Cavity Wall or Solid Wall? Free UK Diagnostic Tool | EcoHome UK

Most UK homeowners don't know what their walls are made of — but it's the single biggest factor in what insulation you can get and how much it costs. Five quick questions tells you with high confidence.

If you have an EPC certificate, the build year is on it. Or check Rightmove / Zoopla listings for your address.

Pebbledash, render or stone

No bricks visible from outside. We'll use the other questions to work out your wall type.

Look at any external wall. Pick the option that matches what you see — we'll explain what it tells us.
Open a window. Measure from the inside surface of the wall to the outside surface. A 50p coin is roughly 27mm wide if you don't have a tape.
Small filled circles (about the size of a pencil) in the mortar between bricks are a sure sign of past cavity wall insulation.

Diagnosis

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How we got there

    Likely insulation type

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    Typical install cost

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    Want to know for certain?

    Our surveyors confirm wall type in a 20-minute on-site visit — no charge, no obligation. We'll also check what grant funding you qualify for.

    Book a free survey →
    TrustMark registered. PAS 2035 compliant. Covering Bristol, Bath, Exeter & the South West.
    How does this diagnostic work? (Methodology)

    The diagnostic uses three reliable indicators that surveyors use in the field, weighted by how strong each one is:

    1. Build year — the strongest single indicator. UK building regulations effectively standardised cavity wall construction from the 1930s onward. Pre-1920 homes are almost universally solid wall (over 95% confidence). 1920–1929 is a transition period.
    2. Brick pattern — stretcher bond (all bricks laid lengthways, no short ends visible) requires the wall to be only one brick thick, which is why a cavity is built behind it. English and Flemish bonds use short bricks ("headers") that tie two skins of brickwork together — this only makes sense in a solid wall.
    3. Wall thickness — a solid 9-inch brick wall measures around 220–240mm. A cavity wall (two skins of brick + 50mm cavity + plaster) measures 260mm or more. Measure at the reveal of a window or door for the most accurate read.

    Stone and pebbledash/render walls are almost always solid wall. Concrete and system-built homes (Wimpey No-Fines, BISF, etc.) are special cases that always need a survey. Timber frame homes have a different construction system that needs different products.

    The diagnostic gives a confidence rating: High when multiple indicators agree, Medium when there's one strong indicator, Low when indicators conflict or you've answered "not sure" to most questions.

    This tool is for guidance only. For a definitive answer, request a free survey.

    What's the difference for my insulation options?

    Cavity wall homes can have cavity wall insulation (CWI) blown into the gap between the two skins. It's quick (2–4 hours for a virgin cavity, longer if old insulation needs extracting), lasts 25+ years, and costs £1,000–£4,500 depending on whether the cavity is virgin, needs extraction, or has rubble to clear, and is often fully grant-funded under ECO4 or GBIS for eligible households.

    Solid wall homes need either:

    • Internal Wall Insulation (IWI) — fitted to the inside face of external walls. £4,000–£8,000 typically. Faster and cheaper than EWI but reduces internal floor area slightly and requires redecorating.
    • External Wall Insulation (EWI) — fitted to the outside of the building and rendered over. £8,000–£15,000 typically. No internal disruption, transforms the look of older properties, often the right call for solid wall homes that need re-rendering anyway.

    Both IWI and EWI deliver similar energy savings. The choice usually comes down to budget, planning constraints, and aesthetics.