Condensation & damp explained
The damp started after we had work done
New windows, insulation, a loft conversion, a new kitchen. It is not a coincidence, and in most cases it is not a botched job either — every one of those improvements either removed ventilation the house was relying on, or moved the cold surfaces to somewhere new. Each trade did their own work properly. Nobody was responsible for the air.
An old house was not designed to be airtight. It was designed around a continuous, accidental air change that nobody ever thought of as ventilation: open flues in every room, sash windows that never quite shut, floorboards over a ventilated void, a back door that whistled. All of that carried moisture out of the building day and night, and it did it whether anyone remembered or not.
Every sensible thing done to a house since has closed one of those routes. That is not a criticism — the house is warmer, quieter and cheaper to run, and none of those improvements should be undone. But the moisture your household produces has not changed. Ten to fifteen litres a day for a family of four, exactly as before, into a building that now has nowhere to put it.
So the damp is not a sign the work was done badly. It is a sign the work was done without anybody being responsible for the air. There is no trade whose job that is. The window fitter fits windows, the insulation installer insulates, the loft company builds the room, and every one of them did what they were asked correctly.
What each job actually changed
Find the one that matches your timeline. If two of them do, that is usually the answer.
New windows
The old ones leaked continuously around the frames and the sashes, and that leak was background ventilation nobody had to think about. A modern sealed unit does not. Where the replacements went in without trickle vents — or with them, but painted over, taped up or never opened — the room lost its air supply on the day of the swap. The principle in the guidance is that a replacement should not leave a room with less background ventilation than it had before, so it is worth asking your installer what was provided. This is the single most common item on this list.
A capped or blocked chimney
An open flue is a permanently running extract in that room, twenty-four hours a day, for free. Capping it is the right thing to do for heat loss and for keeping the rain out, and it is genuinely worth doing — but it removes a substantial and continuous air change from the room that most often turns out to be the one with the mould in it. Where a chimney is capped it should be capped with a vented cowl and left with an airbrick into the room, so the flue itself can still breathe.
A loft conversion
The biggest single change on this list, and the one that most reliably causes a problem the following winter. A cold, freely ventilated roof space becomes a heated, occupied, insulated room — so the whole thermal line of the building moves, the eaves ventilation path is usually built over, and the house gains a bedroom and very often a shower room with no external wall behind it. That last detail decides what the ventilation job costs, and almost nobody is told about it in advance.
Loft insulation topped up
More insulation at ceiling level makes the loft above it colder, which is the point. It also means any warm moist air leaking up from the house — around a hatch, through downlight holes, or from a bathroom fan discharging into the roof space — now condenses on a colder surface than it used to. And when insulation is pushed out into the eaves it blocks the ventilation path the roof relies on. Both show up as damp in the loft rather than in the rooms, so people find them late.
Wall insulation, internal or external
Done well it raises the surface temperature of the wall and makes condensation less likely, which is the intended result. What it can do, if the detailing at the edges is not right, is move the problem to the junctions it cannot cover — window reveals, the joint between wall and ceiling, the line where a floor meets an outside wall. Mould that appears in a clean straight line around the edge of a room after insulation is usually this.
A new kitchen
The one nobody suspects. A great many kitchen refits replace a hood that ducted outside with a recirculating one that does not, because it is easier to install and it looks better. A recirculating hood filters grease and smell and returns every bit of the steam to the room. An uncovered pan on a rolling boil is around a litre of water an hour, and the kitchen has just lost the only thing that was removing it.
An extension
What used to be an external wall is now an internal one, which changes where the cold surfaces are and often leaves an awkward cold junction where old meets new. Extensions also commonly take the airbricks or the through-draught that the original layout depended on, and the room they open into gets deeper and further from any window.
Draught-proofing and a new front door
Individually small, collectively significant. Letterbox brushes, draught excluders, sealed floorboards, a solid composite door where a rattly timber one used to be. Each is sensible and each closes a route. It is very common for a house to have had no single major job done and still to have quietly lost most of its air change over fifteen years of small improvements.
Why nobody caught it at the time
Building regulations do cover ventilation, and a good deal of this work does trigger a requirement — but three things mean it very often goes through anyway:
- Much of the work is not inspected. Replacement windows, a kitchen refit, draught-proofing and topping up loft insulation typically do not bring an inspector to the house, so nobody stands in the room and asks where the air is coming from.
- Sign-off is not the same as measurement. Where ventilation is specified, it is frequently signed off on the basis that a fan of the right rating was installed — not on the basis that somebody put a meter on the grille and read the rate it actually delivers. Those two things are routinely different by a factor of two.
- The effect arrives a season later. Work finished in June produces mould in January, by which point the connection is not obvious to anybody and the trade who did it is long gone.
That gap is not really anybody’s fault either. It is just that air is the one part of a building nobody owns, and it is the part that only announces itself once the weather turns.
The good news, and it is genuine
A problem with a date on it is much easier to solve than one that has always been there. If you can say it started the winter after the loft went in, or we had never had it until the windows were done, you have already narrowed it further than any instrument will — you know what changed, so the survey is confirming a hypothesis rather than hunting.
It also usually means the fix is proportionate. Where a specific route was closed, the answer is often to reopen a controlled version of it: trickle vents that work, a vented cap on a capped flue, an airbrick reinstated, an extract that actually extracts. Where the whole house has quietly tightened up over a decade of improvements, continuous ventilation with heat recovery is what keeps the warmth you paid for and takes the moisture out anyway — which is the point of doing it properly rather than opening a window.
What we will not do is tell you to undo the work. The insulation, the windows and the loft room were all worth having. They just need the other half of the job finishing.
Start with what you can actually see
Most people arrive here with a symptom rather than a diagnosis, and the symptom is usually enough to narrow it down. Each of these explains what is happening and what genuinely fixes it — no form, no email address, just the answer.
My windows are wet every morning
Water running down the glass from October to March, and pooling on the sill. What it means, when it is ordinary and when it is not — and why the glass is the only surface in the room honest enough to show you.
Read the explanationThe mould keeps coming back
You cleaned it, you sprayed it, you painted over it, and six weeks later it returned in the same shape. Why that shape is the diagnosis, why anti-mould paint runs out, and how to tell which of the three damps you have.
Read the explanationThere is mould in the bathroom, and a fan
A fan running is not a fan working. The eight things that go wrong — starting with the one you can check yourself today with a torch and a loft hatch, which is the worst of them and the most common.
Read the explanationIt started after we had work done
New windows, insulation, a loft conversion, a new kitchen. Not a coincidence and usually not a botched job — every trade did their own work correctly, and nobody was responsible for the air.
Read the explanationQuestions people ask after building work
It is extremely common, and it usually means the new windows are doing their job. The old ones leaked air continuously around the frames and sashes, and that leak was ventilation nobody thought of as ventilation. Seal it and the moisture your household was already producing has nowhere to go, so it starts appearing on the coldest surface in the room. Slightly counter-intuitively, the glass itself is now warmer than it was, so you may find the condensation has moved off the window and onto the reveals, the corners or the wall behind furniture — which is worse, because those absorb it and grow mould rather than just showing you a puddle. The fix is to restore controlled background ventilation, not to regret the windows.
The principle in the guidance is that replacing windows should not leave a room with less background ventilation than it had before, so where the original windows had vents the replacements are expected to have them, and fitting them is the normal expectation on a modern replacement. It is worth asking your installer directly what was provided and what the original had. Whichever the answer, the practical point stands on its own: if your windows have no trickle vents, or they have them and they are painted shut, taped over or simply never opened, then the rooms they serve have no background air supply. That is a cheap thing to establish and often a cheap thing to put right.
Because it changes more about a house than any other common job. A cold, freely ventilated roof space becomes a heated, insulated, occupied room, so the whole thermal line of the building moves upward and surfaces that were never inside before now have warm moist air against them. The eaves ventilation path that used to keep the roof dry is very often built over. The house gains a bedroom — two people breathing for eight hours a night — and frequently a shower room too, usually in the middle of the plan with no external wall behind it, which rules out a simple wall fan and means an inline fan and a duct run. None of that is bad building work. It is a genuinely different building afterwards, and its ventilation needed designing rather than inheriting.
Insulation done well reduces condensation, because warmer surfaces sit further above the temperature at which moisture drops out of the air. What it can do is move the problem to the places it does not cover. Mould appearing in a clean line around window reveals, along the wall-to-ceiling joint or where a floor meets an outside wall after wall insulation is usually a cold junction the insulation could not bridge. In a loft, adding insulation makes the space above it colder, so any warm air leaking up from the house — around the hatch, through downlight holes, or from a bathroom fan that discharges into the roof — condenses more readily than before, and insulation pushed out into the eaves blocks the airflow the roof depends on. So: not a reason to avoid insulating, and a good reason to look at the air at the same time.
Yes. Fitting out a wet room triggers a requirement for mechanical extract at the rates in Approved Document F, and an openable window does not satisfy it. The gap worth knowing about is between specifying and proving: a job is very often signed off because a fan of the correct rating was installed, rather than because anyone put a meter on the grille and read what it actually delivers. Those two figures routinely differ by a factor of two once a real duct run is attached. Ask whoever did the work for the commissioning figures. If there are none, that is worth knowing before the first winter rather than after it.
That depends entirely on what was specified, what was signed off and who agreed to what, and it is not something a ventilation contractor should be advising you on. What we can tell you is what is actually happening, in writing, with readings and dated photographs — which room is producing the moisture, what the existing extract genuinely moves against what it should, and which surfaces are cold enough to condense on. That report is useful whoever ends up paying for the remedy, and it is a far better starting point for a conversation with a builder than an argument about whose fault the mould is. In a great many cases it also shows the work itself was fine and something separate needs adding, which is a much easier conversation for everybody.
No, and we would talk you out of it. The insulation, the windows, the draught-proofing and the loft room are all worth having, they are all making the house cheaper and more comfortable to run, and removing them to solve a moisture problem would be an expensive way to make a worse house. What is missing is the other half of the job. A tighter house needs deliberate ventilation instead of accidental ventilation, and done properly that means controlled, continuous air change at a few watts — with heat recovery, if it is worth keeping the warmth you have just spent money trapping.
Not sure where to start?
Send us a message with what you have noticed at home. We will tell you honestly whether you need a survey, a system, or neither.
Or see what it costs — unlike most in this trade, we publish our prices.
Trained on the systems we install
Certificated — and here they are
Most trade sites show you a row of logos. These are the actual certificates, held by this business. Tap either one to read it, or download the PDF.
- 12-month workmanship guarantee On top of the manufacturer’s warranty, which we register in your name. If anything we installed fails because of the way we installed it, we come back and put it right. What it covers.
- Company no. 12092552 Registered in England & Wales. Trading as Cavalli Ventilation since July 2019, and in ventilation since 2003.
- Airflow measured on every install A calibrated anemometer at the grille, recorded and handed over. The only figure that counts is the one measured in your room.