Houses built before the 1920s usually have solid walls, a single thick skin of brick or stone with no gap inside. These lose more heat than any cavity wall, which makes insulating them the largest fabric improvement an older home can make, though also the most expensive and disruptive. There are two ways to do it, internal and external, each with real trade-offs in cost, disruption and how the house ends up looking. And there is a serious pitfall with older breathable walls that can cause damp if you get the materials wrong. This guide covers the methods, the money, the grants and the cautions.
The short answer. Solid walls can be insulated from the inside, with insulated boards on the inner face of external walls, or from the outside, by wrapping the house in insulation and a new render. Internal is cheaper, around £5,000 to £15,000, but eats floor space and means redecorating. External performs better and leaves rooms untouched but costs more, often £10,000 to £25,000. Both can save £300 to £500 a year on gas heating, with payback over many years, so treat it as a long-term comfort and value investment, done after the cheaper fixes.
Why solid walls leak so much heat
A solid wall is a single layer of brick or stone, typically nine inches of brick or thicker stone, with nothing inside to slow heat escaping. A cavity wall, by contrast, has two skins with a gap that can be filled with insulation. The solid wall has no such gap, so heat passes straight through the masonry.
In numbers, an uninsulated solid wall loses heat at roughly twice the rate of an uninsulated cavity wall, and several times the rate of a well-insulated modern wall. In a typical pre-war house the walls are the single biggest route for heat to escape once the loft is done, ahead of windows and floors. That is why insulating them offers the largest single fabric saving available to an older home, and why it is worth doing properly even though it is expensive.
If your house has a cavity instead, the job is far cheaper and easier. Our cavity wall insulation guide covers that case. Solid-wall work is a different order of cost and disruption, which is why it sits at the end of the queue, not the start.
There is a comfort dimension too, not just a bill one. Solid external walls run cold to the touch in winter, and a cold wall radiates heat away from your body even when the room air is warm, so the room feels chillier than the thermostat says. Insulating the walls keeps the inner surface warm, which is why an insulated solid-wall house often feels comfortable at a lower thermostat setting, saving a little more on top of the raw heat-loss figure.
Internal versus external: two ways to do it
You can insulate a solid wall from the inside or the outside. They achieve a similar result but the experience and the cost are very different.
Internal wall insulation fixes insulated plasterboard or a battened stud layer to the inner face of each external wall. It is cheaper per room and can be done one room at a time, but it eats a little floor space, around 60mm to 100mm off each external wall, and it is disruptive inside. Skirting boards, sockets, radiators and window reveals all have to be moved or reworked, and the room needs redecorating afterwards. It is the practical choice where external changes are not allowed or the budget is tighter.
External wall insulation wraps the outside of the house in an insulating layer finished with a new render or cladding. It performs better because it covers the whole envelope without the cold bridges you get around internal fittings, it keeps the masonry warm so condensation risk drops, and it leaves every room untouched and full-size inside. The downsides are the cost, which is considerably higher, and that it changes the look of the building, which can be a problem on a period frontage or where planning rules apply.
| Approach | Rough cost | Floor space | Disruption | Performance |
|---|---|---|---|---|
| Internal (IWI) | £5,000 to £15,000 | Lost from each room | High inside, room by room | Good |
| External (EWI) | £10,000 to £25,000 | Unchanged | Low inside, scaffolding outside | Best |
Indicative whole-house costs and vary widely with house size, finish and region. Get several quotes. Worked examples.
The saving on offer
Because solid walls leak so much heat, the potential reduction in heating demand is large, often the biggest single fabric saving available to a pre-war house. A typical detached or semi-detached solid-wall home might save somewhere around £300 to £500 a year on gas heating once the walls are insulated, with terraced houses saving less because they have fewer exposed walls.
| House type | Rough yearly heating saving | Typical cost | Rough payback |
|---|---|---|---|
| Detached solid-wall house | £400 to £500 | £12,000 to £25,000 | 25 to 50 years |
| Semi-detached | £300 to £400 | £8,000 to £15,000 | 20 to 40 years |
| Mid-terrace | £150 to £300 | £5,000 to £12,000 | 20 to 40 years |
At 7.33p per kWh gas, Ofgem cap July to September 2026. Savings and costs are broad estimates and depend heavily on the existing state of the house. Worked examples; get a proper survey.
The flip side of that big saving is the upfront cost, high enough that the simple payback runs over many years rather than a handful. Solid-wall insulation is best thought of as a long-term investment in comfort, warmth and property value as much as a quick money-saver. It is also far cheaper to fold into work you are doing anyway, such as a re-render, a new extension or external repairs, since the scaffolding and disruption are already paid for. If you are moving to a heat pump, insulating the walls first lets the pump run at a lower flow temperature and cuts its running cost.
Mind the breathability
This is the pitfall that catches people, and it can cause expensive damage. Old solid walls were built to let moisture move through them and dry out, a property called breathability. They handle damp by absorbing it and releasing it again rather than sealing it out.
Wrap a breathable old wall in the wrong, vapour-tight materials and you can trap moisture inside the masonry. The result is damp held in the wall, rotting timbers built into it, blown plaster and mould, which costs far more to put right than the heating you saved. This is a real risk with internal insulation in particular, where moist room air can reach a cold spot behind the boards and condense.
With older and historic buildings especially, use breathable insulation systems specifically designed for solid walls, such as wood-fibre or other vapour-open boards and lime-based finishes, rather than the cheapest modern foam board on the shelf. Take advice from someone who understands traditional construction, get the detailing right around floors, reveals and party walls, and make sure the house is properly ventilated afterwards. Our notes on draught-proofing and ventilation explain why you must not seal a house up completely without giving moisture a way out.
Grants, and doing the cheap stuff first
Solid-wall insulation is the most expensive fabric job, so two things should shape when you do it.
First, look for help with the cost. Schemes change, but the long-running ECO obligation has funded solid-wall insulation for eligible households through energy suppliers, and various national and local grant programmes come and go. It is worth checking what is currently open before paying the full price yourself, because the sums involved are large enough that a grant changes the whole calculation. An energy performance certificate assessment will usually flag solid-wall insulation as a recommended measure, which can help with eligibility.
Second, do the cheap, high-return work first. Given the cost, solid-wall insulation should usually come after you have done the loft, sealed the draughts and worked through the quick wins. Those give fast returns for little money. Walls are the big, expensive finale. Do them in that order and you are never spending thousands on walls while pennies are still leaking out of an unsealed letterbox and an uninsulated loft. For the full sequence, the main heating hub sets out the order of jobs.