External wall insulation (EWI) and internal wall insulation (IWI) solve the same problem — heat escaping through solid walls — in opposite ways. EWI wraps the outside of the house; IWI lines the inside. Choosing the wrong one wastes money and, with IWI, can cause moisture problems. This guide compares them directly so you can see which fits your property.
Quick verdict
- Choose EWI if you have a solid-wall home outside a conservation area and want the biggest thermal gain in one project, with no loss of internal space.
- Choose IWI if you own a listed building, a conservation-area home, or a flat where you can't change the exterior — or if you want to insulate one room at a time.
- Sometimes both: EWI on the rear and sides, IWI on a protected front elevation.
Not sure which applies? A free survey settles it in one visit by checking your wall construction and any planning restrictions.
EWI vs IWI: side-by-side
| Factor | External Wall Insulation (EWI) | Internal Wall Insulation (IWI) |
|---|---|---|
| Typical cost | £90–£140/m² · £12,000–£20,000 for a 3-bed semi | £50–£90/m² · £8,000–£15,000 for a full 3-bed |
| Thermal performance | Highest — cuts wall heat loss ~70%, no cold bridging | Very good, but cold bridges at floors/party walls are harder to treat |
| Internal space | None lost — insulation is outside | Loses ~50–100mm off each external wall |
| Appearance | Changes the exterior (new render finish) | No external change — frontage preserved |
| Planning | Restricted in conservation areas / listed frontages | Almost always permitted — ideal for heritage homes |
| Disruption | External works + scaffolding; home stays liveable | Room-by-room internal works; redecoration needed |
| Can be phased? | Usually done in one project | Yes — easy to do one room at a time |
| Moisture risk | Low — keeps the wall warm and dry | Needs a vapour-open build-up on older walls to avoid trapping damp |
| Best for | Solid-wall semis & terraces outside conservation areas | Listed / conservation homes, flats, phased projects |
How they actually differ
Both measures target solid walls — the 9-inch brickwork found in most pre-1930s housing, which has no cavity to fill and can lose 30–40% of a home's heat. The difference is which side of the wall the insulation sits on.
EWI fixes insulating board to the outside and finishes it with a weatherproof render. Because the masonry stays inside the warm envelope, it also protects the brickwork and removes the cold bridges that cause damp patches. The trade-off is purely visual — and regulatory, in protected areas.
IWI fixes insulated board or a breathable system to the inside of external walls, room by room. It's the only practical option where the outside can't be touched, but it leaves the masonry cold, which is why the moisture detail has to be right.
When to choose EWI
- Solid-wall home (typically pre-1930s) outside a conservation area.
- You want the maximum thermal gain in a single project.
- Your render or pebbledash is tired and due for refurbishment anyway.
- You don't want to lose any internal floor space or redecorate inside.
Read the full external wall insulation guide or see our EWI service and costs.
When to choose IWI
- Listed building or conservation-area home where EWI isn't permitted.
- A flat or maisonette where you don't control the whole façade.
- You want to spread the cost and insulate the coldest rooms first.
- The brick frontage must be preserved for its appearance.
Read the full internal wall insulation guide or see our IWI service and costs.
Cost compared
IWI is cheaper per square metre because it avoids scaffolding and render, but the gap narrows once you account for redecoration and the fact that EWI treats the whole house at once. As a rule of thumb for London and the South-East:
- EWI: £90–£140/m², or about £12,000–£20,000 for a typical 3-bed semi.
- IWI: £50–£90/m², or about £8,000–£15,000 for a full 3-bed house — and a single room from £1,500 if you phase it.
Every quote we give is free, fixed and itemised after a survey, so you can compare the two options for your own property on like-for-like terms.
EWI vs IWI: FAQs
Is external or internal wall insulation better?
Neither is universally better — it depends on the property. External wall insulation (EWI) gives the best thermal performance and loses no internal space, but changes the look of the building, so it suits solid-wall homes outside conservation areas. Internal wall insulation (IWI) is the right choice for listed buildings, conservation-area homes and flats where the exterior can't be altered.
Which is cheaper, EWI or IWI?
Per square metre, IWI is usually cheaper (around £50–£90/m² versus £90–£140/m² for EWI), because it avoids scaffolding and render. But EWI treats the whole house in one go, while IWI is often phased room by room. For a full 3-bed house, IWI typically runs £8,000–£15,000 and EWI £12,000–£20,000.
Does internal wall insulation cause damp?
Only when the wrong system is used or the detailing is poor. Older solid walls need a vapour-open build-up (such as breathable wood fibre) so moisture isn't trapped behind the insulation. Correct specification and PAS2030-certified installation matter more with IWI than with any other measure.
Do I lose room space with wall insulation?
With EWI, no — the insulation goes on the outside. With IWI you lose roughly 50–100mm off each external wall, which is a small reduction in floor area in the affected rooms only.
Can I have EWI on a conservation-area home?
Usually not on elevations visible from the street, because EWI changes the external appearance. Some homeowners apply EWI to rear and side elevations and IWI to the protected frontage. We check your address against local planning records during the free survey.