Home efficiency
Solid-wall insulation: internal or external?
Solid walls lose more heat than cavity walls and can't be filled. They can be insulated from inside or outside: both are large projects, best done alongside other work on the house.
Updated 3 October 2026 · energy.org.uk editorial team
The two options
- Internal wall insulation fixes insulated boards to the inside of outside walls, or builds a stud wall filled with insulation. Energy Saving Trust puts the cost for a typical three-bedroom semi-detached home at around £12,000. Rooms lose some floor space, and fittings such as radiators, sockets and skirting have to come off and go back.
- External wall insulation fixes insulation to the outside of the walls, finished with render or cladding. Energy Saving Trust puts it at around £15,000 for the same home. It doesn't reduce room space, but it changes how the house looks and may need planning permission.
Check
What to check first
- Whether your walls are solid. Homes built before about the 1920s usually are.
- Damp. Solve any damp first. Insulation can trap moisture if the materials or details are wrong; internal insulation makes the wall itself colder, so moisture can build up inside it.
- Planning and conservation rules, especially for external insulation, listed buildings and conservation areas.
- Timing. Doing it during other work, such as re-rendering or replastering, reduces the extra cost and disruption.
Do
Practical next steps
- Ask for a whole-house assessment that covers moisture, ventilation and the junctions around windows, floors and roof.
- Use an installer trained for the system they propose, and ask about guarantees.
- Check grants in the support finder; some schemes fund solid-wall insulation for eligible homes.
- Do cheaper measures first if they are still needed; see a sensible order for improvements.
Take care
Important limitations
- A simple payback figure is misleading here: the cost is large, and much of the value is comfort and reduced condensation as well as lower bills.
- Costs are Energy Saving Trust typical figures, not quotes; they vary a lot with the house.
- Poor detailing can cause damp and mould, so design matters as much as the insulation itself.