Cavity Wall Insulation
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Cavity wall insulation survey

Before anything comes out of your walls, someone should look inside them. A survey drills a small hole through a mortar joint, puts a borescope camera into the cavity and records what the fill is doing: whether it is dry, wet, slumped, patchy or bridged by debris.

Typically £50 to £300, often free with a quote

Guide prices checked November 2026, for England, Scotland and Wales.

Ask for a cavity survey

When a survey is the right first step

  • You have damp inside and do not know why. Cavity fill is one cause among several. Leaking gutters, failed pointing, a bridged damp-proof course and condensation all look similar from inside a room.
  • A valuer or surveyor has mentioned the insulation. A borescope report with photographs is usually what a lender or buyer’s solicitor wants to see next.
  • You want to make a guarantee claim. The guarantee provider will investigate, but your own record of what is in the wall helps if the claim is disputed.
  • You are not sure the walls should ever have been filled. A survey can tell you the cavity width, the type of fill and whether the wall faces severe driving rain.

What a survey costs

ItemWhat it coversGuide price
Borescope inspectionSeveral test holes per elevation, photographs of the cavity£50 to £300
Wall tie surveyMetal detector and borescope check of tie condition, often with a report£250 to £450
Free survey with a quoteMany extraction firms look before pricing at no charge£0

Guide prices before VAT, from published UK price guides read November 2026. A report written for a lender usually sits at the top of the range. Work out your own house.

Not sure the insulation is the problem? Start with someone looking inside the wall.

Ask for a cavity survey

What the camera actually shows

A borescope is a thin camera on a flexible lead. The surveyor drills a hole of around 10 to 22 mm through a mortar joint, feeds the camera in and looks up, down and sideways. Holes are usually made on each elevation, high and low, because insulation fails unevenly: the wall facing the prevailing weather tends to be worst, and slumping shows at the top of a wall rather than the bottom.

What a good survey records

  • The cavity width, measured rather than guessed. Many houses built before the 1980s have cavities of 50 to 65 mm. Historic England notes that cavities narrower than 50 mm carry a higher risk of water crossing the fill.
  • The fill type: polystyrene bead, blown mineral wool, urea formaldehyde foam or boards.
  • Whether the fill is dry, damp or saturated, and where. A photograph of beads stuck together with water or wool lying in a wet mat is the evidence a claim or a lender needs.
  • Voids: gaps under windows, at the top of the wall or where the original installer missed a drilling point.
  • Bridging: mortar snots on wall ties, builders’ rubble at the base of the cavity, or a cavity tray that has failed above a window.
  • The condition of the wall ties, because corroded ties are made worse by wet insulation.

What a survey cannot tell you on its own

A camera sees a few metres from each hole, not the whole wall. A good surveyor combines what the camera shows with what is outside: the exposure of the site, the state of the pointing, gutters and downpipes, render cracks, ground levels against the wall. BRE guidance on cavity walls treats the exposure of the wall to wind-driven rain as one of the main risks for filled cavities, so expect the surveyor to ask about it.

Ask for the report in writing, with photographs labelled by elevation. If you go on to claim on a guarantee or sell the house, that report is worth more than the conversation on the day.

Thermal imaging

Some surveyors add a thermal camera on a cold day. It can show cold bands where fill has slumped, which helps decide whether one wall or the whole house needs attention. It does not replace looking inside the cavity.

Where our information comes from

Prices are typical ranges from published UK contractor and trade price guides, read November 2026, before VAT. Guidance is summarised in our own words; it is general information, not advice on your particular house.

How the job gets done

Most surveys take an hour or two and leave holes no bigger than a pencil, filled with matching mortar.

  1. Look outside first

    Exposure, pointing, gutters, render, ground levels and any signs of past drilling.

  2. Drill test holes

    Through mortar joints, on each elevation, high and low.

  3. Camera into the cavity

    Fill type, condition, voids, bridging and wall ties, photographed.

  4. Written findings

    What is wrong, which walls, and whether extraction is needed at all.

Questions homeowners ask

How do I know if my cavity wall insulation has failed?

Damp patches on inside walls (often low down, or around windows), mould that keeps coming back, walls that feel cold, or a valuer flagging it. The only way to be sure is to look inside the cavity with a borescope.

Who should do the survey?

An extraction specialist will survey before quoting, often for free. If you want an opinion from someone who is not also pricing the work, a chartered building surveyor can inspect and report. For a guarantee claim, the guarantee provider arranges its own inspection.

Does a survey damage the wall?

It leaves a few small holes in mortar joints, which are filled afterwards. Ask for the mortar colour to be matched.

Can I check the cavity myself?

You can buy a cheap inspection camera, but drilling an external wall and reading what you see is not a DIY job. If you have a guarantee, disturbing the wall yourself could complicate a claim.

Will a survey tell me if I have a guarantee?

No. Ask the original installer or CIGA, which can confirm whether it issued a guarantee for your address. See our page on CIGA guarantee claims.

Keep reading

Tell us what the walls are doing

Three short steps. Your postcode, the house and what you are seeing are enough to start. The specialist confirms the detail with a borescope survey before any price is fixed.

Costs you nothing. We hold on to what you send until you have agreed to the extraction specialist we suggest.

Step 1 of 3

Where is the house?
Ask for a cavity survey