Guide
External vs internal wall insulation
External wall insulation usually performs better, keeps room sizes and renews the outside, but changes the look and needs scaffolding. Internal insulation keeps the façade, can be done room by room and costs less per m², but loses floor space and needs careful detailing to avoid condensation.
Side-by-side comparison
| External (EWI) | Internal (IWI) | |
|---|---|---|
| Guide cost | £105–£210 per m² | £75–£130 per m² |
| Room space | No loss | ≈50–100mm per wall |
| Disruption inside | Minimal | Room out of use 3–5 days |
| Cold bridges | Well treated | Harder at joists and party walls |
| Appearance | Changes outside | Outside unchanged |
| Planning | Sometimes needed | Rarely needed |
Why external usually performs better
Wrapping the outside keeps the whole wall warm, including where floors and internal walls meet it. Internal insulation leaves those junctions as cold bridges unless they're returned and detailed, which is where condensation tends to form.
When internal is the better choice
Listed buildings, conservation-area fronts, flats where the outside belongs to the freeholder, and single rooms during a renovation. Many owners combine the two: internal on the front, external on the rear and sides.
Questions
Can I do both?
Yes — a mix is common on terraced houses with attractive fronts.
Which is better for damp?
External, generally, because the wall stays warmer and drier. Internal needs careful moisture design.
Do both need building control?
In England and Wales, yes, once more than half of a wall's area is treated (or more than a quarter of the whole building envelope).