What the pattern is telling you
Where the growth appears narrows the cause down faster than anything else you can do without equipment. Find your pattern in the table, then read the note underneath it.
| Where it grows | What it means |
|---|---|
| Corners of external walls, up near the ceiling | Condensation at a cold bridge |
| Around window reveals | Cold bridge and poor ventilation |
| Behind wardrobes and sofas on external walls | No air movement, cold surface |
| An isolated patch mid-wall | Penetrating damp through a defect outside |
| A level band low down with salts | Rising damp |
| The ceiling below a bathroom | Failed shower or bath waterproofing |
| Spread across a ceiling with a stain | Roof leak |
| Uniformly through a room | Whole-house humidity and ventilation |
Spotted black growth in corners and around reveals, worse in winter, with no salt bloom, is condensation — a ventilation problem, not a waterproofing one. An isolated patch that darkens after a front is penetrating damp, and that is a building fault we can fix.
Mould inside walls and cavities
Growth is not always on the surface you can see. Where moisture has been in a wall for a long time, mould colonises the plaster, the cavity face, the back of plasterboard and any timber in there. The signs are a persistent musty smell with nothing visible, discolouration bleeding through fresh paint within weeks, and plasterboard that feels soft or sounds dull.
Surface treatment does nothing for this. The material has to come off, the cause has to be fixed, and it has to dry before anything goes back.
A smell with nothing visible is worth following up — see musty smell in the house.
The three levers
- Reduce the water. Fix leaks and damp; extract at source in bathrooms and kitchens; dry laundry outside or in a vented machine.
- Raise the surface temperature. Mould grows where the surface is coldest. Insulation and dealing with cold bridges does more than any spray.
- Move the air. Growth needs still air. Pull furniture a few centimetres off external walls, keep trickle vents open, do not block airbricks.
If the water is coming from a building fault rather than from the air, none of the three will hold, and that is the point at which it becomes our problem rather than a housekeeping one.
More on mould
- How to remove mould from walls — doing it properly, and what not to use
- Black mould on ceilings — why ceilings are different
- Mould in bathrooms — the room it starts in
- Is black mould dangerous? — what the health evidence supports
- Mould-resistant paint — what it does and does not fix
Mould is the symptom; the wall is wet for one of four reasons, and only two of them are fixed by a membrane. The diagnosis-first route through that is damp proofing in Cape Town.
Common questions
Why does mould keep coming back on the same wall?
Because that spot is still the coldest and wettest surface in the room. Cleaning removes the growth, not the condition. Until the surface is warmer, drier or better ventilated, the spores in the air reoccupy exactly the same place.
Is mould on walls always caused by damp?
It is always caused by moisture, but not always by a building fault. Condensation from everyday living produces most household mould. The distinction matters because a waterproofing system fixes one and does nothing for the other.
Can mould grow behind wallpaper or paint?
Yes, and it commonly does — the adhesive is a food source and the surface behind stays damper. Discolouration bleeding through fresh paint within a few weeks usually means growth behind the film rather than on it.
Does bleach kill mould on walls?
It kills what is on the surface and removes the stain, which is why it looks effective. On a porous surface the roots survive below and it regrows. It also adds water to a wall that already has too much.