Solid Wall Insulation: Internal vs External

Approximately 7-8 million solid-walled homes in England are among the hardest to insulate and the most expensive to heat. Victorian and Edwardian terraces, pre-1920 semis and detached houses, and many interwar properties have solid brick or stone walls with no cavity. The only way to improve thermal performance significantly is to add insulation to either the outside or the inside of the wall. Both approaches work. Both have significant trade-offs.

Why Solid Walls Are a Problem

A typical uninsulated solid brick wall (215mm) has a U-value of around 2.1 W/m2K. A cavity wall with mineral wool insulation achieves around 0.5 W/m2K. That's a factor of four difference in thermal performance. The walls are typically the single largest source of heat loss in a solid-walled property, more significant than the roof in many cases.

Adding insulation to solid walls is expensive and disruptive in a way that loft insulation or cavity fill is not. The costs reflect this: solid wall insulation is typically £8,000-£25,000 for a standard terraced or semi-detached house, compared to £300-£600 for loft top-up insulation.

External Wall Insulation (EWI)

External wall insulation fixes insulation boards to the outside of the wall, covers them with a reinforcing mesh, and applies a finish render or cladding. The finished wall is thermally excellent, protects the existing structure from weather, and doesn't reduce internal floor area.

Typical U-value achievable: 0.28-0.35 W/m2K with 90mm insulation boards. The wall gets noticeably thicker on the outside: a typical installation adds 100-150mm to the external wall thickness.

Advantages:

• No loss of internal floor area
• No disruption to internal decoration or fitments
• Keeps the thermal mass of the wall inside the insulation, improving comfort and reducing overheating risk
• Protects the structure from weather and can improve the external appearance
• Avoids condensation risk within the wall (the wall stays warm)

Disadvantages:

• Changes external appearance significantly (planning permission often required)
• Not suitable for listed buildings or many properties in conservation areas
• Requires windows and doors to be extended or refitted to accommodate the increased wall thickness
• Needs careful detailing around ground level (junction with DPC), at eaves, and at window reveals
• Typically more expensive than IWI

Internal Wall Insulation (IWI)

Internal wall insulation fixes insulation boards or a studwork framework with insulation fill to the inside face of the external walls, covered with plasterboard. It doesn't change the external appearance of the house.

Typical U-value achievable: 0.3-0.35 W/m2K with 70-80mm insulation.

Advantages:

• No change to external appearance; no planning permission required in most cases
• Suitable for conservation areas and, in some cases, listed buildings
• Generally lower cost than EWI

Disadvantages:

• Reduces internal floor area (typically 70-90mm per wall treated, so a room loses 140-180mm of width if two walls are treated)
• Disrupts all internal finishes: skirting boards, architraves, electrical sockets, radiators all need moving
• Creates cold bridges at floor and ceiling junctions if not carefully detailed
• Risk of interstitial condensation if the vapour control layer is not correctly installed
• Pushes the existing wall into a colder zone, potentially causing condensation in the wall structure

Breathable Insulation for Older Buildings

For Victorian and older solid masonry buildings, breathable insulation systems are often recommended. These use insulation materials with good vapour permeability (wood fibre board, calcium silicate board, natural cork) rather than the vapour-impermeable PIR or EPS boards typical of modern IWI systems.

A breathable approach allows moisture that enters the wall to dry to the interior, rather than being trapped. It's more forgiving of imperfect vapour barriers, more compatible with lime plasters, and generally preferable in buildings with variable moisture behaviour. It typically achieves lower U-values per millimetre of insulation depth, requiring thicker boards for equivalent performance.

Grant Funding for Solid Wall Insulation

Solid wall insulation is funded under ECO4 for eligible lower-income households and under the Great British Insulation Scheme for broader eligibility. The cost, even with grant support, is often still substantial for the homeowner in higher-income bands. As of 2025, solid wall insulation under GBIS is available with partial subsidy for properties with an EPC of D or below that are outside the lower-income ECO4 threshold.

Costs without grant funding typically run to £100-£160 per square metre for EWI and £60-£100 per square metre for IWI installed. For a typical 3-bedroom terraced house with 80 square metres of external wall area, that's £8,000-£12,000 for IWI or £12,000-£20,000 for EWI before any grant contribution.