Mould is one of the most common and most mishandled household problems in South Africa. It appears on walls, ceilings, window frames, and in cupboards — often first noticed as a musty smell or a dark patch that seems to grow back no matter how many times you wipe it down. The instinct is to paint over it or scrub it with bleach and hope it stays away. This approach rarely works, because mould is a symptom of a moisture problem, not the moisture problem itself. Without fixing the underlying cause, the mould returns — often worse, and often having spread inside the wall structure where you cannot see it.
This guide explains the different types of mould problems common in South African homes, how to identify whether the cause is condensation, rising damp, or penetrating damp, what treatment approaches work and which do not, when to call a professional, and how to prevent recurrence.
Mould vs Mildew vs Efflorescence — Knowing What You Are Dealing With
Not everything dark on a wall is mould, and the treatment differs based on what it actually is.
Mould is a living fungal organism that grows in conditions of elevated humidity and organic material to feed on (paint, plaster, dust, paper behind wallpaper). It appears as black, green, or grey patches and produces spores. It has a musty smell and can cause respiratory problems, particularly for people with asthma or allergies. Mould on a surface indicates that moisture is present or was recently present.
Mildew is a type of mould — the terms are often used interchangeably in a household context. It is typically grey or white and is more common on shower grout and bathroom surfaces. Same cause (moisture), same treatment approach.
Efflorescence is not mould. It is white, crystalline, and appears on masonry walls — it is the residue left as moisture moves through brickwork and evaporates, depositing salts on the surface. Efflorescence indicates moisture movement through the wall (possibly rising damp or penetrating damp) but is not itself a health risk. It cannot be cured by treating the surface — only by stopping the moisture movement.
Identifying which you have matters for treatment. A black or green fuzzy growth on a wall or ceiling is mould. White crystalline deposits on external-facing masonry is likely efflorescence. A persistent grey film in a bathroom that returns weekly is mildew.
The Three Causes of Indoor Mould in South Africa
Mould cannot be permanently treated without addressing its cause. The three main causes in South African homes are different problems with different solutions.
Condensation (the most common cause): Warm, moist air meets a cold surface and the moisture condenses, creating the damp conditions mould needs. This is most common in bedrooms and bathrooms in winter — particularly on external-facing walls, around windows, and in the corners where cold air collects. The classic sign is that mould appears on or near windows and on the coldest walls, and is worst in winter or in rooms with poor ventilation.
Condensation mould is addressed through ventilation and insulation, not by waterproofing the wall. Open windows daily, use extractor fans in bathrooms and kitchens, avoid drying washing indoors in winter, and consider insulating cold external walls. Anti-condensation paint (which has insulating properties) helps on the wall surface but will not solve the problem if the room is not ventilated.
Rising damp: Ground moisture wicking up through walls. The mould appears at low level — the bottom metre of walls — and is usually accompanied by a tidemark staining and crumbling plaster. Rising damp is treated by damp-proofing professionals (chemical DPC injection and render replacement), not by surface treatment.
Penetrating damp: Water entering through cracks, leaking gutters, missing roof tiles, or porous render. The mould follows the water source and may appear anywhere — not just at low level. Fixing penetrating damp requires finding and sealing the water entry point first, then treating the internal affected area.
Surface Treatment — What Works and What Does Not
If the moisture source is controlled (condensation through ventilation, or penetrating damp through external repair), the surface treatment can be effective. If the moisture source is not controlled, no surface treatment lasts.
Bleach solutions: A diluted bleach solution (1 part bleach, 4 parts water) kills mould on non-porous surfaces (tiles, glass, sealed paint). It does not penetrate porous surfaces like plaster effectively — it bleaches the surface colour but the mould structure inside the plaster survives. Use on grout, tiles, and sealed surfaces. Wear gloves and ensure ventilation.
Anti-fungal cleaners: Purpose-made anti-fungal sprays (available at hardware stores) are more effective than bleach on painted plaster because they penetrate slightly better. Follow the manufacturer's contact time before wiping. Effective for surface mould on painted walls where the underlying moisture cause is condensation and has been addressed.
Anti-mould paint: Paints containing fungicide (sold under various brand names at hardware stores) slow regrowth significantly on surfaces where the moisture cause is controlled. They do not solve the problem on their own if the cause is not addressed — but as the final step after treating the cause and the surface, they extend the period before regrowth.
Painting over mould without treating it: This does not work. Paint over living mould and the mould bleeds through within weeks, often with a stain that is worse than the original. Always treat and clean the surface before repainting.
When to Call a Professional
Surface mould in a well-ventilated bathroom that responds to cleaning and improved ventilation does not require a professional. Call a professional when: the mould covers an area larger than roughly 1m² (indicating a systemic moisture problem); it returns within weeks of treatment despite ventilation improvement; it appears on multiple walls including internal walls (suggesting the problem is inside the wall structure); there is a musty smell throughout the house that you cannot locate; or a household member has respiratory symptoms that improve when they leave the house.
A damp-proofing specialist can diagnose rising damp versus penetrating damp versus condensation using a moisture meter and professional assessment. This is worth doing before spending money on treatments that address the wrong problem. A building inspector can identify structural causes that are not visible from the interior.
For health-threatening mould infestations — black mould (Stachybotrys) is the most severe type — professional remediation is essential. Disturbing a large mould colony without proper containment and protective equipment can release significant quantities of spores into the air, which is worse than leaving it undisturbed while you arrange professional removal.
Preventing Mould From Recurring
Ventilation is the single most effective prevention measure for condensation mould. Open bedroom windows for at least 30 minutes each morning even in winter. Run bathroom extractor fans during and for 15 minutes after showers. Do not dry washing indoors in winter.
For homes with persistent condensation problems in winter, a dehumidifier in the affected rooms reduces ambient humidity significantly. Running a dehumidifier in a damp bedroom can make the difference between an annual mould problem and none at all.
For properties with rising damp or penetrating damp, prevention requires fixing the structural cause — not surface treatment. An annual inspection of gutters, roof condition, and the seal around window frames (before winter rains) catches the most common entry points for penetrating damp before they become internal mould problems.
Quick Checklist When You Find Mould
- Identify the cause first — condensation, rising damp, or penetrating damp — before spending money on treatment
- For condensation mould, improve ventilation before treating the surface
- For rising or penetrating damp, fix the external cause before treating the internal surface
- Never paint over mould — treat the surface first, then paint
- Use anti-fungal cleaner on porous surfaces; bleach solution on tiles and grout
- Finish with anti-mould paint as the last step after the surface is completely dry
- Call a professional if the affected area is larger than 1m², if it returns quickly after treatment, or if it is inside walls
- Run a dehumidifier in winter for persistent condensation problems in specific rooms
If mould in your home is being caused by rising damp, penetrating damp, or a structural water ingress issue, a qualified contractor can diagnose and fix the source — KiesSlim makes it easy to find damp-proofing and building specialists in your area based on real reviews from other homeowners.