Cavity Wall vs Solid Wall Insulation — Costs, EPC Impact and Which Houses Qualify

Why Wall Insulation Has Such a Large EPC Impact

Walls account for roughly 35% of heat loss in a typical uninsulated UK home — more than any other element. Addressing this is therefore the single largest potential gain for properties with inadequate wall insulation, often delivering 8–15 SAP points and moving a property up one to two EPC bands. The challenge is that wall construction varies widely across the UK housing stock, and the appropriate solution — and cost — varies accordingly.

How to Identify Your Wall Construction

**Cavity walls** (common in post-1930 properties): Two layers of brick with a gap (cavity) of 50–100mm between them. You can often identify cavity walls by measuring the total wall thickness — typically 280–330mm for a brick cavity wall. Properties built from the early 1930s through to the mid-1990s (when cavity fill became standard during construction) are most likely to have unfilled cavities.

**Solid walls** (typical in pre-1930 properties): A single thick leaf of brick or stone with no cavity. Victorian and Edwardian terraces and pre-war semis almost always have solid walls. Measuring the wall thickness through a window or door reveal gives a clue — solid brick walls are typically 215–225mm; solid stone walls are often 350–600mm.

**System-built or non-traditional construction**: Some post-war properties (1945–1975) were built using concrete panels, steel frames, or timber frames. These have their own insulation characteristics and may require specialist surveying.

Cavity Wall Insulation

How It Works

A qualified installer drills a grid of small holes (typically 22mm diameter) through the outer leaf of brick, injects insulation material (mineral wool, EPS bead, or polyurethane foam), and plugs the holes. The work typically takes 2–3 hours for a semi-detached property.

Cost

EPC Impact

Typically **6–10 SAP points** — often enough for a full EPC band improvement (e.g. E→D or D→C).

Grant Availability

Cavity wall insulation is one of the primary measures covered by ECO4 and the Great British Insulation Scheme. Many households qualify for free installation.

Caveats

Not all cavities are fillable. Properties in exposed coastal locations (high wind-driven rain) may require a specialist assessment; some cavity walls have already been filled during construction; and properties with wall ties in poor condition may need remediation first. An installer survey (usually free) will confirm suitability.

Solid Wall Insulation

For properties without a fillable cavity, insulation must be applied either to the outside face of the wall (external wall insulation, EWI) or to the inside face (internal wall insulation, IWI).

External Wall Insulation (EWI)

A layer of insulation board (typically 60–100mm of EPS, mineral wool, or PIR) is fixed to the outside of the wall and finished with a render coat or cladding. EWI does not reduce interior floor space, improves weatherproofing, and can modernise the external appearance of the property.

**Cost**: £8,000–£20,000 for a typical semi-detached (the variation reflects wall area, render finish, and scaffolding requirements). This is a significant investment and is one of the main reasons solid-walled properties are harder to bring to EPC C.

**EPC impact**: Typically **10–18 SAP points** — often sufficient for two EPC band improvements in properties starting from F or G.

Internal Wall Insulation (IWI)

Insulation boards or a stud wall filled with mineral wool are fixed to the inner face of external walls. IWI avoids scaffolding costs but reduces room dimensions slightly (typically 75–100mm per external wall) and requires replastering and redecoration.

**Cost**: £5,000–£12,000 for a semi-detached, depending on the extent of work and the level of finishing included.

**EPC impact**: Similar to EWI — **10–16 SAP points** — depending on insulation thickness.

Grants for Solid Wall Insulation

ECO4 and the Great British Insulation Scheme both cover solid wall insulation for eligible households. Given the high cost, grant funding makes a substantial difference — eligible households can receive free or heavily subsidised external or internal wall insulation.

Which Should You Prioritise?

For properties with unfilled cavity walls, cavity fill is almost always the right first step — the cost-to-SAP-gain ratio is exceptional. For solid-walled properties, the decision between EWI and IWI depends on planning considerations (conservation areas often restrict external alterations), available space, and whether scaffolding is already being erected for other works (which reduces the marginal cost of EWI).

Use the [EPC Improvement Calculator](/epc-improvement-calculator) to model the EPC band impact and estimated cost of wall insulation for your specific property type and current rating.