Homes built with solid walls (no cavity) lose heat much faster than cavity-wall properties. These homes are often colder, harder to heat, and more expensive to run — especially during winter. Internal Wall Insulation (IWI) is one of the most effective ways to upgrade these older, solid-wall homes and improve overall energy efficiency.

Under the ECO4 scheme, eligible households can receive free internal wall insulation, helping transform cold, draughty rooms into warm, comfortable spaces while significantly reducing heat loss.

How Internal Wall Insulation works

IWI involves fitting insulated boards directly onto the inside of your external walls. This creates a new insulated layer that stops heat from escaping through the solid wall structure.

A typical IWI installation includes:

• Thermal insulation boards
• A vapour-control layer to prevent moisture
• A new plasterboard finish ready for decorating

Because the insulation is installed internally, it does not affect the exterior appearance of the house — making it ideal for older homes, terraced properties, or houses in conservation areas.

IWI under the ECO4 scheme

ECO4 funds Internal Wall Insulation when:

• The home has solid walls (no cavity to fill)
• The property has a low EPC rating (D–G)
• Insulation is needed to reduce heat loss and raise the EPC rating
• The upgrade forms part of the whole-house improvement plan

A surveyor will assess your walls, measure heat loss, and determine the best thickness and type of insulation  for your home.

Benefits of Internal Wall Insulation

Installing IWI can:

• Make rooms noticeably warmer
• Reduce cold spots and draughts
• Lower heating bills
• Improve EPC rating
• Reduce damp and condensation issues
• Increase comfort and air quality

IWI is especially effective in older stone, brick, or solid-walled homes that lose heat quickly and struggle to stay warm. When combined with loft or floor insulation, the improvement can be dramatic.

What to expect during installation

Internal Wall Insulation is installed room-by-room, meaning some internal replastering and redecorating is required. Installers aim to keep disruption minimal, and all work is completed to TrustMark standards.

Your installer will:

• Prepare the walls
• Fit the insulation boards
• Seal and finish the joints
• Install a new plasterboard surface
• Leave the wall ready for painting or decorating

Installation quality

All IWI installations under ECO4 are carried out by certified, TrustMark-accredited installers, ensuring high-quality workmanship and full compliance with government standards.

Internal Wall Insulation is one of the most effective ways to upgrade solid-wall homes, dramatically improving warmth, comfort, and long-term energy performance — all fully funded for eligible households under the ECO4 scheme.