Solid wall insulation

An older house can take a lot of heating.

Homes built before the 1920s are more likely to have solid walls, with no gap inside to fill. Those walls can still be insulated, from the inside or the outside. Research by the Energy Saving Trust found that around 33% of the heat lost from an uninsulated home escapes through its walls.

Is your wall solid?

On bare brick, the pattern of the bricks gives a clue. In a solid wall, some bricks are laid across the wall, so you see their short ends from outside. In a cavity wall, the bricks usually all run lengthways.

You can also measure the thickness of the wall at an open door or window. A brick wall between 260mm and 350mm thick is likely to have a cavity. A thinner one is probably solid. Stone walls tend to be solid whatever their depth.

If the walls are rendered or clad, a surveyor, architect or experienced installer can tell you what is behind.

Inside or outside

External insulation is a layer fixed to the outside of the wall, then rendered or clad. It keeps the size of your rooms, and it goes on without disrupting the household. It often means extending the roof edge and moving rainwater pipes and meter boxes.

Internal insulation goes on the inside of the outside walls. Installers use rigid boards, a stud wall filled with insulation, or an insulating plaster. It usually costs less than external insulation. It takes a little floor space, the rooms are out of use while the work goes on, and skirting boards and door frames come off and go back.

The Energy Saving Trust suggests fitting either kind alongside other work. External insulation can share scaffolding with a new roof or windows. Internal insulation fits well with redecorating.

Damp and fresh air

Traditional brick, stone, earth and lime let moisture pass in and out. Insulation changes how moisture moves through an old wall. Internal insulation makes the outer part of the wall colder, and moisture can build up inside it.

Installers can avoid this with breathable materials and breathable paints and finishes. They should repair the roof, gutters, brickwork and pointing first, and leave airbricks and trickle vents open. Insulation can also cut the air flow into rooms, so the installer should check whether extra ventilation, such as extractor fans, is needed.

Planning and listed buildings

In England, outside Conservation Areas, National Parks, Areas of Outstanding Natural Beauty and the Broads, cladding can go on a house without planning permission if the materials look similar to the original ones. Inside those areas you need to apply first. A listed building needs listed building consent for any significant work, inside or out.

In Wales, external wall insulation needs planning permission if it sticks out more than 16cm from the wall. Your local planning authority can tell you what applies to your house.

Finding an installer

TrustMark is the government-endorsed quality scheme for work in and around the home. Its website lists certified insulation installers. The Energy Saving Trust suggests getting at least three quotes.

In October 2025, the National Audit Office reported problems with wall insulation fitted under government schemes. The government is offering free audits, carried out by TrustMark, to homes that had external wall insulation under ECO4 or the Great British Insulation Scheme. The Great British Insulation Scheme ended on 31 March 2026.