windows: 1.4 W/m²K
- • Doors: 1.4 W/m²K
31% CO₂ reduction vs Part L 2013
Future Homes Standard 2025
Further improvements expected including ~0.15 wall target, no gas boilers, and 75-80% CO₂ reduction. Consider building ahead of regulations.
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