Cavity wall insulation problems, and the table that says which walls should have had it
A cavity exists to stop water crossing it. Filling one is a deliberate change to that, and the regulations already publish which walls the change was recommended for. What that table says, what the 25 year guarantee actually covers, and how to tell whether the fill is your problem.
By The Damp GuyLast updated
The short version
A cavity wall works by separation. The outer leaf is meant to get wet, and the gap is what stops a wet outer leaf becoming a wet inner one. Filling that gap is a change to how the wall handles water, so the regulations set conditions on it rather than waving it through. Approved Document C carries a table giving the maximum exposure zone each insulated construction is recommended for, and the numbers run lower than most people expect. Blown fill into a 50mm cavity behind plain facing brick is recommended up to zone 2 of 4, and only where the mortar joints are tooled flush. Where the joints are recessed, every injected fill in that table drops to zone 1, the sheltered zone, at every cavity width it lists. None of which means the insulation caused your damp. It means there was a published test for your wall, and the question is whether anyone applied it.
What filling a cavity actually changes
It changes the one thing the cavity was built to do. That is not an argument against doing it, and it is the reason the rules attach conditions.
Approved Document C puts the purpose of a cavity in one sentence. An external cavity wall meets the requirement if the outer leaf is separated from the inner leaf by a drained air space, or in any other way that stops precipitation being carried to the inner leaf. Read it twice. The job is separation, and the gap is how it is done.
The same section says the cavity should be at least 50mm wide and should be bridged only by wall ties, by cavity trays that stop moisture being carried across, and by cavity barriers, firestops and closures where they are needed. That is a short list of permitted bridges. Where a cavity is only partly filled, the residual gap should not be less than 50mm.
So a full fill is a deliberate departure from the drained air space, and the regulations handle it by attaching conditions instead of banning it. The insulation systems are designed for it. CIGA describes its own systems as designed so that any water entering the cavity can still drain away.
None of that is the same as saying every wall suits it. Approved Document C says that when the cavity of an existing house is being filled, special attention should be given to the condition of the external leaf, and it gives state of repair and type of pointing as the examples. It then names BS 8208-1:1985 as the guidance. That is an assessment, of your wall, before the work. Whether it happened is a fair question to ask of whoever holds the paperwork.
One thing worth separating out before you go further. Water crossing the structure and water condensing on a cold surface are two different mechanisms with two different fixes, and this page is about the first. If the picture is black spots in corners and around windows that come and go with the weather outside, the condensation side of it is the better place to start. Insulation is also one of the two honest levers on that problem, which is the argument on heating a cold house versus insulating it.
The four fills, and the one the regulations hold hardest
Three of these get injected into a wall that already exists. The fourth was built in and never involved a drill. Approved Document C treats them differently, and it treats one of them harder than the rest.
The last column is the useful one. It is the maximum exposure zone Approved Document C Table 4 recommends for the most ordinary British case there is: facing brickwork with no render and no cladding, mortar joints tooled flush, cavity 50mm. Four zones exist, from 1 sheltered to 4 very severe.
| The fill | What it is | What has to happen before it goes in | Highest zone recommended, plain facing brick, 50mm cavity |
|---|---|---|---|
| Mineral wool | a cotton wool like material produced from either rock wool or glass wool, blown in through drilled holes. Used in the majority of installations | certification from an appropriate body, installation to the appropriate installations code, the wall assessed for suitability first, and an installer operating under an Approved Installer Scheme | Zone 2 of 4 |
| Polystyrene bead | small expanded polystyrene beads, normally injected with a binder | the same conditions as mineral wool. Approved Document C does not separate the two | Zone 2 of 4 |
| Urea-formaldehyde foam | a water based chemical system that foams inside the cavity. Introduced into the UK in the late 1960s | material to BS 5617:1985, installation to BS 5618:1985, the wall assessed for suitability first, and an installer under an Approved Installer Scheme | Zone 1 of 4 |
| Rigid board or batt | built into the wall as it was being built, leaving a residual cavity of not less than 50mm. Nothing was injected into anything | current certification from an appropriate body or a European Technical Approval, and work carried out to that document | Zone 3 of 4 |
Mineral wool and polystyrene bead sit in the same row of Table 4, "injected fill, not UF foam", so the regulations draw no distinction between them on exposure. If you have spent an evening reading that bead is worse than wool or the other way round, that is not a distinction Approved Document C makes.
Urea-formaldehyde foam is the one held hardest. It has its own row set, it sits a zone lower than everything else at the same cavity width, and it carries two named British Standards of its own: BS 5617:1985 for the material and BS 5618:1985 for putting it in. CIGA is blunt about it too, saying its three main systems are all suitable for the purpose and, except for UF foam, can be used in all areas of the UK. UF foam was introduced here in the late 1960s, so a house filled with it has almost certainly been filled for decades.
The fourth row is on the table to stop a common mix-up. A rigid board or batt built into the wall as it went up, with a residual cavity of at least 50mm left behind it, is a partial fill. The drained air space is still there. It is recommended a full zone higher than any injected fill at the same width, and nothing was ever blown into it.
Recessed pointing puts every injected fill in zone 1
This is the finding, and it comes from reading the whole of Table 4 rather than the one row that applies to you. It is also the thing you can check from the pavement.
Table 4 gives a zone limit for each combination of insulation method, cavity width and outside finish. The finishes it separates are impervious cladding, a rendered finish, facing masonry with tooled flush joints, facing masonry with recessed mortar joints, and flush sills and copings.
Take every injected-fill row in the table. There are 8 of them, covering cavity widths from 50mm to 150mm, split between foam and everything else. In the recessed mortar joints column, all 8 say the same thing: zone 1. The sheltered zone. A wider cavity does not lift it, and neither does a different material.
Which is a quiet way of saying that the way the mortar was finished on the outside of your house is, by the table's own arithmetic, decisive. Recessed joints hold water at every course. The table treats them as the hardest case there is, and it does not offer a construction that works around them.
Two things stop this being a verdict on your wall. Nobody is obliged to rebuild an existing house to current guidance, and Table 4 is a recommendation for what gets installed rather than a retrospective test of what already has been. And a zone is not fixed by your postcode alone. Approved Document C says to take the mapped zone and then adjust it: add one where local conditions accentuate wind effects, such as an open hillside or a valley funnelling wind onto the wall, and subtract one where the walls do not face into the prevailing wind. One wall of a house can sit two zones harsher than another.
The mapped zones themselves, and the wind-driven rain thresholds behind them, are set out on the penetrating damp guide, where the exposure zone argument starts. Read this page and that one together and you have the whole of what the regulations offer a homeowner on this: a rating for your weather, and a table of what construction that rating needs.
How to tell whether the fill is your problem
Nobody can tell you what is happening inside your cavity from a description of it, and nothing on this website can either. What you can do is watch the wall, because the three kinds of damp behave differently.
Start where CIGA starts, because it costs nothing and it is the first thing the guarantee body itself tells people to do. Check there is no obvious cause. It gives blocked gutters and earth piled up against the wall as its two examples. A great many damp inner leaves have a cause you can see from the garden, and a filled cavity nearby that had nothing to do with it.
Then watch the timing. Water crossing the structure follows driving rain and eases in a dry spell. Condensation follows cold surfaces and shut windows, so it is worst in still cold weather. That difference is free to observe and tells you more than any single reading does. If the patch is really black mould growing on a cold surface, it is a surface humidity problem and the cavity may be irrelevant to it.
The pattern that points at the cavity is a damp inner leaf with no defect visible outside, on a wall that is otherwise sound. That is worth saying plainly: it is the one damp pattern where there is nothing to see. It is also the pattern a bridged cavity, a failed cavity tray, mortar dropped on a wall tie during construction, and debris in the cavity all produce. Filled or empty, that list is the same list.
So the question that actually separates them is documentary rather than visual. Was this wall assessed for suitability before it was filled, and what did the assessment conclude? Approved Document C says the assessment should happen, names the standard, and singles out the state of repair and the type of pointing on the outer leaf. Somebody holds that paperwork.
If you do reach for an independent opinion, who is paying for the survey decides a great deal about what it finds. That holds here more than anywhere, because the two obvious people to ask are a firm that extracts insulation and a firm that installs it.
What the guarantee actually says
A 25 year guarantee covering defects in materials or workmanship, transferable with the house, capped at £20,000 of rectification. The published terms are more specific than almost anybody repeats them.
The Cavity Insulation Guarantee Agency was set up in 1995, after consultation with what was then the Government's Energy, Environment and Waste Directorate, to provide an independent, uniform and dependable guarantee covering defects in materials and workmanship. It describes itself as an independent body. It is also the guarantee body for the industry it guarantees, and its registered installers are its members. Both of those things are true at once and the second is worth holding in mind when you read the first.
Here is what its own published summary of cover says, in its own numbers.
- It runs 25 years, and that is a notification window. Any claim must be notified to CIGA within 25 years of the installation date.
- It covers defects in materials or workmanship. Not a wall that turned out to be a poor candidate, and not damp from another cause.
- It transfers on sale. The guarantee remains valid for subsequent owners or occupiers of the property, and the clock does not restart when you move in.
- Rectification is capped at £20,000. CIGA states that as the current maximum value of rectification work.
- There is one stated exclusion, and it is broad. The guarantee is not valid if the insulation has been altered or disturbed. Ordinary later building work is the thing most likely to do that.
- Report to the installer first, then escalate. Problems go to the installer as soon as practicable, and CIGA must be told if the matter is not resolved satisfactorily within 2 months. The installer is to make contact within 24 hours, and if they cannot resolve it within 41 working days you can go to CIGA.
Once a concern is with CIGA, it publishes performance targets for its own process. They took effect from 1 September 2017 and they are specific.
- Acknowledge the concern, or refer it to the installer: 2 working days
- Contact the householder to arrange an inspection: 5 working days
- Send the inspection report and the proposed scope of works: 27 working days
- Obtain quotes, once the householder has agreed the scope: 2 working days
- Complete rectification in a simple case: 50 working days
- Complete rectification in a complex case: 90 working days
- Respond where the householder rejects the proposed resolution: 5 working days
- Implement an arbitration award: 5 working days
The split between a simple case and a complex one is where extraction appears. Simple means only a minimal rectification works have been identified i.e. Refill of void areas only and no decorative works/drying out period. Complex means significant rectification works have been identified i.e. total cavity clearance with drying out period for subsequent decorative works; appointment of specialist to assess before further action; blocks of flats. So clearing a cavity out is inside the scheme rather than outside it, and where CIGA decides there is a defect the remedial work is carried out free of charge.
Disagree with the outcome and the route is arbitration. The Centre for Effective Dispute Resolution runs the scheme, approved by the Chartered Trading Standards Institute, and CIGA states that the arbitrator's decision is final and legally binding on both parties. Read that last clause before you use it, because it binds you as well.
Three things nobody tells you, and each has caught people out. A CIGA guarantee is not automatic: CIGA names Enterprise ECO, IWA ECO, Qualitymarkprotection as other guarantee providers, so a blank in one database proves nothing about the others. CIGA no longer issues retrospective guarantees for installations carried out between 1995 and 1 January 2012 where none was bought at the time. And a replacement certificate costs £60.00 including VAT, although CIGA says it needs only the property address and postcode to search its database, so you do not need to buy one to raise a concern.
Where the installer has stopped trading, CIGA directs the complaint to the supplying System Designer. Which is the honest answer to the question everyone actually has, and it is why the guarantee is worth more than the installer's own promise: the scheme outlives the company.
What the guarantee body's own figures show
CIGA publishes quarterly complaint statistics on its own website, quarter by quarter, going back to 2015. Almost nobody writing about cavity wall insulation cites them, so here are the last six.
| Quarter | Claims handled | Water penetration share of claims received | Claims resolved |
|---|---|---|---|
| Q1 2025 | 679 | roughly 30% | 410 |
| Q2 2025 | 427 | roughly 24% | 318 |
| Q3 2025 | 402 | roughly 25% | 299 |
| Q4 2025 | 412 | roughly 30% | 275 |
| Q1 2026 | 901 | roughly 40% | 500 |
| Q2 2026 | 391 | roughly 33% | 297 |
Add the six quarters up and you get 3,212 claims handled and 2,099 resolved. The word in every quarter's commentary is "roughly", and it is CIGA's word, so the percentages are approximations the body itself has published rather than anything computed here.
Water penetration is the only complaint type CIGA puts a number on in its commentary, and across those six quarters it runs between 24% and 40% of claims received. That is the sentence worth taking away. It does not say water penetration is the largest category, because CIGA never says that and the underlying counts are not published. It says the guarantee body has chosen to report one number every quarter, and the number it reports is how much of its postbag is water getting through a wall.
The categories CIGA charts alongside it are worth reading as a list of what people actually ring up about: water penetration, condensation & mould, voids & unfilled areas, escape of insulation, disturbance of insulation, poor performance, non traditional building, blocked ventilators, blocked cavity. Named on the chart legend, every quarter.
One more figure, and it is small enough to be worth stating precisely. Arbitration referrals run at a handful a quarter, between one and ten in the six quarters above. In Q3 2025, CIGA reported that of 3 published awards, 2 had failed and one had succeeded in part. That is the whole published record of how an arbitration lands. Three cases is not a rate and nothing here pretends otherwise, but three published outcomes is three more than any other guarantee scheme in this trade puts in front of a homeowner.
The thing genuinely not published anywhere is what proportion of claims are upheld. CIGA publishes volumes, types, resolution counts and arbitration counts. It does not publish how many concerns it decided were defects and how many it decided were not, and no other body publishes it either. That absence is worth knowing about before you form a view from somebody's anecdote.
If it has to come out
Extraction is not a small job and it is not the automatic answer. The trade body for damp work says removal should happen only if the evidence dictates it, and it has an incentive to say so, and it is still right.
How it is done depends on which fill you have, which is the practical reason to know the answer to that question.
- Loose beads and blown fibres: vacuumed back out through drilled holes.
- Foam and bonded materials: sections of the wall taken out so the fill can be broken up.
That second line is the one to sit with. Loose bead and blown fibre come back out through holes. Foam and bonded material do not, so parts of the outer leaf have to be taken down and rebuilt. The same publisher puts the duration at a few days to a week, depending on how much has to come out.
On money, there is one published set of figures in this entire market and it comes from a company that sells home retrofit surveys and installation. Stated in September 2026, it puts removal from a whole house at £1,500 to £3,000, re-insulating a typical semi at £2,500 to £4,500, and the two together at £4,000 to £7,500. The same page notes that the same publisher adds that scaffolding or a complex job pushes all three ranges up.
Treat that as one company's price list and not as a market rate. No second publisher has put a range in public, which is itself worth noticing on a job this common. And hold it against the guarantee: if a guarantee applies and the body decides there is a defect, the work is carried out free of charge, which makes the £20,000 rectification ceiling a real number rather than a theoretical one.
The Property Care Association, whose members do damp work and who sell training and certification to contractors, warns in its own published article about opportunistic insulation removal companies and says removal should follow the evidence rather than lead it, because fixing the ventilation or something else may be the better answer. A trade body saying the popular fix is often the wrong one is worth a hearing on that basis alone.
What to ask, and in what order
You are not expected to argue Approved Document C with anybody. Ask, then listen to how the answer lands.
- Was this wall assessed before it was filled, and what did it conclude? Approved Document C says the suitability of the wall is to be assessed before the work is carried out, and names the standard. This is the first question, and it is documentary.
- Which fill is in there? It decides the Table 4 row, which of the two removal methods applies, and roughly what era the installation is from.
- What exposure zone is this wall in, after the modifiers? Mapped in Approved Document C, then adjusted up for an open hillside or a funnelling valley and down where the wall does not face the prevailing wind.
- Are the mortar joints recessed or tooled flush? You can answer this one yourself, from the pavement, and the table treats it as decisive.
- Which guarantee scheme, and is the certificate findable? CIGA is not the only provider. A scheme with published terms and published timescales is worth more than one without, and you can read both before you need them.
- Has the obvious cause been ruled out first? Blocked gutters and ground piled against the wall, in CIGA's own words. Nothing about the cavity matters until those are off the table.
- Who benefits from the answer I am being given? The firm that extracts insulation and the firm that installs it will not give you the same diagnosis, and neither of them is lying. They are looking at the same wall through different order books.
About this resource
The regulations here are Approved Document C, "Site preparation and resistance to contaminants and moisture", 2013 edition, Section 5, including the whole of Table 4. It is published free and every line of it is checkable. The guarantee terms, the published timescales and the quarterly claim figures come from CIGA's own website, read on 11 September 2026, and CIGA is the guarantee body for the industry it guarantees. The order of redress comes from the government's own consumer guide of October 2019 and from Ofgem, which administers the ECO scheme and states plainly that it does not regulate the insulation industry. BS 8208-1:1985, BS 5617:1985, BS 5618:1985 and BRE Report 262 are named by title and scope only, because all four are paywalled.
No figure for how many British homes have a filled cavity, because nothing publishable supports one. No proportion of claims upheld, because nobody publishes it. No claim about spray foam in a masonry cavity, because neither the regulations nor the guarantee body covers such a system. This site does not survey, treat or quote for work. It cannot tell you what is happening inside your wall. That needs somebody who has stood in front of it, and preferably somebody with no stake in the answer.
Common questions
How do you know if your cavity wall insulation has failed?
Mostly you do not, from the inside. A damp inner leaf with no obvious defect outside is a pattern, not a verdict. CIGA's own first instruction to a worried householder is to check there is no obvious cause first, and it gives blocked gutters and earth piled up against the wall as the examples. After that the useful question is not about the insulation at all. It is whether the wall was assessed for suitability before it was filled, because Approved Document C says that assessment should happen and names the standard for it.
Which houses are not suitable for cavity wall insulation?
Approved Document C does not publish a list of house types. It publishes a table of wall constructions against exposure zones, and it says that when the cavity of an existing house is being filled, special attention should be given to the condition of the external leaf, giving its state of repair and its type of pointing as the examples. Read off that table, the answer is a combination rather than a category: this construction, at this cavity width, with this outside finish, in this zone. Published advice elsewhere adds high-rainfall areas, solid walls that have no cavity at all, and brickwork with significant cracks or damage that needs repairing first.
What are the downsides of cavity wall insulation?
The cavity exists to stop water crossing from the wet outer leaf to the dry inner one. Filling it is a deliberate change to that arrangement, which is why the regulations set conditions on it rather than waving it through. The government's own consumer guide is clear that most cavity wall insulation has been completed in suitable homes to good standards, and that genuine issues have arisen in a small number of instances. The categories people report are published on CIGA's own complaint charts: water penetration, condensation and mould, voids and unfilled areas, escaped insulation, blocked ventilators and a blocked cavity among them.
Can I get my cavity wall insulation removed for free?
Where a guarantee applies and the guarantee body decides there is a defect in the materials or the workmanship, the remedial work is carried out free of charge. CIGA's published definition of a complex case names total cavity clearance with a drying-out period, so extraction sits inside the scheme rather than outside it. Two catches. Having cavity wall insulation does not mean having a CIGA guarantee, and CIGA names three other providers. And CIGA no longer issues retrospective guarantees for installations carried out between 1995 and 1 January 2012 where none was bought at the time.
How can I prove I have cavity wall insulation?
The certificate is the proof, and it is the first thing to look for. If it has gone, CIGA says it needs only the property address and postcode to search its guarantee database, so you can raise a concern without the document. A duplicate certificate costs £60.00 including VAT and needs proof of ownership plus a recent utility bill. Worth knowing before you pay for one: it is not the only scheme. CIGA names Enterprise ECO, IWA ECO and Qualitymarkprotection as other guarantee providers, so an absence from CIGA's database is not proof there is no guarantee.
What happens to cavity wall insulation after 25 years?
Nothing happens to the insulation. Something happens to the paperwork. CIGA's published summary of cover says any claim under the guarantee must be notified within 25 years of the installation date, so the 25 years is a notification window and not a service life. The material is not on a timer. The claim is. That matters most to the people this catches, which is anyone who bought a house with a filled cavity some years into somebody else's guarantee, because the guarantee transfers with the property and the clock does not restart when you move in.