Condensation, Damp or Mould? How to Tell

Condensation, penetrating damp, rising damp and plumbing leaks all get called damp, yet each needs a different fix. This guide gives a step-by-step check, a comparison of the four, what humidity readings can and cannot tell you, which professional to call and the records worth keeping once it is fixed.

Condensation, penetrating damp, rising damp and plumbing leaks all get called damp, yet each needs a different fix. Condensation is the most common, so it is the place to start. No single sign proves a cause, though: where the damp shows, when it gets worse and what it looks like point to the answer together.

There is a black patch in the corner of the bedroom, or a stain spreading up the wall behind the sofa. Treating one problem as another is how people end up paying four figures for work that was never going to fix anything, while the actual cause carries on quietly.

So the first job is not to fix it. It is to work out which one you have.

The short version

Condensation is the most common cause of damp and mould in homes and usually the cheapest to deal with.
Penetrating damp and leaks come from a defect, such as a gutter, roof, seal or pipe; the fix is the defect rather than the wall.
Rising damp is often misdiagnosed; RICS says failure of an original damp-proof course is extremely rare.
No single symptom proves a cause: where it shows, its pattern and what makes it worse point to the answer together.
Mould is a symptom, so cleaning it without changing the conditions only buys a few weeks.
An independent surveyor who does not sell treatments is the safest first opinion when the cause is not obvious.
In England, Awaab's Law has applied to social housing since 27 October 2025; no date has been set for private renting.

The four things people call damp

Type	Where it shows	Pattern and what makes it worse	Likely fix	Who to call
Condensation	Windows, cold corners, behind furniture, less-heated rooms; black spot mould	Worst in winter and after showers, cooking and drying washing indoors	More ventilation, steadier heating, less moisture at source	Usually nobody; a surveyor if it persists after the cheap fixes
Penetrating damp	A defined patch at any height, usually on an outside wall	Appears or darkens in and after rain, dries back in dry spells	Repair the defect outside: gutters, pointing, render, roof, seals	A roofer or builder; a surveyor if the source is unclear
Rising damp	Ground floor walls only, an even tide mark, salts, crumbling plaster	Fairly constant, not tied to the weather	Remove any bridging first; treatment only after independent diagnosis	An independent building surveyor
Leaks	Ceilings under bathrooms, around pipes and radiators, under sinks	Grows when water is used, at any time of year	Fix the pipe, waste, seal or fitting	A plumber; an electrician too if water is near electrics

The two questions that do most of the work are how high and when. Damp that stops at a consistent height on a ground floor wall behaves differently from a patch that appears at first floor level after a storm. Both behave differently from mould that comes back every November in the same cold corner.

Two causes can also work together. A wall made damp from outside is colder than it should be, so it attracts condensation as well.

Which one is it? A step-by-step check

Work through these in order. Each answer narrows it down, but none of them proves a cause on its own. If two point different ways, treat that as a reason to get an independent opinion rather than to pick the cheaper answer.

If water collects on the inside of windows on cold mornings and black spots appear in corners, behind furniture or in rooms you heat less, start with condensation.
If a patch appears or darkens during or after rain and dries back in dry spells, look outside for penetrating damp: gutters, downpipes, pointing, render, roof tiles, flashing and window seals.
If a stain is on a ceiling under a bathroom, kitchen or tank or grows when water is used upstairs, suspect a leak from a pipe, a waste, a seal or a shower tray, whatever the weather.
If water is coming through near a light fitting or socket, do not touch the switches. Turn the power off at the consumer unit if you can do it safely and call an electrician as well as a plumber.
If the damp is on a ground floor wall only, with a fairly even tide mark rarely much above a metre, white salts and crumbling plaster, rising damp is possible. Check first for bridging, such as a raised flower bed, path or render above the damp-proof course. Look for leaking pipes low down too.
If it is upstairs, high on a wall or changes with the weather, it is not rising damp, whatever a damp meter reads.
If it returns to the same cold spot every winter and improves when you ventilate and heat the room, it is very likely condensation.
If the two week test below changes nothing, or the signs point more than one way, stop guessing and get an independent survey.

Warning signs and where they show

The signs

A musty smell, often the first clue, especially in cupboards and behind furniture.
Black or grey spots of mould.
Water running down windows and cold surfaces.
Peeling wallpaper, bubbling paint or plaster that sounds hollow or crumbles.
A tide mark or white powdery salts on a wall.
Brown or yellow stains on a ceiling.
Skirting boards or floorboards that feel soft, look swollen or smell of rot.

What the location suggests

Where it shows	Usually points to	Check first
Window reveals, corners where wall meets ceiling, behind wardrobes	Condensation on cold surfaces	Ventilation, heating, furniture against outside walls
Bathroom walls and ceiling	Condensation from steam, or a failed seal if it hugs the bath or tray	The extractor fan and the sealant
A ceiling under a bathroom or tank, around pipes, under sinks	A leak	Pipes, wastes, tray seals, radiator valves
Top floor ceilings, around a chimney breast, where roof meets wall	Roof, flashing, chimney or gutters	Look from the ground in rain; a roofer for anything higher
Beside a downpipe, under a windowsill, on a wall facing driving rain	Penetrating damp	Gutters, downpipes, pointing, render, sills
Low on a ground floor wall	Rising damp, bridging or a leak	Ground levels and paths outside, pipes along the wall

These are clues, not verdicts. A stain under a bathroom can be condensation dripping from a cold pipe as easily as a leak, which is why the pattern over a few weeks matters as much as the spot.

Condensation, which is nearly always the answer

Warm air holds more moisture than cold air. When warm, moist indoor air touches a cold surface, it cools and lets go of that moisture as water. That is condensation and it is the most common cause of damp and mould in British homes.

A modern home produces a surprising amount of moisture: showering, cooking, drying clothes indoors, even breathing. In summer it ventilates away without anyone noticing. In winter, with the windows shut and the walls cold, it lands on the coldest surface it can find.

That is why condensation shows up where it does: window reveals, the corners of external walls, behind wardrobes pushed flat against an outside wall and in rooms that are heated less than the rest of the house. The black spotted mould that follows is a consequence, not a cause.

It is also why condensation responds to things that cost very little. More ventilation, steadier heating and stopping moisture at the source do more than any treatment applied to the wall.

What humidity readings can tell you

A cheap digital hygrometer shows the relative humidity in a room. It is a useful clue rather than a diagnosis. The government's damp and mould guidance gives no target figure and nor does RICS's consumer guide. A range of roughly 40% to 60% is commonly recommended for homes.

Readings that sit well above that for long stretches, especially in bedrooms overnight or after showers, point to too much moisture and too little ventilation. A normal reading does not rule out penetrating damp, rising damp or a leak, because those bring water in through the structure rather than through the air. RICS does give a heating figure: keep rooms warm, ideally between 18°C and 21°C.

Penetrating damp and leaks

Penetrating damp is water getting in from outside. It is a building defect rather than a lifestyle problem.

The usual culprits are dull and findable: a blocked or leaking gutter, a cracked downpipe, failed pointing, damaged render, a slipped tile, flashing that has lifted or a window seal that has given up. Cavity wall insulation that has slumped or been badly installed can also bridge a cavity and carry water across it.

The tell is that penetrating damp is localised and responds to the weather. A patch that darkens during a wet week and dries back afterwards is pointing at something outside. Its position usually points at what.

Go outside and look during heavy rain, from the ground. It feels unscientific and it is remarkably effective: an overflowing gutter or a downpipe splashing against a wall is obvious while it is happening and invisible three days later. Do not climb a ladder to check a gutter or a roof; that is a job for someone with the right access equipment.

Plumbing leaks belong in the same family: water from a pipe, a waste, a radiator valve or a failed seal round a bath or shower tray. They ignore the weather and tend to show below or beside the source, often on a ceiling. A stain that spreads after someone has had a shower upstairs is the classic sign. Deal with it promptly, because a slow leak can rot timber and damage a ceiling long before it looks dramatic.

Rising damp and why it is rarer than you think

Rising damp is groundwater drawn up through masonry by capillary action. It is real and it does happen, particularly in older buildings where a damp-proof course was never fitted or has been bridged by a raised path, a flower bed or a rendered plinth.

It is also diagnosed far more often than it occurs. RICS's consumer guide says rising damp is often misdiagnosed and that failure of an original damp-proof course is extremely rare.

Two things drive the misdiagnosis. The symptoms overlap with condensation and penetrating damp closely enough that a quick look cannot separate them. The handheld meters commonly used to diagnose it are accurate in timber but can read high in masonry and plaster because of salts and other conductive materials. RICS warns against anyone who uses only a damp meter to diagnose dampness, especially in hard walls and solid floors.

Genuine rising damp has a fairly consistent tide line, rarely climbs much above a metre, is confined to the ground floor and usually comes with salt deposits and blown plaster. If what you have does not look like that, be sceptical.

What actually helps with condensation

In rough order of how much difference they make for the effort involved:

Use the extractor fans and check they work. Hold a sheet of paper to the grille: it should stay there. Many do not, because the filter or the ducting is blocked. Run them during and for a while after showering and cooking.
Stop drying washing on radiators. A load of washing holds a lot of water and all of it ends up in the air. If it has to be indoors, do it in one room with the door shut and a window open or a fan running.
Cover pans and use the cooker hood. Cooking is one of the biggest sources of moisture in a home.
Leave trickle vents open. They are there for exactly this and are very often taped shut or painted over.
Heat the house more evenly. Cold rooms are where condensation lands, so a rarely heated spare room with the door shut is often the worst room in the house for mould.
Move furniture off external walls. A gap of a few centimetres behind a wardrobe lets air reach the wall and keeps it warmer.
Open windows wide for a few minutes after a shower or cooking. Short bursts of strong ventilation remove a lot of moisture without cooling the walls much.

If all of that is being done and the problem persists, the next step is usually improving ventilation properly rather than doing more of the same. Fitting a new extractor fan means electrics near water, so it is a job for a registered electrician.

Dealing with the mould itself

Mould is the symptom. Removing it without changing the conditions means it returns, usually within weeks and usually in the same place.

When you do clean it, wipe rather than brush or vacuum, since disturbing it dry releases spores into the room. Use a product intended for the job, ventilate while you work and wear gloves and a mask. Soft furnishings that are heavily affected are often not worth saving. If the mould covers a large area or keeps coming back quickly, get the cause looked at before cleaning again.

Persistent damp and mould are worth taking seriously rather than living with. The government's guidance on the health risks of damp and mould explains that they can affect breathing, particularly for people with asthma or other lung conditions. They also damage plaster and timber over time. If mould keeps returning after you have dealt with ventilation and heating, that suggests the cause is in the building rather than in how the home is used.

When to call a professional and which one

Call someone when the two week test changed nothing, a patch keeps growing, plaster is crumbling, timber feels soft, damp follows rain or a ceiling is stained or sagging. If a ceiling is bulging with water, keep people out of the room and call a plumber straight away. If a survey for a home you are buying has flagged damp, house survey levels explained covers what to ask next.

An independent building surveyor. The right first call when the cause is unclear. RICS suggests an AssocRICS, MRICS or FRICS building surveyor with experience in building pathology, who should explain how they reached the diagnosis.
A Property Care Association member. The PCA is the trade body for damp control, timber preservation and structural waterproofing and audits its members. Many members also sell treatments, so ask for a written report explaining the cause and the evidence for it.
A roofer. For slipped tiles, flashing, valleys, chimneys and anything else at height.
A builder or gutter specialist. For gutters, downpipes, pointing, render and ground levels against the walls.
A plumber. For leaks from pipes, wastes, radiators, tanks or shower trays.
An electrician. If water has reached lights, sockets or the consumer unit.

How to check a tradesperson covers the checks before anyone starts.

If you rent

Tell your landlord in writing, with dated photos. Keep a copy. The government's guidance for rented housing providers says damp and mould should always be dealt with promptly and that blaming them on how tenants live is unreasonable.

In England, Awaab's Law has applied to social housing since 27 October 2025. It requires social landlords to deal with emergency hazards and with damp and mould hazards that present a significant risk of harm within fixed timeframes. Further hazards are due to be added from 30 November 2026. The government has said it will extend Awaab's Law to private renting and consult on the details, but no date has been set. If a private landlord in England does not act, GOV.UK says to contact the environmental health team at your council. Awaab's Law is England only; tenants in Wales, Scotland and Northern Ireland have different rules and their local advice services can explain them.

Write down what you found

Damp is slow, which makes memory a poor instrument. A photograph with a date on it is worth far more than a recollection of whether the patch was bigger last winter.

While you are working it out, keep the photographs, the dates, what you changed and any hygrometer readings. Once it is fixed, keep:

The diagnosis. The surveyor's or specialist's report, including how they reached it.
Before and after photos. Dated, from the same spot, with the repair visible.
Invoices that say what was done. Where, with what materials and by whom, not just "damp works".
Any guarantee and its terms. Note who backs it and ask whether it is insurance-backed, because a guarantee from a firm that has stopped trading may be worth little.
Building regulations paperwork. If the fix included building work that needed it.
Messages with your landlord. If you rent, every report and reply, with dates.

Three things depend on this record: knowing whether the problem is actually improving, giving a surveyor evidence rather than an anecdote and being able to answer honestly when a buyer asks. "We had it looked at, here is the report and here is what was done" is a very different answer from a shrug; how to prove work was done on your home covers the rest of that evidence.

It is the same record as the maintenance history. homehogs can keep the photos, the report and the invoice against the room they relate to, so next winter's comparison is a scroll rather than a search.

Sources

Last fact-checked 5 October 2026.

Keep the evidence, not just the memory

homehogs keeps the photographs, the surveys and the work you had done against the room itself, so next winter you can see whether it is actually getting better.

Start free

Alex Wilson

Founder

Common questions

How do I tell condensation from rising damp?

Height and timing are the quickest tests, though no single sign proves either. Condensation appears on cold surfaces anywhere in the house, including upstairs, is worst in winter and after showering, cooking or drying washing indoors and usually shows as water on glass and black spot mould in cold corners. Rising damp is confined to ground floor walls, rarely climbs much more than a metre, has a fairly even tide line often with salts and crumbling plaster and does not vary much with the weather. Condensation is far more common.

Is black mould on walls dangerous?

Persistent mould is worth dealing with rather than living with. Government guidance explains that damp and mould can affect breathing, particularly for people with asthma or other lung conditions. Mould also damages plaster and timber over time. Clean small patches by wiping rather than brushing or vacuuming, since disturbing mould dry releases spores. Ventilate the room while you work. The important part is fixing the conditions that caused it, because mould cleaned off a wall that is still cold and damp comes back within weeks.

Do I need a damp proof course?

Usually not. It is worth being sceptical of anyone who says you do without a thorough diagnosis. RICS says rising damp is often misdiagnosed and that failure of an original damp-proof course is extremely rare. It also warns against anyone who relies only on a damp meter, which can read high on masonry because of salts. If the damp is upstairs, above about a metre or varies with the weather, it is not rising damp. Get an opinion from an independent building surveyor before committing to any injection work.

Why does mould keep coming back in the same corner?

Because that corner is usually the coldest surface in the room, so it is where moisture in the air condenses first. External corners, window reveals and walls behind furniture pushed flat against them are the usual spots; rooms heated less than the rest of the house are the usual rooms. Cleaning the mould treats the symptom. Warming the surface, improving ventilation and moving furniture a few centimetres off the wall treat the cause.

What humidity should a house be to avoid condensation?

There is no official UK figure: the government's damp and mould guidance does not give one. A relative humidity of roughly 40 to 60 percent is commonly recommended for homes. Readings that stay well above that point to too much moisture and too little ventilation. A cheap hygrometer will show you the reading, but it is a clue rather than a diagnosis, because a normal reading does not rule out a leak or penetrating damp. RICS suggests keeping rooms heated, ideally between 18 and 21 degrees Celsius.

Who should I call about damp in my house?

It depends on the likely cause. For a diagnosis when the cause is unclear, an independent building surveyor, ideally a RICS member with experience in building pathology, is the safest first call. Property Care Association members specialise in damp, though many also sell treatments, so ask for a written report. For a defect you can see, call the trade that fixes it: a roofer for the roof or chimney, a builder for gutters, pointing and render, a plumber for leaks and an electrician if water has reached any electrics.

Does Awaab's Law apply to private landlords?

Not yet. In England, Awaab's Law has applied to social housing since 27 October 2025, requiring social landlords to deal with emergency hazards and with damp and mould hazards that present a significant risk of harm within fixed timeframes. The government has said it will extend it to private renting and consult on the details, but as of October 2026 no date has been set. Private tenants in England should report damp in writing and, if the landlord does not act, contact the council's environmental health team.