Cavity Wall Insulation Calculator
Calculate the U-value for cavity wall insulation and check Part L compliance. Whether you are installing blown insulation, rigid batts, or partial fill boards, our calculator helps you meet Building Regulations targets of 0.28 W/m2K for extensions.
U-Value Calculator
Check Part L 2021 compliance for your insulation project
Tips for Cavity Wall Insulation
- Cavity wall insulation requires a minimum 50mm cavity width for full-fill
- Properties built after 1920 typically have suitable cavity walls
- Check for existing insulation before installing - doubled up insulation wastes money
- Ensure all wall ties are in good condition before filling the cavity
Cavity Wall Insulation Cost Estimates
Professional installation costs by property size (2025 prices):
| Wall Area | Blown Fill | Partial Fill | Property Type |
|---|---|---|---|
| 50m2 | GBP 400-600 | GBP 600-900 | Small terrace |
| 80m2 | GBP 600-900 | GBP 900-1400 | Semi-detached |
| 120m2 | GBP 900-1300 | GBP 1400-2000 | Detached |
| 160m2 | GBP 1200-1800 | GBP 1800-2600 | Large detached |
ECO4 grants may cover full cost for eligible households.
Common Mistakes to Avoid
Installing in unsuitable walls
Problem: Water penetration and damp problems
Solution: Survey for exposed locations, cracks, and failed pointing first
Blocking weep holes
Problem: Trapped moisture causes internal damp
Solution: Clear all weep holes after installation
Ignoring wall tie condition
Problem: Wall tie corrosion accelerates after filling
Solution: Get wall tie survey before installation
Not checking for existing insulation
Problem: Waste of money, no benefit
Solution: Use borescope inspection to check cavity
DIY blown insulation
Problem: Incomplete fill, voids, settlement
Solution: Always use certified installer with guarantee
Understanding Cavity Wall Insulation
Cavity wall insulation fills the gap between the inner and outer leaves of cavity walls, typically 50-100mm wide. It is one of the most cost-effective energy improvements, with typical payback periods of 3-5 years.
If your property has solid walls (pre-1920s construction), see our internal wall insulation calculator or external wall insulation calculator for alternative solutions.
Calculating Cavity Wall U-Values
The achievable U-value depends on cavity width and insulation type. A typical 50mm cavity filled with mineral wool achieves approximately 0.50 W/m2K. Wider 100mm cavities or better-performing materials can achieve 0.30 W/m2K or better.
Example: Semi-Detached House (80m2 wall area)
Before: Uninsulated cavity (U-value approx 1.5)
After: 65mm blown mineral wool (U-value approx 0.45)
Installation cost: GBP 600-900
Annual saving: GBP 180-280
Payback: 3-4 years
For new build projects requiring the stricter 0.18 W/m2K target, visit our new build wall calculator for partial-fill insulation options.
Frequently Asked Questions
Related Calculators
Other Calculators
ECO4 Grants Available
Free cavity wall insulation may be available through government schemes.
Check EligibilityNeed Professional Advice?
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