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Is black mould always caused by condensation?
In short: No. Most black mould in homes does come from condensation, but mould grows wherever a surface stays damp, so penetrating damp, leaks and rising damp can cause it too. If mould keeps coming back in the same place, or appears alongside staining, tide marks or crumbling plaster, there is probably more going on than condensation.
Why condensation is the usual culprit
Mould needs moisture, and the commonest source is the air inside the house: cooking, washing, showering, drying clothes and breathing all add moisture, which settles on the coldest surfaces. That is why condensation mould appears in the top corners of outside walls, around windows, behind furniture and in bathrooms, and why it is worst in winter.
When the mould points to something else
- It sits in one patch that lines up with a downpipe, gutter, window sill or damaged render outside: likely penetrating damp.
- It is low on a ground-floor wall with a tide mark and salts: possibly rising or bridged damp.
- It appears near pipework or a bathroom on the other side of the wall and keeps spreading: possibly a leak.
- It is worse after rain than in cold, dry weather: rain is getting in.
A wall that is damp for any of these reasons is also colder, so it attracts condensation too. Treating only the condensation then gives a partial fix at best.
Why mould comes back after cleaning
Cleaning and treating the surface kills the mould you can see, but if the wall stays damp, new mould grows. Painting over mould without treating it usually lets it grow back through. A lasting fix means dealing with the moisture first, then treating the mould, replacing any damaged plaster or sealant and redecorating.
Getting it diagnosed
If mould keeps returning, a damp survey will tell you whether it is condensation, another type of damp or both. It costs £119, deducted from any treatment if you go ahead with our quote. For landlords, our damp and mould reports cost £99.
Frequently asked questions
Is black mould always caused by condensation: FAQs.
Is black mould dangerous?
Mould can affect health, particularly for people with asthma, allergies or other breathing problems, and for babies and older people. It is worth dealing with promptly rather than living with it.
Will mould-resistant paint stop mould?
It helps on surfaces prone to condensation once the cause has been dealt with, but it won't stop mould on a wall that stays damp.
How long does mould take to come back?
If the cause is still there, often within weeks, especially in winter.