Cavity or Solid Wall?
Answer three quick questions and we'll tell you what type of external walls your home probably has — and which insulation applies. Takes under a minute.
Get your wall type confirmed for free
A borescope survey takes minutes and tells you exactly what you have — and whether you qualify for grant-funded insulation under ECO4 or GBIS.
Book a free survey →How does this work? (Heuristics & sources)
This tool combines three well-established heuristics used by surveyors as a first-pass indicator of wall construction type. They are indicative only — only a physical inspection (ideally a borescope drill) can confirm wall construction with certainty.
Heuristic 1 — Build date
| Era | Typical construction | Why |
|---|---|---|
| Before 1920 | Almost always solid brick (~225 mm / 9 in) | Cavity construction was not yet common practice in the UK |
| 1920–1944 | Mixed — cavity becoming more common | Transitional period; cavities were introduced in the 1920s but adoption was patchy |
| 1945–1990 | Usually cavity (uninsulated) | Post-war housebuilding strongly favoured cavity construction; insulation not mandatory until later |
| After 1990 | Almost always cavity, often already insulated | Building Regulations required insulated cavity walls from the early 1990s onward |
Heuristic 2 — Wall thickness
A solid brick wall is one brick wide: approximately 225 mm (9 in) of brick plus internal plaster, giving a typical reveal depth of around 230–260 mm. A cavity wall has two half-brick leaves separated by a 50–100 mm gap, giving a total of roughly 270–330 mm. Measuring at a window or door reveal is the simplest DIY check.
Heuristic 3 — Brick bond (pattern)
| What you see | Bond name | Likely wall type |
|---|---|---|
| All bricks show only their long face (stretchers) | Stretcher bond | Cavity — only achievable with two separate leaves |
| Short ends (headers) appear every few rows | English or Flemish bond | Solid — headers tie the single-leaf wall together |
| Rendered, pebble-dashed or stone-clad | Cannot determine | Survey required |
| Concrete, timber frame or other non-brick | Non-traditional | Survey required — different rules apply |
Scoring: each signal (age, thickness, bond) independently votes "cavity", "solid" or "unclear". The tool counts votes and takes the majority. Where signals conflict or are unreadable the verdict is "needs a survey". Rendered and non-traditional walls always return "survey needed" regardless of other signals.
Sources: Energy Saving Trust — Cavity wall insulation; Energy Saving Trust — Solid wall insulation; DESNZ — Hard-to-treat cavities guidance; Historic England — Insulation in traditional buildings.
This tool provides an indicative guide based on well-established heuristics. Only a physical survey (ideally including a borescope inspection) can confirm wall construction with certainty. Construction can vary even within the same street or building.
What is the difference between CWI, IWI and EWI?
Cavity wall insulation (CWI) — mineral wool or polystyrene beads are injected into the cavity through small drill holes in the mortar. Takes 2–4 hours with no interior disruption. Often grant-funded under ECO4 or GBIS.
Internal wall insulation (IWI) — insulated boards are fixed to the inside face of external walls. More affordable than EWI but reduces floor area slightly (typically 70–100 mm per insulated wall) and requires moving sockets, skirting boards and redecorating afterwards.
External wall insulation (EWI) — insulation is fixed to the outside and rendered or clad over. More expensive than IWI but no interior disruption and can dramatically improve the appearance of older properties. Often the preferred choice where external render needs replacing anyway. May require planning permission.
EcoHome UK installs all three measures and will advise on the best option for your specific property during a free survey.