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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
Next up: related pages
How We Calculate This
Frequently Asked Questions
Both can achieve similar U-values (0.25-0.35 W/m²K) depending on insulation thickness. EWI typically uses 60-100mm of EPS or mineral wool, achieving 0.25-0.30 W/m²K on a solid wall. IWI achieves similar values but with less thermal mass benefit. The key difference is that EWI wraps the building in a continuous thermal envelope, eliminating more cold bridges.
EWI changes the external appearance of a building, so it may need planning permission — especially in conservation areas, on listed buildings, or where it overhangs a public footpath. In many cases, it falls under permitted development, but always check with your local planning authority. Internal insulation never needs planning permission.
It can if installed incorrectly. IWI moves the dew point (where moisture condenses) into the wall structure. Without a proper vapour barrier on the warm side, moisture from the room can penetrate into the cold wall behind the insulation and condense, leading to mould and damp. A continuous vapour control layer is essential with IWI.
Typically 60-100mm per insulated wall. For a 4m x 4m room with two external walls, you lose about 0.5-0.8 m² of floor area (3-5% of the room). This also affects window reveals (making them deeper), skirting boards, radiator pipes, sockets and switches — all of which need repositioning.
With EWI, yes — all work is external. You may lose use of a garden area for scaffolding but internal disruption is minimal. With IWI, you need to clear each room completely. Sockets, switches, radiators and skirting must be removed and repositioned. Most people work room-by-room to maintain livability.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.