When there was no leak
Nothing leaked, and it still grew. That is a humidity problem, not a plumbing one.
Air holds a fixed amount of water at a given temperature. Where warm humid air meets a cold surface, water condenses on it, and growth follows on the surface rather than inside the structure. That is why external walls, window reveals and unheated rooms are where it appears first.
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How to tell condensation growth from a leak
Condensation growth sits on the surface, is widespread rather than concentrated, and favours cold spots: the top corners of external walls, behind wardrobes on outside walls, around window frames. A leak produces a defined patch that gets worse toward its center, and the material beneath is wet rather than the surface alone.
Move a wardrobe away from an external wall and look behind it. If that is the worst patch in the room, the problem is air, not plumbing.
Why opening a window is not always the answer
It depends on what the outside air is carrying. Where the outdoor dew point is high — the Gulf coast and the humid south for much of the year — bringing outside air in adds water to the house rather than removing it. In a dry or cold climate the same action works.
EPA advises keeping indoor humidity in the range of about 30 to 60 percent. Where the outdoor air cannot get you there, that is a job for equipment: extraction at source, mechanical ventilation, or a dehumidifier.
The county pages on this site show the local July dew point, which is the single number that tells you which of these two situations you are in.
The three sources of moisture in an ordinary house
People, cooking and washing. A household generates several litres of water vapour a day through breathing, showers, drying clothes indoors and cooking. In a well-sealed house with no extraction, that has nowhere to go but onto the coldest surfaces.
Drying laundry indoors is the largest single contributor and the easiest to change.
Extraction fans matter more than their size suggests, and one that vents into a loft rather than outside is worse than none at all.
What to change, by situation
Condensation is the one mold problem with a behavioural answer as well as a building one.
- 01You own and live here. Deal with the three sources, check the extraction actually vents outside, and measure indoor humidity before buying equipment.
- 02You own it and rent it out. Condensation is the case landlords blame on tenants and tenants blame on the building. Usually both contribute. Provide working extraction before arguing about lifestyle.
- 03You live in a humid climate. Ventilating with outdoor air will not get you under 60 percent for much of the year. Plan for dehumidification rather than airing.
- 04You rent here. Report it in writing, note whether extraction fans work, and keep a record of what you do to ventilate. That record is what answers the lifestyle argument.
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What to ask before you agree to anything
Ask these of whoever turns up. A contractor who answers them plainly is the one you want; a contractor who will not is telling you something.
- 01Is this condensation or a leak, and what are you basing that on?
- 02Have you measured indoor humidity, and at what time of day?
- 03Do the extraction fans actually vent outside? Have you checked?
- 04What would you change about the building rather than about how we live?
- 05Is insulation or a cold bridge part of the problem here?
- 06What licence do you hold for this work?
Have this ready before you call
Three facts move the first minute past the formalities: what happened, when it started, and whether water is still moving. These add the detail that decides what gets loaded on the van.
- Where the growth is: external walls, window reveals, behind furniture
- Whether laundry is dried indoors
- Whether extraction fans exist in the kitchen and bathroom, and whether they work
- Whether the affected rooms are heated
- Roughly what the indoor humidity reads, if you have a meter
- Whether it is worse in winter
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Where these figures come from
Every number on this page is published by someone other than us, and each link goes to their own text rather than to a summary of it.