Internal vs External Wall Insulation — UK
Comparing internal wall insulation (IWI) and external wall insulation (EWI) for UK homes — U-value improvement, cost, disruption, planning and condensation risk.
Side-by-Side Comparison
| Property | Internal (IWI) | External (EWI) |
|---|---|---|
| Installed cost per m² | £50-90 | £100-160 |
| Total cost (3-bed semi) | £4,000-8,000 | £8,000-16,000 |
| U-value achieved | 0.25-0.35 W/m²K | 0.25-0.30 W/m²K |
| Cold bridge elimination | Partial — junctions are difficult | Excellent — continuous wrap |
| Room size impact | Loses 60-100mm per wall | No internal space lost |
| Internal disruption | High — rooms cleared, services moved | Low — external scaffolding only |
| External appearance | Unchanged | Changed (rendered finish) |
| Planning permission | Not required | May be required |
| Condensation risk | Higher — needs VCL | Lower — wall stays warm |
| Installation time | 1-2 weeks (room by room) | 2-4 weeks (whole house) |
UK Cost Comparison (2026 Prices)
For a 3-bedroom semi-detached house (approximately 80 m² of external wall):
| Cost Element | Internal (IWI) | External (EWI) |
|---|---|---|
| Insulation materials | £1,200-2,400 | £2,400-4,000 |
| Render / plasterboard finish | £400-800 | £1,600-2,800 |
| Scaffolding | N/A | £1,000-1,800 |
| Labour | £2,000-4,000 | £3,500-6,000 |
| Services (sockets, pipes) | £400-800 | N/A |
| Total | £4,000-8,000 | £8,500-14,600 |
| Annual energy saving | £200-400 | £250-500 |
When to Choose Each
Choose Internal (IWI) When
- External appearance must be preserved
- Listed building or conservation area
- Budget is limited
- Only one or two rooms need insulating
- Mid-terrace (shared party walls)
Choose External (EWI) When
- Whole house needs insulating
- Minimal internal disruption wanted
- Improving external appearance too
- No planning restrictions
- Best thermal performance needed
Related Calculators
- U-Value Calculator — calculate wall U-values with insulation
- 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.