Key takeaways
- Cavity wall insulation costs £600–£1,200 for a typical 3-bed semi; external wall insulation (EWI) costs £12,000–£25,000.
- Cavity wall insulation only works on cavity-walled homes (typically post-1920); solid-wall homes need EWI or internal wall insulation (IWI).
- Cavity wall insulation cuts wall U-value from ~1.5 to ~0.5 W/m²K; EWI on a solid wall takes you from ~2.1 to ~0.30 W/m²K.
- EWI is rarely worth the premium just for energy savings — payback is 25–40 years on energy alone. It pays back faster when you'd be doing render or external repair work anyway.
- Planning permission isn't usually needed for EWI on a non-listed, non-conservation-area home, but check your specific case before quoting.
- Internal wall insulation (IWI) is the third option for solid-wall homes — cheaper than EWI but eats internal floor area and risks condensation if done badly.
Wall insulation is the single biggest fabric upgrade most UK homes need. Walls account for roughly 30–35% of heat loss in an uninsulated house, more than the roof or windows. But the right approach depends entirely on whether your house has a cavity, and that one fact splits the entire decision tree.
This article covers four approaches: cavity wall insulation (cheap, fast, only works on cavity walls), external wall insulation (expensive, transformative, works on solid walls), internal wall insulation (the in-between option), and the no-action case. We'll cover when each is worth it and when it isn't, with installed pricing as of 2026.
Quick wall-type check
Houses built before about 1920 are typically solid wall — a single 9-inch (225mm) brick layer with no air gap. From the 1920s onwards most UK homes were built with cavity walls — two leaves of brick or block separated by a 50–100mm air gap, originally for damp resistance.
You can tell from the brick pattern: solid walls show a mix of header bricks (short ends) and stretchers (long sides) in regular patterns. Cavity walls show all stretchers because the headers don't span both leaves. Wall thickness is the other tell: under 270mm at a window reveal usually means solid; over 270mm usually means cavity.
If you're not sure, the EPC for your property will state the wall construction in the constructional summary. Or any installer will check during a survey before quoting.
One trap: some 1920s and 1930s homes have what's called a "narrow cavity" of 30–40mm — too narrow to take blown insulation effectively. These houses sit awkwardly between cavity-walled and solid-walled and need careful surveying. Insulating a too-narrow cavity can cause damp problems if the insulation bridges moisture across the cavity. A reputable installer will measure cavity width before quoting and decline the job if it's under 50mm.
Cost comparison
| System | Cost (3-bed semi) | Cost per m² | Disruption |
|---|---|---|---|
| Cavity wall insulation (mineral fibre or EPS bead) | £600–£1,200 | £8–£15 | Low — 1 day, drilling small holes |
| External wall insulation (EWI, 90mm EPS + render) | £12,000–£18,000 | £100–£140 | High — 2–4 weeks, scaffolding |
| External wall insulation (EWI, 100mm mineral wool + brick slip) | £18,000–£25,000 | £140–£200 | High — 3–6 weeks, scaffolding |
| Internal wall insulation (IWI, 60mm PIR + plasterboard) | £6,000–£12,000 | £70–£120 | Medium-high — disrupts each room |
The 15-to-1 cost ratio between cavity and EWI is the single most important number in this comparison. Cavity wall insulation is one of the highest-return energy retrofits available — payback is typically 3–5 years, sometimes faster. EWI's payback on energy alone is 25–40 years, which is essentially saying "this isn't really an energy-economics decision."
Government grants close some of the gap. ECO4 (Energy Company Obligation) provides funding for low-income or vulnerable households on solid wall insulation, sometimes covering 100% of EWI cost. Local authority schemes vary and come and go. If you're eligible — typically means-tested or in receipt of certain benefits — these schemes change the calculation entirely. Outside the eligible group, you're paying market rate.
U-value uplift comparison
U-value is how much heat passes through a wall per square metre per degree of temperature difference. Lower is better. Building Regulations Part L now requires new walls at 0.18 W/m²K, but retrofit gets you to "as good as practical" rather than new-build standard.
| Wall type | Before | After | % reduction |
|---|---|---|---|
| Cavity wall, uninsulated | ~1.50 W/m²K | ~0.50 W/m²K (with insulation) | 67% |
| Solid wall, no treatment | ~2.10 W/m²K | ~0.30 W/m²K (with EWI 90mm) | 86% |
| Solid wall, no treatment | ~2.10 W/m²K | ~0.35 W/m²K (with IWI 60mm) | 83% |
| Pre-1990 cavity (partial fill) | ~0.70 W/m²K | ~0.40 W/m²K (with full fill) | 43% |
EWI on a solid-wall home delivers a bigger absolute U-value improvement than cavity insulation — you're starting from a worse position so there's more to fix. But the cost-per-watt-saved is dramatically lower for cavity work.
Quick maths to sanity-check this: a 100m² wall area at U=1.5 loses roughly 1,500W per 10°C delta-T. After cavity insulation at U=0.5, it loses 500W — a 1,000W reduction. The same 100m² wall area on a solid-wall home goes from 2,100W to 300W — an 1,800W reduction. The EWI delivers nearly twice the energy saving in absolute terms, but at 17x the cost. That's the core arithmetic.
When EWI is genuinely worth the premium
EWI's £15,000–£25,000 outlay rarely pays back through energy savings alone. The cases where it does make economic sense are:
- You need render replacement anyway. If your existing render is failing and you'd be paying £6,000–£10,000 to strip and re-render, the marginal cost of insulation under the new render is much smaller — often £6,000–£12,000 instead of £15,000.
- You're doing a major refurb. Adding scaffolding to an existing project is cheaper than mobilising it from scratch. EWI as part of a wider renovation project lands closer to £80–£100/m² installed.
- You're targeting EPC A or net-zero. If you're going for low operational carbon as a strategic goal, EWI is the only realistic path on a solid-wall home short of demolition.
- Cold-bridge elimination matters to you. EWI wraps the entire fabric including window reveals and lintels. IWI doesn't — cold bridges remain at floor and ceiling junctions.
- You'll stay 15+ years. Long tenure justifies the long payback, particularly if you value comfort uplift (which EWI delivers strongly).
If none of those apply, the right answer for a solid-wall home is often "live with it for now, do other upgrades first, and revisit when the render needs work."
The comfort uplift from EWI is the most consistent feedback we hear from people who've done it. Solid-wall homes have cold internal surfaces that pull radiant heat from your body even when air temperatures are warm. EWI eliminates this. Many homeowners report turning their thermostats down 1–2°C after EWI without feeling colder — which closes some of the energy-economics gap because the saving against the actual lived experience is bigger than the modelling suggests.
Internal wall insulation: the in-between option
IWI is roughly half the cost of EWI but introduces real complications. You lose 60–80mm of internal floor area along every external wall. You have to redecorate every room you treat. And condensation risk is higher — moisture from internal living can hit the now-cold internal face of the masonry and condense, leading to mould.
The most reliable IWI builds use a vapour-permeable insulation like wood fibre with proper hygrothermal modelling, not the cheap PIR-and-plasterboard approach that some installers still push. Done well, IWI works. Done badly, it's a long-term mould risk.
One IWI strategy that often makes sense: do it room-by-room rather than whole-house. Treat one bedroom this year, the lounge next year, and so on. This spreads the cost and disruption across multiple years and lets you live with each completed room before committing further. The PAS 2035 retrofit standard explicitly supports phased programmes if a coordinator is involved — this gives you access to better grants in some schemes too.
Planning permission and conservation
For most UK homes, EWI does not need full planning permission — it falls under permitted development as long as the materials and finish are similar in appearance to existing. Exceptions:
- Listed buildings — yes, listed building consent required
- Conservation areas — likely planning application, sometimes refused for façade change
- Article 4 Direction areas (some terraced streets) — full planning required
- Front elevation of a house facing a highway — varies by authority
Always check with your local planning authority before getting an EWI quote, because a refusal mid-project is expensive.
Disruption you should plan for
Cavity: One day, two installers, drill holes from outside, blow in insulation, plug the holes. Most of your day is uninterrupted. The dust and noise are minimal.
EWI: Scaffolding for 2–6 weeks. Front and back of your house become a building site. Window cills extend, drainpipes get re-routed, satellite dishes come down and go back up. Boundary fences may need temporary removal. Your neighbours will know.
IWI: Each treated room becomes uninhabitable for 1–2 weeks. Furniture moved, sockets re-fitted, skirting boards replaced. Decoration costs sit on top of the headline IWI cost.
FAQ
Will cavity insulation cause damp?
If your existing cavity is exposed to driving rain and the insulation creates a bridge to the inner leaf, yes it can. Pre-installation surveys check for rainfall exposure, render condition, and any existing damp issues. About 5–10% of homes are unsuitable for full-fill cavity insulation. A reputable installer will refuse the job rather than risk it.
Can I install EWI in stages, e.g. just the back?
You can, but the cost-per-m² climbs because you can't amortise the scaffolding and mobilisation across the full elevation. Practically, most projects do all four sides at once.
What about timber-frame houses?
Timber-frame homes have insulation in the frame itself; you don't add cavity or EWI in the same way. Upgrading involves opening up sections from inside or out, which is a different specification entirely.
Is graphite EPS better than standard EPS?
Yes — about 15–20% better thermal performance for the same thickness, hence why most modern EWI specifications use it. Cost premium is small (5–10%).
Will EWI change how my house looks?
Yes, noticeably. Walls become 100–150mm thicker, window reveals deepen, brickwork is hidden behind render or brick slip. If your house has architectural details (string courses, brick patterning, window trims), EWI either covers them or requires expensive replication. This is one reason heritage homes often refuse EWI.
How long does each system last?
Cavity insulation: 25+ years if installed correctly and the cavity stays dry. EWI: 25–35 years for the insulation, with render or brick slip needing periodic maintenance. IWI: 25+ years if the wall stays dry on the cold side.
What this means for your decision
If you have a cavity wall and it's not yet insulated, do it. The economics are unambiguous and it's the highest-return retrofit available. If you have a solid wall, the answer is more nuanced — EWI is the right call when you're already doing render work or a deep retrofit, IWI is workable for one or two rooms with proper detailing, and accepting the wall as-is for now while doing other upgrades is sometimes the most rational choice.
Get a personalised payback estimate with the insulation payback calculator on EcoSavingHub — it factors in your wall type, floor area, postcode, and current heating fuel.
Want to model your specific home? Use our free ROI calculators →