Vivid Ready

Most damp in a British house is condensation, not a leak, and a house being heated less produces far more of it. Find out which one you have before you spend anything, because the fixes are opposites: a leak needs sealing and condensation needs opening up.

This page matters more in the situation this app is written for than it does in ordinary times, and it matters in a way that catches people out. A household heating one room, drying clothes indoors, cooking on a stove and keeping the windows shut against the cold is producing more water vapour than usual and giving it fewer places to go. Mould follows within weeks.

It is also the damp problem people most often misdiagnose, spend money on, and fail to fix.

Which damp have you got

Telling them apart

	Condensation	Penetrating damp	Rising damp
Where	Coldest surfaces, corners, window reveals, behind furniture, north walls	One patch, on an outside wall or ceiling	Ground floor only, low down
Shape	Diffuse, often speckled black mould	A defined patch, sometimes with a tide line	A horizontal tide mark, typically under a metre
When	Worse in winter, worse overnight, worse after cooking or showering	Worse during and after rain, often wind-direction specific	Fairly constant, worse in wet seasons
Smell	Musty	Musty, sometimes earthy	Earthy, often with salt deposits
The fix	Ventilation and heat	Find and fix the defect outside	Rare, and a specialist job

Most of it is condensation. In British housing it is by a wide margin the commonest cause of visible mould, and it is the one that responds to behaviour rather than to builders.

Penetrating damp is a defect outside: a cracked render, a failed gutter, a slipped tile, a blocked airbrick, a bridged cavity, ground level built up above the damp proof course. Follow it back to the outside and look. See roof leaks if it is coming from above.

Rising damp is much rarer than the industry selling injection creams suggests. A tide mark under a metre with salting is worth investigating properly. Most "rising damp" diagnoses turn out to be a bridged damp proof course, a raised path, or condensation.

A quick test for condensation: tape a piece of aluminium foil or a small mirror tightly over the damp patch, sealed at all four edges, and leave it a few days. Moisture on the room side of the foil means condensation. Moisture behind it, against the wall, means it is coming through the wall.

Why a colder house is a wetter one

The physics is simple and the consequence is counter-intuitive.

Warm air holds more water vapour than cold air. When warm, moist room air touches a cold surface, the air right at that surface cools, its capacity drops, and the water comes out as liquid. The room's overall humidity can look perfectly reasonable while the corner behind the wardrobe on the north wall is soaking.

Mould does not need the room to be humid. It needs the surface to be. Growth starts at around 80% relative humidity at the surface, which happens at a cold spot long before the room feels damp.

A household of four generates roughly 10 to 15 litres of water vapour a day through breathing, cooking, washing, bathing and drying clothes. That water goes somewhere. In a heated, ventilated house it leaves. In a cold, sealed one it lands on the walls.

So the two things that reduce mould are the two things a cold house tends to do less of: keeping surfaces warm enough that water does not condense on them, and moving the moist air out. Doing only one is much weaker than doing both.

The things that actually work

Roughly in order of how much difference they make per unit of effort.

Stop making so much of it:

Dry washing outside whenever the weather allows, even in winter, even if it takes two days. A load of wet washing is around two litres of water and putting it on a radiator in a closed room puts all of it into the house.
If you must dry indoors, do it in one room with the door shut and the window open, or with a dehumidifier running.
Lids on pans. A rolling uncovered pan is a humidifier.
Vent a tumble dryer outside, properly, not into a bucket of water.

Get it out:

Ten minutes of cross-ventilation, twice a day, windows open on opposite sides of the house. Short and wide beats a small window on the latch all day: it exchanges the air without cooling the structure, and it is the single most effective habit on this page.
Extractor fans on during and for fifteen minutes after every shower and every meal. If yours is feeble or has never worked, that is a genuinely worthwhile repair.
Shut the bathroom and kitchen door while in use, not open. Containing the moisture and extracting it beats letting it spread through the house.
Do not block airbricks, trickle vents or old chimney vents. People block them to stop draughts and it is the commonest way a manageable house becomes a mouldy one.

Warm the surfaces:

Steady low heat beats short hot blasts. Walls have thermal mass; a two hour burst warms the air and leaves the surfaces cold, which is the exact condition for condensation.
Pull furniture 50 mm off external walls, and do not push a wardrobe or a sofa flat against a north-facing wall. Still air behind furniture is where mould starts.
Insulate cold spots if you can: loft insulation first, then anything obviously uninsulated.

Cleaning mould off

A proper fungicidal wash does the job better than bleach on paintwork and plaster. Follow the contact time on the bottle.
Bleach whitens more than it kills on anything porous. The mould disappears visually and the growth is still in the plaster, which is why it comes back in the same place.
Soft furnishings, curtains and mattresses that are properly affected are usually not salvageable. Wash what can be washed hot.
Then fix the cause. Cleaning without changing the conditions gets you the same corner again in six weeks, and each time it comes back faster.
Do not paint over it. Anti-mould paint is a useful last coat on a dried, treated, ventilated wall. Over live mould on a damp wall it is a way of hiding the problem for one season.

When it is a health problem

Mould is not merely unsightly, and this is the part that justifies the effort.

Damp and mouldy homes are associated with wheezing, coughing, respiratory infections and worsening asthma. The people most affected are infants, older people, anyone with a respiratory or immune condition, and anyone already unwell, which in the scenario this app describes is a meaningful part of a household.

Take it seriously if: a child in the house develops a persistent cough that improves when they are elsewhere, an asthmatic's symptoms worsen at home, or anyone is sleeping in a room with visible mould. Move the sleeping arrangements before you fix the wall, because the wall will take weeks.

The one warm room problem

The scenario cards and the hypothermia page both, correctly, tell you to put everybody in one heated room. This page is the bill for that.

One room containing the whole household, cooking, breathing, drying clothes and sleeping, with the door shut and the window shut, will run at very high humidity within a day. The windows stream, the outside wall goes cold and wet, and mould appears in the corners inside a fortnight.

Do it anyway, and manage it:

Air it hard, twice a day. Everybody in coats, windows wide for ten minutes, then shut. You lose surprisingly little heat from the structure and you dump a great deal of moisture.
Do not dry clothes in the sleeping room. Use a different, colder room with its window open, and accept that it takes longer.
Keep a gap behind beds and furniture on the external wall.
Wipe the condensation off the windows each morning rather than letting it run onto the sills and into the frames.
If you are running any kind of flueless heater, be aware that burning fuel produces water as well as carbon monoxide. See ventilating indoor burning, which is the page that outranks this one every time.