Cavity vs Solid Wall Insulation — UK Guide
Comparing cavity wall insulation and solid wall insulation for UK homes — applicability, methods, cost, condensation risk and U-value improvement.
Side-by-Side Comparison
| Property | Cavity Wall Insulation | Solid Wall (Internal) | Solid Wall (External) |
|---|---|---|---|
| Applies to | Post-1930 cavity walls | Pre-1920 solid walls | Pre-1920 solid walls |
| Method | Blown fibre/foam into cavity | Insulated boards fixed inside | Insulation + render outside |
| Cost (3-bed semi) | £500-1,500 | £4,000-8,000 | £8,000-16,000 |
| Installation time | 4-6 hours | 1-3 weeks | 2-4 weeks |
| Disruption | Minimal (exterior drilling) | High (rooms cleared) | Low (external only) |
| U-value before | 1.0-1.6 W/m²K | 2.0-2.2 W/m²K | 2.0-2.2 W/m²K |
| U-value after | 0.35-0.55 W/m²K | 0.25-0.35 W/m²K | 0.25-0.30 W/m²K |
| Annual saving | £150-300 | £300-500 | £350-550 |
| Payback period | 2-5 years | 8-15 years | 15-30 years |
UK Cost Comparison (2026 Prices)
For a 3-bedroom semi-detached house:
| Insulation Type | Cost | Annual Saving | Payback |
|---|---|---|---|
| Cavity wall (blown fibre) | £500-1,000 | £150-250 | 2-4 years |
| Cavity wall (foam) | £800-1,500 | £200-300 | 3-5 years |
| Solid wall (internal) | £4,000-8,000 | £300-500 | 8-16 years |
| Solid wall (external) | £8,000-16,000 | £350-550 | 15-30 years |
When to Choose Each
Cavity Wall Insulation When
- You have unfilled cavity walls (post-1930)
- Cavity is 50mm+ wide
- Walls not severely exposed to rain
- Best value energy improvement available
- Check ECO4 eligibility for free installation
Solid Wall Insulation When
- Pre-1920 solid brick or stone walls
- No cavity available to fill
- Internal (cheaper, preserves appearance)
- External (better performance, less disruption)
- Major renovation or extending anyway
Environmental Considerations
Wall insulation delivers the highest carbon savings of any home improvement after loft insulation. A solid-walled house loses about 35% of its heat through the walls. Insulating solid walls saves approximately 1,500-2,500 kg of CO2 per year from gas heating. Cavity wall insulation saves 500-1,000 kg/year. Both pay back their embodied carbon within 1-3 years.
Related Calculators
- U-Value Calculator — calculate wall U-values
- Insulation Thickness Calculator — required thickness for target U-value
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.