Solid walls — either solid brick, stone, or pre-1920s two-leaf construction without a cavity — account for around one third of all UK homes. They lose roughly twice as much heat per square metre as a modern cavity wall, with an uninsulated U-value typically between 1.6 and 2.1 W/m²K. Insulating them is one of the highest-impact improvements you can make, but the method you choose has lasting consequences for your property’s appearance, floor area, and moisture management.
There are two routes: applying insulation to the outside face of the wall (external wall insulation, or EWI) or fixing it to the inside face (internal wall insulation, IWI). Both can hit the Building Regs Part L target of 0.30 W/m²K or better — but neither approach is universally right.
What Makes a Wall “Solid”?
Victorian and Edwardian terraces, semi-detacheds, and earlier stone cottages are the most common candidates. You can usually confirm by measuring wall thickness at a window reveal: solid brick runs at around 220–225 mm; a cavity wall of the same period sits at 260–300 mm. A builder or energy assessor can confirm with a borescope or thermal imaging survey (typically £150–£400).
External Wall Insulation (EWI)
EWI involves mechanically fixing and bonding insulation boards — usually EPS (expanded polystyrene), mineral wool, or phenolic foam — directly to the outside of the wall, then applying a reinforced render or cladding finish over the top.
Advantages
- No loss of internal floor area
- Treats the whole wall in one pass — no cold-bridging at floor/ceiling junctions
- Protects the original wall from weathering; can extend its life by decades
- Drying potential improves (wall stays warm and dry)
Disadvantages
- Changes external appearance — often requires planning permission in conservation areas or for listed buildings
- Window and door reveals need extending; flashings and sills must be re-detailed
- Scaffolding required throughout
- Impractical on some party-wall or terrace situations without neighbour cooperation
Typical EWI costs (2026, England)
| Property type | Approx. wall area (m²) | Installed cost (EPS render system) |
|---|---|---|
| Mid-terrace, 2 bed | 55–70 | £8,500–£13,000 |
| Semi-detached, 3 bed | 90–120 | £13,000–£18,000 |
| Detached, 4 bed | 140–180 | £18,000–£26,000 |
Mineral wool and phenolic systems cost 20–35% more than EPS but offer improved fire performance and slightly better U-values per mm of thickness.
Internal Wall Insulation (IWI)
IWI fixes insulation boards or a studwork/batts system to the inside face of external walls. Common solutions include dry-lining with PIR board direct to wall (using dot-and-dab or mechanical fixing), or a timber/metal stud frame filled with mineral wool then boarded.
Advantages
- No change to external appearance — suitable for listed buildings and conservation areas
- Can be done room by room, spreading cost and disruption
- No scaffolding required
- Cheaper overall installed cost
Disadvantages
- Reduces floor area (typically 60–100 mm per wall, or 75–140 mm for stud + insulation)
- Causes significant internal disruption: all skirting, architraves, window boards, radiators, sockets, and light switches must be moved
- Cold bridges remain at floor and ceiling junctions unless carefully detailed
- Vapour control is critical — if the original masonry gets cold and wet, interstitial condensation can cause mould
Typical IWI costs (2026)
| Method | Material cost (per m²) | Installed cost (per m²) |
|---|---|---|
| PIR board, dot-and-dab | £18–£28 | £45–£70 |
| Stud + mineral wool | £12–£22 | £55–£80 |
| Woodfibre (breathable) | £28–£48 | £75–£110 |
For a 90 m² semi-detached (around 45 m² of external wall treated internally, allowing for windows and doors), total installed costs typically range from £4,000 to £13,000.
U-Value Comparison
| System | Insulation thickness | Achieved U-value (W/m²K) |
|---|---|---|
| Uninsulated solid brick | — | 1.8–2.1 |
| EWI — 90 mm EPS | 90 mm | 0.28–0.30 |
| EWI — 60 mm phenolic | 60 mm | 0.25–0.28 |
| IWI — 60 mm PIR | 60 mm | 0.28–0.32 |
| IWI — 100 mm woodfibre | 100 mm | 0.28–0.35 |
Building Regs Part L (for existing dwellings, Approved Document L1B) sets a 0.30 W/m²K target where technically and economically feasible. Both systems can meet this, though thicker insulation may be needed with IWI to compensate for cold bridging.
Grants and Funding (2026)
The Great British Insulation Scheme (GBIS) and ECO4 both cover solid wall insulation for eligible households (broadly: lower EPC ratings, lower income households, or those receiving means-tested benefits). Check the government’s Simple Energy Advice portal or use Ofgem’s eligibility checker. Grants can cover the full cost in some cases; partial contributions are common. The Warm Homes Plan (successor to ECO4, phased in 2025–2026) extends eligibility further.
Even without a grant, a well-insulated solid wall can reduce space heating demand by 30–40%, with payback periods of 15–25 years at current gas prices — shorter if you’re moving to a heat pump (which needs a low heat-loss envelope to perform efficiently).
Planning and Building Regs
EWI almost always needs a Party Wall Agreement if the works involve scaffolding over a neighbour’s land or affect a shared wall. It may require planning permission in Article 4 direction areas, conservation areas, or for listed buildings — check with your local planning authority before proceeding.
Building Regs approval is required for solid wall insulation (it is a notifiable alteration under Part L). Your installer should submit a Building Notice or Full Plans application; many specialist EWI and IWI contractors are registered with a competent person scheme (such as CIGA or TrustMark) that allows self-certification.
Which Should You Choose?
Choose EWI if: your home is a detached or semi-detached in a non-restricted area, you can tolerate the external change in appearance, and you want the best thermal performance with minimum internal disruption.
Choose IWI if: your home is listed, in a conservation area, a mid-terrace where only party walls are exposed, or you need to phase works room by room. Use a vapour-open system (woodfibre or breathable mineral wool with a smart membrane) if the original wall is stone or lime-pointed brick that needs to breathe.
In either case, get a hygrothermal assessment (WUFI modelling or equivalent) if there is any doubt about moisture risk, and always use a specialist contractor with experience of the wall type and local climate.
