Energy & EPC

Cavity vs Solid Wall Insulation: How to Tell Which You Have and What Each Costs

Wall insulation type is one of the most important factors affecting a property's energy efficiency and EPC rating. Many homeowners don't know whether their walls are cavity or solid — and the distinction determines which insulation solutions are available and what they cost.

Published: 19 Mar 2026 · Updated: 19 Mar 2026 · 7 min read

Why Wall Type Matters for Insulation

Around 35% of a typical home's heat is lost through the walls, making wall insulation one of the most impactful energy efficiency measures available. However, the appropriate insulation solution depends entirely on the construction of the walls — and the two main types (cavity and solid) require completely different approaches.

Getting this wrong is costly: filling a solid wall with cavity wall insulation is impossible; attempting to install cavity insulation in a wall with a compromised or already-filled cavity causes damp problems. Understanding your wall type is the essential first step.

How to Identify Your Wall Type

**Age of the property** is the most reliable starting point:

Construction period	Likely wall type
Pre-1920	Solid brick or stone (most common)
1920–1940	Transitional — some early cavities, but many still solid
1940–1995	Cavity wall (most common)
Post-1995	Cavity wall, often with partial insulation already installed

**The brick pattern test:** Look at the external brickwork. A **stretcher bond** (all bricks running lengthways) suggests a cavity wall — the inner and outer leaves are separate. A **Flemish bond** or **English bond** (alternating "headers" — bricks running width-on through the wall) indicates a solid brick wall.

**The wall thickness test:** Measure the depth of a window reveal (the internal face of the window opening). A cavity wall typically measures 270–350mm from outer face to inner face. A solid wall is typically 215–225mm for single-brick construction, or 330mm+ for double-brick.

**Thermal imaging surveys:** A professional thermographer can image external walls at dawn (the coldest period) when temperature differentials reveal insulation patterns, filled cavities, and thermal bridges. Useful for confirming suspected partial fills or localised voids.

Cavity Wall Insulation

For homes with an unfilled cavity, cavity wall insulation is one of the most cost-effective energy efficiency measures available. The installer drills small holes (approximately 22mm) at regular intervals in the mortar joints of the external wall, injects insulating material (typically mineral wool, EPS bead, or polyurethane foam), and plugs the holes.

**Cost:** £600–£1,800 for a typical semi-detached or terraced house. Larger detached properties can be £1,200–£2,500.

**Typical energy saving:** £150–£400 per year depending on property size and heating fuel.

**EPC impact:** Can gain 2–5 rating points on the SAP score, often representing a move of one EPC band.

**Important caveats:** Cavity wall insulation should only be installed by a CIGA-registered (Cavity Insulation Guarantee Agency) installer. Not all cavities are suitable — walls with defects, high exposure to wind-driven rain, or non-standard construction may not be suitable. A pre-installation survey by an independent surveyor is recommended.

**Grant funding:** Cavity wall insulation is funded under the ECO4 (Energy Company Obligation) scheme for qualifying low-income or low-EPC households. Check eligibility at Simple Energy Advice (GOV.UK).

Solid Wall Insulation

Solid walls cannot be injected with insulation — the wall is a single mass with no air gap. Two approaches exist:

**External Wall Insulation (EWI):** Insulation boards (typically EPS, mineral wool, or phenolic foam) are fixed to the outside of the building and covered with a render finish or cladding system. The result is a well-insulated wall with no loss of internal space.

Cost: £8,000–£25,000+ for an average semi-detached house
Energy saving: £300–£700+ per year
EPC impact: Very significant — often 2–4 bands
Planning permission: Usually not required under permitted development, but required for listed buildings, some conservation areas, and flats

**Internal Wall Insulation (IWI):** Insulation is fixed to the inside face of external walls and covered with plasterboard. Cheaper than EWI but reduces internal floor area by approximately 70–100mm per wall.

Cost: £4,000–£15,000 depending on scope
Energy saving: Similar to EWI
Complications: Window reveals deepen, radiators and skirting boards must be relocated, services (pipes, sockets) require repositioning

Checking Your EPC Before and After

The existing EPC for any property in England and Wales can be viewed via Property Passport UK or the official EPC Register. The recommendations section of the EPC will identify whether the assessor recorded the walls as solid or cavity (filled or unfilled) and will recommend the appropriate insulation measure. Comparing the current SAP score with the "potential" score on the EPC shows how much improvement is theoretically achievable.

After insulation work is installed, commissioning a new EPC confirms the improved rating and creates the official record needed for green mortgage applications or tenancy compliance purposes.