Cavity Wall vs Solid Wall Insulation: UK Standards and Real-World Costs

Around 8 million UK homes have uninsulated cavity walls, and roughly 6 million more have solid walls with no external or internal insulation. After energy price rises in 2022–2023 changed the payback calculation significantly, both government schemes and private installer demand surged. This guide focuses on what actually happens once a crew arrives at your property — not the grant application process, which changes regularly and is best checked via the current Energy Saving Trust guidance.

How to Tell What Wall Type You Have

The simplest test: measure the thickness of an external wall at a window or door reveal. A cavity wall — standard construction from roughly 1920 onwards — will typically be 270–300 mm thick: 102 mm brick outer leaf, 50–100 mm cavity, 100 mm block inner leaf. A solid brick wall will measure around 220 mm (nine-inch brick) or 330 mm (thirteen-inch, common in Victorian terraces). Pre-1920 stone construction varies widely.

If you're genuinely uncertain, a thermal imaging survey in winter — when the inside of the house is warm — will show clearly where heat is escaping fastest. Surveyors can also drill a small test hole to look directly into the wall structure.

Cavity Wall Insulation: What the Process Involves

Cavity fill is the least disruptive form of wall insulation. An installer drills a regular pattern of holes in the outer masonry — typically 22 mm diameter, at roughly 1-metre centres — and injects mineral wool fibre, blown EPS beads, or polyurethane foam under pressure. The holes are re-pointed to match the existing mortar. Interior decoration is untouched.

The critical issue is suitability. Cavity fill in an exposed location, or where the existing masonry has cracks or poor pointing, can allow injected material to act as a moisture bridge — connecting the damp outer leaf to the inner wall. The British Standard BS 8208 covers assessment of exposure level before installation. A reputable installer will carry out a full survey; if they don't, treat that as a red flag.

Solid Wall Insulation: The Two Routes

Solid wall properties — typically pre-1920 construction, or some 1940s no-fines concrete houses — cannot be cavity filled. The two options are internal wall insulation (IWI) and external wall insulation (EWI). Each involves significant trade-offs.

Internal Wall Insulation

Insulated plasterboard (PIR foam bonded to plasterboard, or mineral wool between timber battens) is fixed to the inside face of the external walls. This is disruptive: every room treated needs skirting boards, architraves, electrical sockets and radiators moved. The cold bridge at the wall-floor junction and around window reveals requires careful detailing to avoid creating new condensation points.

The depth added — typically 70–100 mm of insulated dry-lining — reduces floor area in every treated room. In a narrow Victorian terrace bedroom, this can matter. On the plus side, IWI can be phased room by room and doesn't require external scaffolding or planning permission in most cases.

External Wall Insulation

EWI bonds insulation board (typically mineral wool or EPS) to the outside face of the masonry, finished with a reinforced render coat. It doesn't reduce internal room area, doesn't disrupt interior decoration, and resolves the thermal bridge at the floor junction more completely than IWI. It also adds weather protection to older masonry.

The constraints are significant. Cost is substantially higher — typically £8,000–15,000 for a semi-detached. It changes the external appearance of the building, which may need approval in conservation areas. Eaves, window sills, rainwater goods and meter boxes all need extending out. In terraced properties, the junction with the neighbour requires coordination.

U-values and UK Building Regulations

UK Building Regulations Approved Document L sets the target U-value for external walls at 0.18 W/m²K for new build. For existing dwellings, the guidance specifies that insulation should be fitted to at least the extent that it becomes cost-effective to do so — defined using a payback period test. A typical solid brick wall without insulation has a U-value around 2.0 W/m²K. With 100 mm of PIR board IWI, this improves to approximately 0.25–0.28 W/m²K.

Thermal Bridging: The Detail That Matters Most

In retrofit insulation, the installation team often installs continuous insulation on the flat wall area and stops at the lintel over a window or door. The result is a cold bridge at every opening: a strip of wall at window head that's significantly colder than the insulated sections. In a house with 12 windows and 4 external doors, this adds up to a substantial area of uninsulated wall and a significant source of heat loss and surface condensation.

Detail drawings for retrofitting around reveals and lintels are available from the Retrofit Academy and BRE publications. If an installer cannot show you their detailing approach for reveals, ask them explicitly before signing a contract.

Author: Daniel Marsh — Building performance specialist. Last updated: March 2026. References: Energy Saving Trust, BRE, Approved Document L (2022 edition).