Solid Wall Insulation, Internal vs External and Which Suits Your Home

What is a Solid Wall?

Properties built before approximately 1920, and many built up to 1945, typically have solid walls: a single leaf of brick or stone with no gap between the inner and outer surfaces. Heat passes directly through the wall with no air gap to slow it down.

Solid walls lose roughly twice as much heat as an insulated cavity wall. For pre-1919 terraces, semi-detached properties, and Victorian and Edwardian townhouses, solid wall insulation is often the single biggest improvement available on an EPC assessment.

The Energy Saving Trust estimates that solid wall insulation can reduce heat loss through walls substantially, though the precise saving varies with property size, location, and current heating system.

Internal Wall Insulation (IWI)

Internal wall insulation is applied to the inside face of external walls. A rigid insulation board, typically mineral fibre or phenolic foam, is fixed to the wall surface and finished with plasterboard and skim.

**Advantages:**

• Does not alter the external appearance of the property
• No planning permission typically required
• Can be done room by room, spreading the cost
• Suitable for properties in conservation areas or with heritage brickwork

**Disadvantages:**

• Reduces floor area in each treated room (typically 80–120 mm loss per wall)
• Requires moving radiators, skirting boards, and electrical sockets
• Window reveals become deeper, requiring extension or boxing out
• Thermal bridging at joist ends if not carefully detailed

External Wall Insulation (EWI)

External wall insulation wraps the outside of the building in a layer of insulation, usually mineral wool or expanded polystyrene, fixed mechanically and chemically, then covered with a render or cladding system.

**Advantages:**

• Does not reduce internal floor area
• Resolves thermal bridging more effectively
• Can improve weatherproofing and external appearance
• Typically delivers higher energy performance improvement

**Disadvantages:**

• Changes the external appearance of the property
• May require planning permission, particularly in conservation areas
• More disruptive to install, requires scaffolding
• More expensive than internal insulation per square metre

Comparing the Two Approaches

PAS 2035 and Solid Wall Insulation

Any solid wall insulation project funded through government schemes, including ECO4 or the Great British Insulation Scheme, must comply with PAS 2035, the national standard for domestic retrofit. This requires a Retrofit Assessor to survey the property, identify any pre-existing damp or structural issues, and specify the correct solution before a Retrofit Coordinator oversees the installation.

This standard exists specifically because poorly installed solid wall insulation, particularly EWI applied to properties with pre-existing damp, has caused serious harm to buildings. Only use TrustMark-registered contractors, and ask to see their compliance documentation.

Planning and Listed Buildings

External wall insulation changes the external appearance of a property and is therefore subject to permitted development rules. In conservation areas, properties with Article 4 Directions, or listed buildings, planning permission is almost always required and may be refused. For listed buildings, Historic England guidance strongly favours approaches that preserve the character and breathability of historic fabric.

Property Passport UK shows the EPC rating and wall construction type for every property, making it straightforward to check whether solid wall insulation is listed as a recommended improvement before commissioning a survey.

Funding Options

Solid wall insulation is one of the most expensive retrofit measures, but funding is available through:

• **ECO4**, Free or heavily subsidised for eligible low-income and fuel-poor households
• **Great British Insulation Scheme**, Broader eligibility than ECO4, covers solid wall insulation
• **Local authority schemes**, Many councils run area-based retrofit programmes

Contact an Energy Saving Trust advice centre or your local authority to understand what you may qualify for before paying privately.