Cavity Wall Suitability Checker

Why this checker exists: the damp problem

Cavity wall insulation is one of Britain's best-value retrofits — about £350 a year saved for a semi-detached home, installed in half a day for under £1,200. It is also, in the wrong walls, one of the most damaging things you can do to a house. Fill a cavity that should never have been filled and wind-driven rain tracks across the insulation to the inner leaf, producing damp that did not exist before, rotting timbers, and repair bills in the tens of thousands. This is not theoretical: the problems peaked in the 2000s––2010s when aggressive sales operations filled unsuitable walls across exposed western Britain, and the redress schemes are still paying out. This checker is deliberately conservative. A false "not suitable" costs you a survey; a false "suitable" can cost you your walls.

The physics is simple. An empty cavity is a drainage and drying zone: rain that penetrates the outer brick runs down the inside face and evaporates, never reaching the inner leaf. Fill the cavity and you create a bridge — some insulation materials wick moisture, and even non-wicking materials reduce the cavity's ability to dry. In sheltered locations with sound brickwork this does not matter; the outer leaf never gets saturated. In severe exposure — Atlantic-facing coasts, hilltops, the western Pennines — the outer leaf can saturate regularly, and a filled cavity becomes a damp delivery system. Exposure, wall condition, and material are everything.

How to tell what walls you have

Most UK homes built between the 1930s and 1980s have cavity walls; most pre-1920s homes are solid; timber frame appears in pockets (notably 1960s–80s estates and modern builds). Quick checks: brick pattern — if every visible brick shows its long face (stretcher bond), the wall is likely cavity; alternating long and short faces (Flemish or English bond) usually means solid. Wall thickness at a door or window reveal: under ~260mm suggests solid, over ~300mm suggests cavity. Age: cavity construction became standard in the 1930s and mandatory-ish after the war, but plenty of 1930s homes are solid — never assume by age alone. If in doubt, select "not sure" and the checker routes you to a survey: a borescope inspection (a tiny camera through a drilled hole) settles it definitively in minutes.

The materials: mineral wool, beads, and foam

Three main fill materials are used in 2026: mineral wool (the traditional choice, blown in as fibres — cheap and effective, but the most moisture-sensitive), EPS beads (polystyrene beads, often bonded with adhesive — the best moisture resistance, the current professional preference for anything but the most sheltered sites), and polyurethane foam (injected, excellent thermal performance, but controversial: it is hard to remove, can complicate future wall-tie replacement, and some mortgage lenders dislike it). If your surveyor recommends foam, ask specifically about wall-tie access and lender acceptance — and get the reasoning in writing. For most suitable walls, bonded bead is the sweet spot of performance, moisture safety, and removability.

Red flags in an installer's survey

A proper survey takes 45–60 minutes and includes: borescope inspection of the cavity, a damp-meter check of internal walls, assessment of exposure and wall finish, and questions about the property's history. Red flags: a "survey" done entirely from the doorstep or via satellite imagery; pressure to sign the same day; no mention of the CIGA guarantee; a quote far below £700 (corners are being cut) or with "today only" discounts; and any reluctance to put the wall-type finding in writing. The industry's rogue era was driven by commission-paid salespeople, not surveyors — the person assessing your walls should not be paid by the job they recommend.

If your walls are not suitable: the alternatives

"Not suitable" is not the end of the retrofit — it just reroutes it. Solid walls take internal wall insulation (typically £5,000–£9,000 for a semi, disruptive but effective) or external wall insulation (£8,000–£15,000, transforms the appearance — check planning in conservation areas). Timber frames need breathable, frame-safe approaches designed by a specialist. And whatever the walls, the cheap wins still apply: loft insulation, draught-proofing, and heating controls do not care what your walls are made of. Run the energy audit to re-rank your options.

The CIGA guarantee and your paperwork

Every compliant install comes with a 25-year CIGA guarantee covering defects in materials and workmanship — including damp caused by the installation. This is not optional and not a paid extra; if it is not offered, walk away. Keep the guarantee certificate with your house paperwork: solicitors and surveyors ask about cavity insulation on every transaction, and a documented CIGA-backed install is an asset while an undocumented one is a question mark. If you already have cavity insulation and suspect damp, CIGA operates an independent complaints and redress process — use it rather than paying a second firm to "fix" the first firm's work.

Extraction: when fill has to come out

Sometimes the checker comes too late — the walls were filled years ago, by a previous owner, and now there is damp. Extraction is possible but it is not the mirror image of installation: it involves drilling a grid of holes and vacuuming or water-flushing the material out, it costs £2,000–£4,000 for a typical house, and it does not always get everything (mineral wool comes out cleanest; bonded bead usually extracts well; foam is the nightmare — it adheres to the masonry and often cannot be fully removed). Before paying for extraction, get an independent diagnosis: not all damp in a filled house is caused by the fill. Leaking gutters, failed pointing, and bridging at the damp-proof course cause identical symptoms, and fixing a £200 gutter is better than a £3,000 extraction. If the fill is the cause, the CIGA guarantee and redress process is your first call — that is what the 25-year guarantee exists for — not a second contractor's invoice.

There is also the question buyers ask: "the survey says the cavity is filled — is that good or bad?" A documented, CIGA-guaranteed install in a suitable property is a selling point — it proves the insulation and the guarantee in one document. An undocumented fill of unknown age and material is a surveyor's query, not a dealbreaker: the standard response is a borescope check and a damp-meter survey, which usually clears it. If you are selling a house with old fill and no paperwork, digging out the CIGA certificate (or getting the installer's records) removes the question before it is asked.

When to do it in your retrofit sequence

Cavity fill sits in Phase B (fabric), alongside loft insulation and draught-proofing — after the sub-£500 quick wins, before windows and heating systems. One sequencing note: if wall-tie replacement is on the cards (common in 1930s–60s cavity homes — your surveyor checks), do the wall ties before the fill, because remedial ties are far harder to install through insulation. And as with all fabric measures, it shrinks the heat pump you will eventually need — see the retrofit planner for the full order.