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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-1940 cavity walls | Pre-1919 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-1919 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
Next up: related pages
How We Calculate This
Frequently Asked Questions
Look at the external brickwork. Cavity walls show stretcher bond (all bricks laid lengthways). Solid walls show alternating headers and stretchers (Flemish bond) or a row of headers every few courses (English bond). Most UK houses built after 1940 have cavity walls, and properties built before 1919 are typically solid wall. A borescope inspection through a drilled hole can confirm.
Cavity wall insulation is one of the most cost-effective energy improvements. It costs £500-1,500 for a typical house and saves £150-300/year on heating bills — payback in 2-5 years. It is quick to install (4-6 hours) with minimal disruption. The Energy Company Obligation (ECO4) scheme may fund it free for eligible households.
In some cases, yes. If the cavity is too narrow (<50mm), the wall is very exposed to driving rain (west-facing, coastal, tall buildings), or the insulation is poorly installed with gaps, moisture can bridge across the cavity. This affects about 2-5% of installations. A pre-installation survey should assess exposure, cavity width and wall condition.
Solid walls can be insulated internally (IWI: insulated plasterboard or studwork, £50-90/m²) or externally (EWI: insulation boards with render, £100-160/m²). There is no cavity to fill, so both methods add thickness. IWI reduces room size; EWI changes the external appearance. EWI gives better thermal performance with fewer cold bridges.
A typical solid-walled 3-bed semi has a wall U-value of 2.0-2.2 W/m²K. Insulating to 0.30 W/m²K saves approximately £300-500/year on gas heating. Payback periods are 10-20 years for EWI and 8-15 years for IWI, depending on cost and fuel prices. Government grants and ECO4 funding can significantly reduce the cost.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.