If your gas bill has been creeping upwards and the living room still feels draughty with the heating on full, the walls are almost always the main culprit. A typical UK home loses about 35% of its heat through uninsulated walls — more than the roof, more than the windows. There are two routes that genuinely move the needle on that loss: cavity wall insulation for homes built from 1930 onwards, and external wall insulation for older solid-wall properties. They work on different houses, cost wildly different amounts, and come with very different grant profiles.
The rest of this page is the straight decision guide — which route fits your house, what it costs, what grants pay, and what most homeowners get wrong. Over a cup of tea on a survey visit, it's a 15-minute conversation. Written down, it takes this page.
The quick-pick: which does my house need?
Homes built from 1930 onwards typically have a cavity — two skins of brick (or brick + blockwork) with a 50–100mm air gap between them. That gap is what cavity wall insulation fills. Older homes — most pre-1930 builds and almost all listed or conservation-area properties — have solid walls, no cavity, and need external (or internal) wall insulation. A 30-second survey of the wall thickness usually settles which camp you are in.
- Wall thickness roughly 250–300mm with a consistent brick pattern? Likely a cavity — CWI is the answer.
- Wall thickness under 240mm, stretcher-and-header brick pattern, or stone? Solid wall — EWI is the answer.
- Timber-frame or modern brick-and-block new-build? Often already insulated, but we still survey.
Cavity wall insulation — cost, saving, payback
Fitting cavity wall insulation on a 3-bed semi in Essex, Suffolk, Kent or Cambridgeshire takes one OMEGA crew half a day. The installer drills a grid of 22mm holes in the mortar joints at roughly metre spacing, injects bonded glass-mineral wool (our default is Knauf Earthwool DriTherm 32) or graphite-enhanced EPS beads, and fills the holes with matching mortar. The whole job is invisible from inside and externally by the time the crew leaves.
Typical fitted price on a 3-bed semi in 2026 sits around £1,450 if you are self-funding. Under ECO4 for eligible households it is £0. Under GBIS (broader eligibility) you typically pay £100–£400 depending on council tax band and property type. The fabric payback on a self-funded install is around 3.1 years against current gas prices — faster than solar, faster than a heat pump, faster than almost anything else you could spend on the house.
External wall insulation — the full-fabric upgrade
EWI is a different order of magnitude. On a 3-bed solid-wall home it is typically £12,800 self-funded, or £0–£3,000 out-of-pocket under ECO4. The system stacks 90–120mm of EPS or mineral-wool slab onto the outside of the walls, renders or clads over it, and rebuilds the window and door reveals. The U-value drops from roughly 2.1 (solid brick) to 0.30 — a 7x improvement. Annual gas savings on a heated-3-bed come in at £740 in 2026 tariffs. More importantly, the walls stay warm — so condensation, mould, and cold-room-corner complaints disappear.
EWI takes 2–3 weeks on site. Scaffolding goes up, windows and pipes are packed out, boards are mechanically fixed and glued, basecoat + mesh + topcoat is applied, and everything is made good. The finished house usually looks better than the starting one — most customers pick a silicone-textured render that is a large visual step up from tired pebbledash or painted brick.
Side-by-side: the commercial case
| Cavity wall insulation | External wall insulation | |
|---|---|---|
| Suits | 1930+ cavity homes | Pre-1930 solid-wall homes |
| Install time | Half a day | 2–3 weeks |
| Disruption | None — external only | Scaffolding, 10–15 days |
| Self-funded cost | ~£1,450 | ~£12,800 |
| ECO4 cost to you | £0 | £0–£3,000 |
| GBIS cost to you | £100–£400 | Not typically covered |
| Annual saving | £395 | £740 |
| Self-funded payback | ~3.1 yrs | ~12 yrs |
| U-value reduction | 1.6 → 0.55 | 2.1 → 0.30 |
| Bonus outcomes | Cuts draughts | Kills damp, modernises facade |
| Lifespan | 25+ years | 30+ years |
What most customers get wrong
Assuming you cannot get CWI because the house is "too old"
Anything built after about 1930 usually has a cavity. We have insulated 1950s semis, 1960s maisonettes and 1970s executive homes in Chelmsford, Ipswich and Canterbury. On survey we borescope the cavity through a test hole to confirm it is open and uninsulated — no guesswork. If you are unsure, book a free survey and we tell you what you actually have.
Treating EWI as just an energy upgrade
It is also a cosmetic and moisture upgrade. Homeowners looking at an EWI quote in isolation often see only the £12k number. The better frame: you were probably going to repaint or render the house anyway in the next 10 years for £4–6k. EWI includes that work — so the real incremental cost vs maintenance is closer to £7k for a 30-year fabric transformation. Plenty of our EWI customers cite "the house looks brand new" as a bigger motivator than the gas saving.
Skipping the retrofit assessment
A PAS2035 retrofit assessment is mandatory on ECO4, and genuinely useful outside it. It models how the whole fabric performs, flags moisture and ventilation risks, and sequences the measures correctly. Skipping it is the fastest way to install EWI on a house that then develops condensation in the kitchen. Our assessment is included on any ECO4 or mixed funding route.
What OMEGA actually installs
- Knauf Earthwool DriTherm 32 blown mineral wool (cavity wall) — BBA 08/4569, 25-year product warranty.
- Graphite-enhanced EPS beads with bonding agent for hard-to-treat cavities.
- Wetherby Silicone-enhanced EWI system — EPS core, alkali-resistant mesh, silicone top coat (BBA 15/5271).
- Mineral-wool slab EWI for fire-regulated flats and listed-adjacent properties.
- ProPERLA thermal-insulating facade paint where planning restricts render (Grade II conservation areas).
Every install includes post-completion thermography to verify the result, TrustMark and PAS2030 paperwork, a 25-year product warranty, and our 5-year installation guarantee.
Which service areas do we cover?
OMEGA fits CWI and EWI across Essex, Suffolk, Kent and Cambridgeshire. In practice that includes Chelmsford, Colchester, Basildon, Southend-on-Sea, Brentwood, Braintree, Ipswich, Bury St Edmunds, Canterbury, Maidstone, Dover, Cambridge, Peterborough and every village in between. If your postcode is in those counties, we are a local installer — not a national brand subcontracting the work.
Cavity wall and external wall insulation FAQs
- Can I do cavity wall insulation myself?
- No — the equipment needed to inject bonded mineral wool or EPS beads at the correct density requires specialist kit and a certified installer. DIY attempts typically leave voids and fail the warranty. The professional route is also the only one eligible for ECO4 / GBIS funding.
- Will external wall insulation cause damp?
- If the home is surveyed properly and the EWI is specified as a full-fabric system with correct ventilation, EWI usually removes damp rather than causing it. The PAS2035 retrofit assessment is the safeguard — it identifies ventilation upgrades needed to keep moisture moving. Skipping that assessment is what causes damp problems.
- How long does cavity wall insulation last?
- Bonded mineral wool and graphite EPS beads installed in an open, dry cavity have a working life of 25+ years — matching the BBA product certification. All our installs come with a 25-year GDG guarantee and our 5-year installation cover.
- Does external wall insulation need planning permission?
- For most English homes, EWI is permitted development and does not need planning permission. Exceptions: listed buildings, most of a conservation area, and article 4 directions. We handle the paperwork either way — our surveyor tells you on the first visit whether you need consent.
- Can I get both cavity and external wall insulation?
- Not on the same wall — they target the same heat loss. But on mixed-fabric homes (e.g. a solid-wall front and a cavity rear extension) we spec each wall to its own optimal measure. This is more common than most homeowners realise.
- Is it worth doing if I already have partial insulation?
- Often yes. Cavities injected in the 1980s with older-generation materials may have slumped or voided. We borescope to check — if there are gaps, we re-fill under warranty. Similarly, 1990s external render without a proper insulation core underneath is effectively decorative; upgrading to a full EWI system delivers the heat-loss benefit for the first time.
Book a free survey — zero pressure
Book a free home energy survey online or ring 0800 229 4094. One of our own surveyors — not a salesperson — spends 45 minutes at the property, borescopes the walls where needed, models the savings against your real gas bills, and tells you honestly whether cavity wall, external wall, or nothing is the right answer for this house. No cost. No obligation. No pressure.
Want to find out how much you could save?
Book a free 45-minute survey. We'll check every active grant, measure your home, and give you a written quote — with no obligation to proceed.
No cost. No obligation. No pressure.