Damp, Condensation & Mould Hub
Damp, condensation and mould are the symptoms most homeowners search for — and the ones most often mis-treated. They are usually one connected problem: humid air meeting cold surfaces that are too poorly ventilated to dry.
Book a Diagnostic SurveyMould and condensation appear where a surface falls below the dew point of humid indoor air. So the cure is rarely a chemical treatment — it is warmer surfaces and controlled ventilation, identified by measuring moisture, surface temperature and air change.
Recommended diagnosis: Damp & Moisture Investigation
Condensation & Moisture
All 12 guidesWhy do I have condensation on my windows?
Condensation on windows is the visible result of two things meeting: humid indoor air and a cold glass surface below its dew point. It is a building-physics problem with a clear cause — and a clear, lasting fix.
Read the guideWhy are my windows wet in the morning?
Windows that are wet every morning are showing overnight condensation. While you sleep, the glass cools to its lowest and you add moisture to the air by breathing — so by dawn the air at the glass tips past its dew point.
Read the guideWhy is there moisture on my walls?
Moisture on a wall has three possible mechanisms — condensation, penetrating damp or rising damp — and they look similar but need completely different remedies. Diagnosing which one you have is the single most important step.
Read the guideWhy is condensation worse in winter?
Condensation gets worse in winter for two reasons working together: surfaces are much colder, and homes are closed up so indoor humidity rises. Both push the air at cold surfaces past its dew point.
Read the guideHow do I stop condensation in my home?
Stopping condensation for good means tackling the two things that cause it together: too much moisture in the air and surfaces cold enough for that moisture to condense on. Wiping windows, running a dehumidifier or opening a window now and then treats the symptom briefly. The lasting fix is to lower indoor humidity with controlled ventilation and to warm the cold surfaces, so the air no longer reaches its dew point indoors.
Read the guideWhy do I get condensation in my bedroom?
Bedrooms are one of the most condensation-prone rooms because, overnight, three things combine: we add a surprising amount of moisture to the air just by breathing and perspiring, the room cools as the heating goes off, and the door and windows are usually shut so the moist air cannot escape. Humid air then meets the cold windows and walls and condenses, which is why you so often wake to wet windows and damp patches. It is a balance of moisture, temperature and ventilation, and each can be adjusted.
Read the guideDoes drying clothes indoors cause condensation?
Yes — drying clothes indoors is one of the biggest hidden sources of moisture in a home, and a very common cause of condensation, damp and mould. A single load of wet washing releases a couple of litres of water into the air as it dries, and if that moisture is not ventilated away it raises humidity throughout the home and condenses on the coldest surfaces. It does not mean you cannot dry clothes inside, but it does mean doing so needs ventilation, or the moisture will show up as condensation elsewhere.
Read the guideWhy do I get condensation in an unused or cold room?
An unused or unheated room often gets condensation precisely because it is cold: its surfaces sit well below the temperature of the rest of the home, while moist air from the heated, occupied rooms migrates into it and condenses on those cold walls and windows. Closing the door and turning off the heating to 'save energy' tends to make it worse, not better, because it creates the coldest surfaces in the house with no ventilation to remove the moisture that still finds its way in. The cure is gentle warmth and ventilation, not closing the room off.
Read the guideWhy has condensation suddenly got worse in my house?
When condensation suddenly gets worse, something in the balance that controls it has changed. Condensation forms when humid indoor air meets surfaces below the dew point, so a sudden increase means either there is more moisture in the air, the surfaces have got colder, or the ventilation that used to remove the moisture has reduced — and often a specific, identifiable change is behind it. Colder weather, a new household routine, a broken extractor, draught-proofing or new windows, or a recent retrofit can each tip the balance. Rather than treating the symptom on the glass, the useful question is what changed, because finding the trigger points straight to the fix.
Read the guideHow do I stop condensation in my bathroom?
Bathroom condensation is one of the most common and most solvable damp problems, because the moisture has a clear source — showering and bathing — and the answer is to remove it at source before it spreads and condenses on the cold walls, ceiling, mirror and window. The key is effective extraction that actually clears the steam, run for long enough, combined with keeping the surfaces warm enough not to condense. Where a bathroom keeps misting up and growing mould, it is usually because the extraction is too weak, badly placed or not run long enough, the room is cold, or moist air is escaping into the rest of the home — all of which can be put right.
Read the guideWhy is there condensation on my walls and ceiling?
Condensation on walls and ceilings — not just windows — is a sign that those surfaces are cold enough to fall below the dew point of the indoor air, and that the air is humid enough to deposit moisture on them. It is more serious than window condensation because walls and ceilings are absorbent and harder to wipe, so the moisture soaks in and feeds mould. It points to two things together: surfaces that are too cold (from missing insulation, thermal bridges or unheated rooms) and humidity that is too high (from too little ventilation). The fix is to warm the surfaces and lower the humidity, not to treat the wall as 'damp'.
Read the guideWhy do my windows only get condensation in some rooms?
Condensation appearing on the windows of some rooms but not others is normal and informative: it reflects differences between those rooms in moisture generation, heating and ventilation. The rooms that mist up are usually those that are more humid (bedrooms, where breathing overnight adds moisture; bathrooms and kitchens, where water vapour is generated), cooler (so the glass is colder), or less ventilated (so the moisture isn't removed). The dry rooms simply have a better balance of those three factors. Reading which rooms condense, and why, points straight to the fix — usually more ventilation and warmth in the affected rooms.
Read the guideMould Problems
All 13 guidesWhy does mould keep coming back?
Mould keeps coming back because cleaning and painting over it treats the symptom, not the cause. Mould grows wherever a surface stays cool and humid for long enough — so until those conditions change, it returns.
Read the guideWhy is there mould behind furniture?
Mould behind a wardrobe or sofa is a classic condensation pattern: a cold external wall, trapped air with no movement, and indoor humidity that climbs against the cold surface until mould takes hold.
Read the guideWhy is there mould in my bedroom?
Bedrooms are one of the most common places for mould because they combine a high overnight moisture load, cold surfaces and often poor ventilation — the exact conditions condensation and mould need.
Read the guideIs black mould caused by condensation, damp or ventilation?
Black mould is almost always a condensation problem — humid indoor air meeting cold surfaces — made worse by inadequate ventilation. Genuine penetrating or rising damp is a far less common cause, though it does happen. Telling them apart is the key.
Read the guideWhy is there mould in my bathroom?
Bathrooms are the highest-moisture room in most homes, so when that moisture is not removed quickly by ventilation it condenses on cold surfaces — the ceiling, the wall above the tiles, the corners and the sealant — and mould follows. Bathroom mould is rarely a cleaning problem; it is a moisture-and-ventilation problem. Stopping it means removing the steam at source and keeping surfaces warm enough not to condense.
Read the guideWhy is there mould on my ceiling?
Mould on a ceiling is usually condensation: the ceiling runs cold — often because the loft or roof above is poorly insulated or air leakage chills it — and humid room air condenses on it, especially in the corners and along the cold perimeter. It can also follow moisture from a room or roof above. Either way, it is a building-physics problem about cold surfaces and moisture, not just a stain to paint over.
Read the guideWhy is there mould on my clothes and belongings?
Mould appearing on clothes, in wardrobes, on shoes and on stored belongings means the air around them is humid and the surfaces are cold and still — usually in a wardrobe against a cold external wall, or in an unheated, poorly ventilated room. The items are not the problem; they are sitting in exactly the conditions mould needs. The fix is to lower the humidity, warm the cold surface and let air move around the belongings.
Read the guideIs black mould in my home dangerous to health?
Black mould can affect health — it can trigger or worsen respiratory problems, allergies and irritation, and the risk is greater for babies, the elderly, and anyone with asthma or a weakened immune system — so it should not be ignored. But cleaning it off treats only the symptom: mould grows because a surface is repeatedly damp, so the lasting protection for your health is to remove the moisture that feeds it. Understanding both the health risk and the real cause is what lets you deal with mould properly rather than fighting it forever.
Read the guideWhy is there mould around my windows?
Mould around windows is almost always caused by condensation: the glass, frames and reveals are the coldest surfaces in the room, so moist indoor air condenses on them, the water runs down onto the sills, frames and surrounding plaster, and those repeatedly damp surfaces grow mould. The reveals and corners around the window are particularly prone because they are colder still and often poorly ventilated behind blinds and curtains. It is a cold-surface and humidity problem, fixed by warming the surfaces and managing moisture, not by cleaning alone.
Read the guideIs mould in my rented home the landlord's responsibility?
Mould in a rented home is, in most cases where it stems from the condition of the building, the landlord's responsibility — but the question turns on what is actually causing it, and that is where tenants and landlords often disagree. If the mould results from a building defect or from inadequate provision the landlord is responsible for — cold, uninsulated walls causing condensation, a leak, or ventilation that does not work — then it is generally the landlord's to remedy. Landlords frequently attribute it instead to 'lifestyle'. The way to resolve that is independent, measured evidence of the cause: showing whether the building, not the occupant's normal use, is producing the conditions in which mould grows.
Read the guideWhy is there mould in my wardrobe or cupboard?
Mould in a wardrobe or cupboard almost always forms for the same reason: it is a still, unventilated pocket of air pressed against a cold external wall, so moist air condenses on the cold surface behind it and mould grows on the back panel, the wall and your belongings. The contents block air movement and trap humidity, while the wall behind — usually an uninsulated external wall — stays cold enough to condense. It is a localised condensation problem, and it is solved by warming that cold surface, letting air move behind the furniture, and reducing the room's humidity, rather than by repeatedly cleaning the mould off.
Read the guideHow do I get rid of mould permanently?
Getting rid of mould permanently means removing the conditions it needs to grow, not just cleaning what you can see. Mould needs a surface that is cold and humid enough for moisture to settle on it; cleaning or painting over it treats the symptom, so it returns within weeks or months because the cold, damp surface is still there. The permanent cure is to warm the surface (so it stays above the dew point) and lower the indoor humidity (so moisture isn't deposited) — usually a combination of insulation, heating and ventilation. Clean the mould, yes, but only as the last step after you have removed the cause.
Read the guideWhy does mould grow in the corner of my room?
Mould forms in the corner of a room because corners are colder than the flat walls around them — they are a thermal bridge — so they fall below the dew point of the room air first and stay humid enough for mould to grow. An external corner has more outside surface losing heat than inside surface to be warmed, and air circulation is poorest right in the corner, so it is consistently the coldest, least-ventilated spot in the room. Add normal indoor humidity and you get the classic patch of mould in the top or bottom corner of an external wall. The fix is to warm that corner and lower the humidity.
Read the guideDamp Problems
All 15 guidesWhat is the difference between condensation, rising and penetrating damp?
Damp is not one problem — it is three. Condensation, penetrating damp and rising damp produce similar-looking stains, smells and mould, but each is driven by entirely different building physics, and each needs a completely different fix. Getting the diagnosis right is the single most important decision you will make, because the cure for one type does nothing for the others.
Read the guideWhy is there damp on my walls?
Damp on a wall is a signal that moisture is reaching the surface and staying there long enough to darken plaster, lift paint or grow mould. In most occupied homes the cause is condensation on a cold wall, but penetrating and — less often — rising damp produce their own distinctive patterns. Reading where and when the damp appears is the first step to fixing it properly.
Read the guideHow do I find the cause of damp?
Finding the cause of damp is a diagnostic process, not a guess. Because condensation, penetrating and rising damp look alike but need opposite cures, the goal is to read the clues your home is giving you and, where it matters, to measure rather than assume. This guide explains the systematic approach a building physicist uses — and how you can begin it yourself.
Read the guideDo I need a damp survey or a building investigation?
If you have damp, you will be offered a 'damp survey' — but these vary enormously, and many are really sales visits for a particular treatment. A building performance investigation is a different exercise: it measures the physics to identify the true cause before anyone recommends a remedy. Knowing which you need can be the difference between solving the problem and paying for work that cannot succeed.
Read the guideHow do I get rid of damp in my house?
Getting rid of damp permanently depends entirely on first knowing which damp you have, because condensation, penetrating damp and rising damp arrive by different routes and need opposite fixes. Treating the wrong one wastes money and leaves the damp in place. The reliable path is to diagnose the moisture source, then apply the matching remedy — ventilation and warmer surfaces for condensation, external repair for penetration, and only a damp-proof course where rising damp is genuinely confirmed.
Read the guideWhy is there damp at the bottom of my walls?
Damp at the bottom of a wall is one of the most misdiagnosed problems in housing, because several very different causes produce the same low-level damp band — true rising damp, penetrating damp, a bridged damp-proof course, high external ground levels, a plumbing leak, or even condensation on a cold low surface. They look alike but need completely different remedies, so the single most important step is to identify which one you have before any treatment, rather than assuming 'rising damp' and applying an injected chemical course that may do nothing.
Read the guideWhy is my chimney breast damp?
A damp chimney breast usually comes from one of three causes specific to chimneys: rain getting in at the top of the stack and tracking down, hygroscopic salts in the brickwork left by decades of combustion that draw moisture from the air, or condensation inside a flue that has been blocked up without ventilation. Often more than one is at work. Because the chimney is a tall, exposed, salt-laden structure passing through the house, it behaves differently from an ordinary wall, so its damp needs diagnosing accordingly.
Read the guideWhy is there damp in my bathroom or around the shower?
Damp in a bathroom or around the shower is usually one of two quite different things: water actually leaking through failed seals, grout or tiling into the wall and floor, or condensation forming because the room is humid and poorly ventilated. They need opposite responses — a leak must be found and sealed, while condensation needs better ventilation and warmer surfaces — so the key is to work out which you have. Because bathrooms generate so much moisture, the two can also occur together, which is why diagnosis matters.
Read the guideIs rising damp real, and do I need a damp-proof course?
Rising damp is real — ground moisture can rise through masonry by capillary action where there is no working damp-proof course — but it is far less common than the volume of 'rising damp' diagnoses and injected damp-proof courses suggests. A great many homes treated for rising damp actually had penetrating damp, a bridged DPC, high ground levels, a leak or condensation, none of which an injected course addresses. So the honest answer is: rising damp exists, but you should only treat it once it has genuinely been confirmed.
Read the guideWhy is there damp coming through my wall?
Damp appearing on the inside of a wall — a patch that darkens, feels wet, or blisters the plaster and paint — is most often penetrating damp: water finding its way through the external wall from outside, usually because of a defect that lets rain in. It is a different problem from condensation, which forms on cold surfaces from indoor humidity, and from rising damp, which comes up from the ground; and because the cause is an external fault, the cure is to find and fix that fault, not to treat the inside surface. The damp patch is a clue to where water is getting in. Tracing it to the actual defect is what allows a lasting repair rather than a cosmetic cover-up that returns with the next rain.
Read the guideShould I be worried about damp when buying a house?
Damp in a house you are buying is worth understanding rather than panicking over — what matters is what is causing it and what it will cost to fix, not the word 'damp' itself. Much of the damp found in homes is condensation, which is manageable and relatively inexpensive to resolve; some is penetrating or rising damp from a defect, which ranges from a simple repair to a significant job; and occasionally it signals a more serious underlying problem. A mortgage valuation and a standard homebuyer survey often flag damp without diagnosing it, which can either scare buyers off a sound house or hide a costlier issue. Getting the cause properly assessed before you commit lets you decide, negotiate or budget with confidence.
Read the guideWhy is my garage or outbuilding damp?
Garages and outbuildings are commonly damp because they combine cold, uninsulated surfaces, often a bare or unsealed floor on the ground, and little or no ventilation or heating — the perfect conditions for condensation and ground moisture. Unlike a heated, ventilated home, a garage stays cold, so moist air readily condenses on its cold walls, floor and the underside of the roof, and any moisture rising from the ground or entering through the structure has no easy way to dry out. The result is a damp, sometimes musty space that can rust tools, spoil stored items and grow mould. Keeping it dry means understanding which source dominates — condensation, ground moisture or ingress — and addressing it.
Read the guideWhy is my internal wall damp?
Damp on an internal wall — one inside the house, not an external wall — is often puzzling, because it cannot be rain getting in directly. The usual causes are condensation on a cold surface, a plumbing or waste leak within or behind the wall, moisture tracking from an adjacent external wall or chimney, or bridging from a wet floor or raised ground. Each leaves different clues and needs a different fix, so the key is to diagnose before treating. What it is almost never is 'rising damp' in the sense the treatment industry sells — and an injected chemical course in an internal wall is one of the more common wasted spends.
Read the guideDo I need a damp-proof membrane?
A damp-proof membrane — a sheet or coating that blocks moisture — is the right answer in some situations and exactly the wrong one in others, so the question is really about diagnosis. Membranes make sense where you must manage water you cannot stop, such as a cellar or a below-ground wall, where a cavity-drain membrane controls and channels it. But applied to an ordinary solid wall suffering condensation or misdiagnosed 'rising damp', a membrane or tanking simply traps moisture in the wall, hides the symptom temporarily, and often pushes the damp elsewhere. Whether you need a membrane depends entirely on what the moisture is and where it comes from — which means measuring before sealing.
Read the guideWhy is my solid floor damp?
Damp in a solid concrete floor usually comes from one of three things: ground moisture rising through a missing, damaged or bridged damp-proof membrane; condensation forming on the cold floor surface; or a leak from plumbing or outside. Older solid floors were often laid without an effective membrane, so ground moisture rises through the slab and shows as damp, cold patches or as moisture trapped under an impervious floor covering. Newer floors should have a membrane, but it can be punctured or bridged. Because the cure differs completely — from improving ventilation to relaying the floor — the cause must be diagnosed before any expensive work.
Read the guideVentilation & Indoor Air Quality
All 13 guidesWhy is my house so stuffy?
A stuffy home is one where the air is not being changed often enough. Carbon dioxide from breathing builds up, moisture and smells linger, and the air feels heavy and stale — all signs that fresh air is not arriving and used air is not leaving at the rate the household needs. Stuffiness is rarely about temperature; it is about ventilation, and it is very fixable once you understand why the air is not moving.
Read the guideWhat are the signs of poor ventilation?
Poor ventilation rarely announces itself directly — instead it shows up as a cluster of everyday symptoms: misting windows, stuffy rooms, lingering smells, recurring mould and air that only feels fresh when a window is open. Recognising these signs early matters, because they point to a home that is not changing its air often enough to stay healthy and dry.
Read the guideHow do I improve indoor air quality at home?
Improving indoor air quality is not about masking smells or running an air purifier — it is about controlling the things that pollute indoor air at source and ventilating the home properly so they are removed. Moisture, carbon dioxide, fine particles and chemical pollutants each have a source and a remedy, and the most reliable way to improve the air is to measure what is actually in it and ventilate accordingly.
Read the guideDo I need MVHR or will extract fans do?
Choosing between MVHR and extract fans is one of the most common ventilation questions — and the honest answer is that it depends on how airtight your home is and what you need the ventilation to do. Extract fans remove moisture at source; MVHR supplies filtered fresh air everywhere and recovers heat. Understanding what each does, and matching it to your home, is the key to ventilating well without wasting money or heat.
Read the guideWhat humidity should my house be?
A healthy home generally sits in a moderate band of relative humidity — broadly around the middle of the scale, neither persistently damp nor uncomfortably dry. When indoor humidity stays too high, it feeds condensation, mould and dust mites; too low and the air feels dry. But the number itself matters less than what it tells you: persistently high humidity is a signal that the home is generating more moisture than its ventilation can remove.
Read the guideWhy isn't my extractor fan clearing the steam?
An extractor fan that runs but leaves the room full of steam is usually not removing anywhere near the air it should — because it is underpowered for the room, badly ducted, poorly positioned, or simply not run long enough. A fan spinning is not the same as a fan extracting. If the steam lingers, the moisture stays in the home and feeds condensation and mould, so it is worth finding why the extract is failing rather than assuming the fan is doing its job.
Read the guideDo I need to open windows if I have MVHR?
With a properly designed and commissioned MVHR (mechanical ventilation with heat recovery) system, you do not need to open windows for fresh air — that is exactly what the system provides, continuously and to every room, while recovering most of the heat from the air it extracts. Opening windows simply bypasses the heat recovery and lets that warmth escape. There are times when opening windows is still useful — for summer cooling, or to clear a one-off strong smell — but for everyday fresh air and moisture removal, MVHR is designed to do the job with the windows shut.
Read the guideWhy is my house too dry in winter?
A home often feels dry in winter for a simple, physical reason: cold outdoor air holds very little moisture, and when that air comes inside and is heated, its relative humidity falls sharply, leaving the indoor air dry. The colder it is outside, the more pronounced the effect. It is the reverse of the summer/condensation problem and usually nothing is wrong — but if the air is uncomfortably dry, the answer is gentle humidity management, not turning off ventilation, which would simply trade dryness for condensation.
Read the guideDo I need a PIV unit to stop condensation and mould?
A positive input ventilation (PIV) unit can help reduce condensation and mould in the right home, but it is not a universal cure, and fitting one before understanding the cause often disappoints. A PIV works by gently introducing filtered air, usually from the loft, into the home to slightly pressurise it and dilute and displace the humid internal air. That can lower whole-house humidity effectively where the problem is generally damp, stale air across the dwelling. But where the mould is driven by a specific cold surface, a localised moisture source, a leak or inadequate extraction at source, a PIV may make little difference. Whether you need one depends on diagnosing why the home is humid and whether the building suits positive input ventilation.
Read the guideWhy is my MVHR not working properly?
When a mechanical ventilation with heat recovery (MVHR) system does not seem to be working — the home feels stuffy or humid, there is condensation, or the unit is noisy or draughty — the cause is rarely a faulty unit. Far more often the system was never properly commissioned, is out of balance, has blocked filters or ducts, or was poorly designed and installed. An MVHR only delivers good air quality and heat recovery if it is set up to move the right amount of air to and from each room and kept maintained; without that, even a good unit underperforms. Diagnosing which of these is at fault — commissioning, balance, blockage, design or maintenance — is what gets the system actually working.
Read the guideDo I need mechanical ventilation in my home?
Whether your home needs mechanical ventilation depends on how much moisture it produces, how airtight it is, and how well it can ventilate naturally. As homes are sealed up to save energy, the accidental draughts that used to carry away moisture and stale air disappear — so a tighter home increasingly needs deliberate, mechanical ventilation to stay healthy. The right answer ranges from improved extract fans, to a positive-input ventilation (PIV) unit, to whole-house mechanical ventilation with heat recovery (MVHR), and depends on measuring the home's actual conditions rather than guessing. The aim is controlled, adequate ventilation that removes moisture and pollutants without throwing away heat.
Read the guideWhy is the air in my bedroom stale in the morning?
Waking to stale, stuffy bedroom air is almost always a sign that the room was under-ventilated overnight — carbon dioxide and moisture from your breathing built up in a closed room faster than the small amount of air change could remove them. A sleeping person exhales CO₂ and water vapour continuously, and in a modern, fairly airtight bedroom with the door and windows shut and no working trickle vent or fan, those levels climb through the night, leaving the air feeling stale and humid by morning — and often misting the windows. It is a ventilation problem with a ventilation solution, and it matters because high overnight CO₂ is linked to poorer sleep quality.
Read the guideHow do I ventilate a home without losing heat?
Ventilating a home without losing heat seems contradictory — fresh air comes in cold and warm air goes out — but it is exactly what controlled ventilation, and especially heat recovery, is designed to solve. The trick is to stop ventilating by accident, through draughts and open windows that dump heat uncontrollably, and instead ventilate deliberately at the right rate, recovering the heat from the air you expel. In a leaky home the priority is to seal the uncontrolled leakage and provide good extract; in an airtight home, mechanical ventilation with heat recovery (MVHR) supplies fresh air and reclaims most of the warmth from the stale air leaving. Either way, the answer is controlled ventilation, not no ventilation.
Read the guide