What Is Solid Wall Insulation?
Solid wall insulation adds a layer of insulating material to walls that have no cavity — the single-skin brick, stone, or concrete walls found in around 30% of UK homes. Most properties built before the 1920s have solid walls, and without insulation they lose twice as much heat as cavity walls. That makes solid wall insulation one of the most impactful energy efficiency improvements you can make.
There are two methods: internal wall insulation (fitted inside) and external wall insulation (fitted outside). Both are covered by the ECO4 grant scheme, which means qualifying households can have either type installed completely free of charge.
Key fact: An uninsulated solid-walled home can lose up to 45% of its heat through the walls. Insulating them can save between £225 and £475 per year on energy bills, depending on property size and fuel type.
Why Solid Walls Matter
If your home was built before the 1920s, it almost certainly has solid walls. Many homes built up to the 1930s and even some post-war properties also have them. You can check by measuring the width of an external wall at a window or door opening — solid walls are typically 23cm (9 inches) or less, while cavity walls are usually wider.
The problem with solid walls is simple: they have no air gap to slow heat transfer. A solid brick wall conducts heat roughly twice as fast as a cavity wall, meaning your boiler works harder and your bills are higher. In older stone-built homes, heat loss can be even greater.
| Feature | Internal Insulation | External Insulation |
|---|---|---|
| Cost without grant | £4,000 – £13,000 | £6,000 – £15,000 |
| Annual savings | £225 – £400/year | £275 – £475/year |
| Room size impact | Loses 50-100mm per wall | No interior impact |
| Exterior appearance | No change | New rendered/clad finish |
| Installation time | 1-2 weeks per room | 2-4 weeks (whole house) |
| Listed buildings | Usually suitable | Often restricted |
| Disruption | Room-by-room, moderate | Scaffolding, minimal inside |
Internal Solid Wall Insulation
Internal wall insulation (IWI) involves adding insulation to the inside face of your external walls. There are two main approaches:
- Insulation boards — rigid insulation panels (typically PIR or phenolic foam) bonded directly to the wall with adhesive and mechanical fixings. This is the thinner option, losing only around 50mm of room depth.
- Stud wall with insulation — a timber or metal frame is built against the wall and filled with mineral wool or rigid insulation, then covered with plasterboard. This is more versatile for uneven walls but takes up more space (around 80-100mm).
Internal insulation is generally cheaper and doesn't require planning permission, making it ideal for terraced houses, flats, listed buildings, and properties where the external appearance can't be altered. The main drawback is losing a small amount of floor space in each room, and you'll need to reposition radiators, light switches, and plug sockets.
Installation Process — Internal
- Survey and assessment — an installer surveys your walls, checks for damp, and measures up
- Preparation — furniture is moved away from walls, carpets may be lifted at edges
- Damp treatment — any existing damp issues are resolved before insulation goes on
- Insulation fitting — boards are bonded/fixed to walls, or stud frame is built and filled
- Plasterboard and finishing — new plasterboard is fitted and skimmed for a smooth finish
- Reinstatement — sockets, switches, radiators, and skirting boards are repositioned
External Solid Wall Insulation
External wall insulation (EWI) involves fixing rigid insulation boards to the outside of your walls, then covering them with a protective render or cladding system. The result is a fresh, weather-resistant exterior finish and a warmer, more comfortable home inside.
- Expanded polystyrene (EPS) — the most common choice, lightweight and cost-effective
- Mineral wool boards — fire-resistant and breathable, good for older buildings
- Phenolic or PIR boards — thinner for the same insulation value, but more expensive
External insulation is more effective overall because it wraps the entire building envelope, eliminates cold bridges, and protects the existing brickwork from weathering. It doesn't reduce room sizes and causes minimal disruption inside. However, it changes the exterior appearance (which may need planning permission) and costs more than internal insulation.
Installation Process — External
- Scaffolding erected — scaffolding is put up around the property
- Wall preparation — existing render or loose material is removed, walls are cleaned
- Insulation boards fixed — rigid boards are mechanically fixed and adhesive-bonded to the walls
- Reinforcement mesh — a fibreglass mesh is embedded in a base coat over the insulation
- Render or cladding applied — a coloured render or decorative cladding is applied as the final finish
- Details finished — window sills, reveals, and roof edges are sealed and trimmed
- Scaffolding removed — final inspection and scaffolding comes down
Planning permission: External solid wall insulation doesn't usually require planning permission unless your property is in a conservation area, is a listed building, or the insulation extends beyond certain limits. Your installer will advise, and ECO4-funded work must comply with all building regulations.
ECO4 — Free Solid Wall Insulation
The Energy Company Obligation (ECO4) scheme runs until March 2026 and funds both internal and external solid wall insulation for qualifying households. ECO4 is the primary route to free solid wall insulation in the UK — the grant covers the full cost of materials, labour, and any necessary building work.
Who Qualifies for ECO4?
You qualify for free solid wall insulation through ECO4 if you receive any of the following benefits:
- Universal Credit
- Pension Credit (Guarantee Credit or Savings Credit)
- Income Support
- Income-based Jobseeker's Allowance (JSA)
- Income-related Employment and Support Allowance (ESA)
- Child Tax Credit (income under £16,385)
- Working Tax Credit
- Housing Benefit
- PIP, DLA, or Attendance Allowance alongside a means-tested benefit
Your home must also have an EPC rating of D or below and have solid walls. Most pre-1920s properties meet both criteria. Check your eligibility now.
GBIS — Another Route to Free Insulation
The Great British Insulation Scheme (GBIS) offers a second pathway to free solid wall insulation. If your home is in council tax bands A-D, you may qualify regardless of your income or benefits. GBIS focuses on homes with the worst energy performance, and solid-walled properties often qualify because of their poor insulation levels.
How Much Can You Save?
Solid wall insulation delivers some of the biggest energy savings of any home improvement. According to the Energy Saving Trust, typical annual savings are:
- Detached house: £375 – £475/year
- Semi-detached house: £255 – £350/year
- Mid-terrace house: £225 – £295/year
- Bungalow: £200 – £280/year
Over a 25-year lifespan (typical for solid wall insulation), that's £5,625 to £11,875 in total savings — and you don't pay a penny if you qualify through ECO4.