Internal and external solid wall insulation

Internal and external solid wall insulation

What is solid wall insulation?

Solid wall insulation is a retrofit solution designed to reduce heat loss through solid external walls, which are common in older UK homes built before the 1920s. Unlike homes with cavity walls, solid walls have no gap to fill, meaning heat escapes rapidly through the building fabric. Insulating these walls can make homes warmer, more comfortable, and cheaper to run, while reducing carbon emissions and boosting energy efficiency.

There are two main ways to insulate solid walls:

• External wall insulation (EWI) installs insulation to the outside of the property, covering the wall with insulating boards and a protective finish.
• Internal wall insulation (IWI) adds insulation to the inside face of external walls, creating a thermal barrier within the home.

Both aim to retain heat inside, but they differ in process, disruption, appearance, cost, and suitability.

How it works

Insulation works by slowing the transfer of heat through the wall. In a typical solid-walled home without insulation, up to 45% of heat can be lost through the walls. Adding insulation, whether inside or outside, traps more heat within the living space, meaning your heating system needs to run less to maintain comfort.

External wall insulation

External wall insulation involves fixing a layer of insulating material to your home’s exterior, then protecting it with a render, cladding or brick slip finish. It creates a continuous thermal layer that wraps the building, helping to eliminate thermal bridges and improve airtightness.

Internal wall insulation

Internal wall insulation is fitted on the inside of the external walls, typically beneath plasterboard or insulated panels. This reduces heat loss and can improve acoustic performance, but can slightly reduce internal space and may require moving fixtures or fittings.

External wall insulation (EWI)

What it is

External wall insulation fixes insulating boards to the outside of your home and covers them with a weather-resistant finish, such as render or cladding.

Things to consider

• Typically more expensive than internal insulation.
• May require scaffolding and careful planning of external features.
• In conservation areas or on listed buildings, external changes may need special approval, even though planning permission is often not required.

Internal wall insulation (IWI)

What it is

Internal wall insulation adds an insulating layer to the inside of external walls. Materials can include rigid boards, insulating plaster, or stud walls with insulation infill.

Things to consider

• Can be disruptive inside the home and may require removal of skirting boards, radiators, or electrical fixtures.
• Internal space is reduced slightly due to the added insulation layer.
• Careful moisture management is needed to prevent condensation issues, especially in older solid wall properties.

Choosing between external and internal insulation – what you need to know

Which option is right for your property depends on your priorities, home construction, and planning requirements:

• External insulation is often preferable where you want to preserve internal space and minimise disruption, or improve the look of your home.
• Internal insulation can be more suitable when exterior changes are restricted (e.g. conservation areas) or where external access is limited.

A professional assessment is essential, as factors such as wall material, moisture characteristics, existing wall condition, and ventilation all influence the optimal solution.

What to expect during installation

External insulation

Installation typically takes several days to a few weeks, depending on property size and finish choice. A survey identifies any remedial work needed before insulation is fixed, boards are applied, mesh and render are added, and final finishes are applied.

Internal insulation

Internal works involve covering wall surfaces, installing insulation panels or stud walls, and finishing with plasterboard and decoration. You may need to temporarily relocate furniture and remove some fixtures.

In both cases, choosing accredited installers with experience in solid wall retrofit ensures quality, reduced risks of moisture problems, and long-term performance.