What you are looking at
Condensation grows on a surface. A leak grows inside the wall.
Two completely different problems look similar on a wall. Condensation forms on cold surfaces and stays on them. A leak soaks the material and works outward, so what you see is the smallest part of it. One is about heating and ventilation; the other is about plumbing and drying.
- 00Move furniture off the outside wallsIf the worst patch is behind a wardrobe on an external wall, the problem is air, not a pipe.
- 01Press the wall firmlySurface growth on sound plaster feels hard. A leak leaves the board soft or crumbling behind the paint.
- 02Look at the shapeCondensation is widespread and favours corners and reveals. A leak is a defined patch, worst in the middle.
- 03Check whether it is worse in winterCondensation usually is. A plumbing leak does not care about the season.
Goes to an independent contractor. Nothing is booked until you say so. Calls may be recorded.
Where condensation always appears first
The coldest surfaces in the room: the top corners of external walls, around window reveals, behind furniture pushed against an outside wall, and in unheated rooms. Warm humid air meets a cold surface and water comes out of the air onto it.
A household puts several litres of water into its own air every day through breathing, cooking, showers and drying laundry indoors. In a well-sealed house with no working extraction, that has nowhere to go but onto the cold spots.
Drying laundry indoors is the single largest contributor and the easiest to change.
Why opening a window is not always the answer
Because you are not exchanging air, you are exchanging water. On a humid day the air coming in carries more of it than the air going out, and the window makes the wall worse. On a cold dry day the same window fixes it. Same action, opposite result, which is why general advice fails here.
EPA advises keeping indoor humidity roughly between 30 and 60 percent. Where outdoor air cannot get you there, the job belongs to equipment: extraction at source, mechanical ventilation, a dehumidifier.
Your county page on this site shows the local July dew point, which is the one number that tells you which situation you are in.
When it is a leak, the visible part is the clue not the extent
Where moisture arrived from inside a wall or under a floor, what reaches the surface is an indicator. A small patch fed by a slow leak in a cavity can sit above a far larger area that stays hidden until the wall is opened.
That is why a contractor reads moisture through the material rather than judging by eye, and why a survey consisting only of thermal camera photographs has skipped the part that produces evidence.
Who fixes which
The two problems land on different people.
- 01You own and live hereCondensation: heating, ventilation, and where laundry dries. Leak: find the source before anything else.
Ask them: is this condensation or a leak, and what are you basing that on? - 02You own and let it outThis is the argument landlords and tenants have most often, and usually both sides contribute. Provide working extraction before debating lifestyle.
- 03You rentReport it in writing, note whether the extraction fans work, and keep a record of how you ventilate. That record answers the lifestyle argument.
- 04You are buyingAsk which it is and ask for evidence. A seller who has dried and documented a leak is in a different position from one who painted over it.
Goes to an independent contractor. Nothing is booked until you say so. Calls may be recorded.
Where these figures come from
Published by someone other than us. Each link goes to their own words rather than to our summary of them.