External wall insulation is insulation board fixed and bonded to the outside of the wall and then rendered over. Our published guide price is £90–£140 per m², against £40–£70 for standard render — the gap being the insulation and the extra detailing it brings. On a solid wall it is the single measure that most changes heat loss, and the insulant must be vapour-open.
- Thickness is the complication: reveals, sills, roofline and every external fitting have to be reworked.
- On a solid wall, use a vapour-open insulant such as mineral wool rather than standard EPS.
- The base detail above the damp-proof course is where a badly done EWI job causes damp.
- Funded work falls under PAS 2035, which brings a required assessment and design process.
What EWI actually is
External wall insulation is a system, not a product. Insulation board is mechanically fixed and bonded to the outside face of the wall, a reinforced basecoat with mesh goes over it, and a topcoat — usually a through-coloured silicone render — finishes it.
You end up with a rendered-looking house, which is why it is often discussed as a rendering job. It is really an insulation job with a render skin, and the differences matter: it adds 100mm or more to the wall, it changes how the wall handles moisture, and it needs detailing at every junction that standard render does not.
The target is solid-walled property. A solid brick wall loses substantially more heat than a modern insulated one and has no cavity to fill, so insulating the outside is the main option that does not cost you internal floor area.
Our published guide prices
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Rendering | £40–£70 per m² | System & access dependent |
| Three-bed semi | £2,500–£5,000 | Full elevation render |
| Detached house | £9,000–£12,000 | Larger area & scaffolding |
| External wall insulation | £90–£140 per m² | Insulated render system |
The gap between £40–£70 per m² for rendering and £90–£140 for insulated render is the insulation plus the additional detailing. If you were going to re-render anyway, that gap is the real decision rather than the full EWI figure.
The insulant choice, and why it is not a detail
Two families dominate, and on a solid wall they behave very differently.
Expanded polystyrene (EPS) is the cheaper and more common option. It performs well thermally and is light and easy to work. It is also relatively vapour-closed, which is fine on a cavity wall and a genuine risk on a solid one.
Mineral wool costs more and is heavier to install. It is considerably more vapour-open, and it is non-combustible, which matters on some buildings.
The reason this matters: a solid wall absorbs driving rain and must release that moisture back out through its surface. Wrap it in a vapour-closed layer and the moisture has nowhere to go, so it accumulates in the masonry. The symptoms are the same as any trapped-moisture problem — internal damp, and eventually the system itself failing from behind.
So on a solid-walled Coventry terrace, a mineral wool system is usually the right specification even though it costs more. If a quote proposes EPS on a solid wall, ask how the wall is expected to dry.
Everything the thickness changes
Adding 100mm or more to every external wall has consequences at every junction, and these are where a cheap EWI job goes wrong.
- Window and door reveals become deeper and need insulating and finishing, or they become cold bridges.
- Sills need extending so they still project beyond the new wall face and throw water clear.
- The roofline may need extending. If the eaves overhang is shallow, the new wall face can end up outside the protection it offers.
- Rainwater goods come off and go back on extended brackets, clear of the new surface.
- The boiler flue must be extended to project correctly — work for a Gas Safe registered engineer, not the rendering team.
- Meter boxes have to be reset flush, not buried.
- Airbricks must be sleeved through. Covering one blocks the ventilation to a suspended floor void.
- The base detail needs a starter track stopping above the damp-proof course with a drip, or the system bridges the DPC.
That list is why EWI quotes vary so much. A quote that mentions none of it has not thought about the job.
What it actually saves
Honest framing matters here, because EWI is often sold on savings figures that assume a worst-case starting point.
The saving depends almost entirely on what you are starting from. A solid-walled, exposed, poorly insulated house with high heating use will see a substantial reduction. A 1990s cavity-walled house with filled cavities and a well-insulated loft will see very little, because the walls were not where the heat was going.
The simple payback on wall insulation alone is long — longer than it is usually presented. What changes the arithmetic is that most people doing EWI also want a new exterior finish. In that case the comparison is not EWI against nothing, but EWI against the cost of re-rendering, and that gap pays back far more quickly.
The benefits people actually report are comfort rather than bills: rooms that warm up faster and hold heat, fewer cold spots, less condensation and mould on external walls, and a quieter house. Those are real and they are hard to put a figure on.
Grants, funding and PAS 2035
Government-backed funding for insulation measures exists, typically targeted at lower-income households, properties with poor EPC ratings, or particular benefit recipients. Schemes change regularly, eligibility criteria shift, and we would not want you to budget on the basis of a scheme described in an article.
What is stable enough to be useful is the framework around funded work. Where EWI is installed through a government-backed scheme, PAS 2035 applies. It requires a retrofit assessment of the property, a designated retrofit coordinator, a design stage, and a defined approach to ventilation and moisture risk.
That last point is the substantive one. PAS 2035 exists partly because earlier insulation schemes produced damp and condensation problems by improving airtightness and insulation without addressing ventilation. The assessment process is there to stop that recurring.
If you are considering funded work, check the current schemes directly through the Government's own energy grants information rather than relying on a contractor's claim about what you qualify for. Unsolicited calls offering free insulation are a well-established pattern worth treating carefully.
Planning and neighbours
EWI changes the external appearance and moves the building line outwards, so permission is a real question rather than a formality.
For most houses it is permitted development. It is not for flats and maisonettes, and listed buildings, conservation areas and Article 4 directions all restrict it. Check the property status before anything is ordered.
There are two practical points beyond planning. On a semi or a terrace, insulating one house and not its neighbour leaves a visible step at the boundary, and the junction needs thinking about. And if the wall is on or near a boundary, the new thickness may encroach — worth establishing early rather than once the boards are going on.
We establish whether the wall is solid or cavity, specify a vapour-open system where it needs one, and price the reveals, sills and roofline work rather than discovering them.
Frequently asked questions
How much does external wall insulation cost?
Our published guide figure is £90–£140 per m², against £40–£70 for standard rendering. The difference is the insulation itself plus the additional detailing — reveals, sills, roofline, rainwater goods, flue and meter boxes all have to be reworked for the extra thickness. Every job is surveyed and quoted in writing.
Is external wall insulation worth it?
It depends on what you are starting from. On a solid-walled, exposed house with high heating use, yes — it is the measure that most reduces heat loss through the walls. On a modern cavity-walled house with filled cavities, the walls were not where the heat was going. It is most compelling when you wanted a new exterior finish anyway.
Will EWI cause damp?
Badly specified, it can; well specified, it usually improves things. The two risks are a vapour-closed insulant on a solid wall that needs to dry outwards, and bridging the damp-proof course at the base. Done correctly — mineral wool on a solid wall, a starter track above the DPC with a drip — the inside face of the wall is warmer, which reduces condensation.
Do I need planning permission for external wall insulation?
For most houses it is permitted development, but it changes the external appearance and the building line so the exceptions matter. Flats and maisonettes are excluded, and listed buildings, conservation areas and Article 4 directions all need checking. Where the wall is near a boundary, encroachment is worth establishing early too.
What is PAS 2035 and does it apply to me?
PAS 2035 is the standard governing retrofit of existing dwellings, and it applies where work is installed through a government-backed funding scheme. It requires a retrofit assessment, a coordinator, a design stage and a defined approach to ventilation and moisture risk. It exists because earlier schemes caused damp problems by insulating without ventilating.
Can I get a grant for external wall insulation?
Schemes exist and are typically targeted at lower-income households, poor EPC ratings or particular benefits, but they change often and eligibility shifts. Check the current position through the Government's own energy grants information rather than relying on a contractor's claim. Be cautious with unsolicited calls offering free insulation.
Insulation first, render second
We establish the wall construction, specify an insulant the wall can live with, and price the reveals, sills and roofline properly. Free survey, fixed written quote.