Mould in a bathroom returns because most treatments address the symptom. Bleach turns the black spots white, the surface looks clean, and three weeks later the spots are back.
The reason is that mould grows from a network of hyphae that penetrates porous surfaces — grout, sealant, plaster. Bleach kills the surface layer and leaves the embedded network alive, so it regrows from the inside.
This guide covers what mould needs, where it starts, and the treatment that actually holds.
What mould needs
Four conditions, all required:
- Moisture — relative humidity above 70%, or liquid water
- Warmth — 20–30 °C is optimal
- Food — dust, soap residue, skin cells, the paper facing on plasterboard
- Time — visible growth takes 1–3 weeks once the other three are met
Remove any one and it stops. Warmth is the hardest to remove in a bathroom, so the practical levers are moisture and food.
| Lever | How | Effect |
|---|---|---|
| Ventilation | Extractor fan, window | Removes moisture at source |
| Heating | Keep the room above 18 °C | Reduces condensation on cold surfaces |
| Cleaning | Remove soap residue and dust | Removes food |
| Sealing | Re-grout and re-seal porous surfaces | Removes the growth medium |
The four places it starts
1. Silicone sealant around the bath and shower
The most common and the most stubborn. Silicone is porous to water vapour, and once mould is inside the silicone itself, no surface treatment reaches it.
The tell: black spots within the silicone, not on top. If scrubbing does nothing, the mould is internal.
The only real fix: replace the silicone. Cut out the old sealant (a silicone remover helps), clean the joint thoroughly, dry it completely — 24 hours with a fan — and re-seal with a mould-resistant sanitary silicone.
Cost: $10–20 for a tube and a gun. Time: 2 hours plus 24 h curing.
Why it works: the growth medium is removed entirely. This is the only treatment in this list that eliminates rather than suppresses.
2. Grout lines
Grout is porous and holds water. Mould grows in the grout and spreads along the lines.
The treatment: an alkaline cleaner, not bleach. Bleach is deactivated by organic matter and does not penetrate.
What works: sodium percarbonate (oxygen bleach) or a purpose-made grout cleaner, applied as a paste, left 30–60 minutes, then scrubbed. Alternatively, a steam cleaner penetrates the grout and kills the hyphae.
The lasting fix: grout sealer, applied after cleaning and drying, every 1–2 years. It fills the pores so water cannot enter.
Cost: $15–25 for cleaner and sealer.
3. The ceiling above the shower
Condensation rises and collects on the ceiling, which is often the coldest surface.
The cause is almost always insufficient ventilation, not a cleaning failure.
The fix, in order:
- Check the extractor fan actually extracts — hold a tissue against it. If the tissue does not stay, the fan is not working or is blocked.
- Run the fan during and for 20 minutes after every shower.
- Leave the door open for 30 minutes after.
- If the ceiling is still wet, a dehumidifier for 30 minutes after showering.
The treatment of existing mould: a fungicidal wash on plasterboard, then a mould-resistant primer and two coats of bathroom paint. Painting over untreated mould means it returns through the paint within weeks.
4. Cold external walls and window reveals
Condensation forms on the coldest surface, which in winter is usually an external wall or a window reveal.
The tell: mould in a vertical band along a wall, or around window frames, not near the shower.
The cause: warm moist air meeting a cold surface. This is a thermal problem, not a cleaning one.
The practical fixes:
| Fix | Cost | Effect |
|---|---|---|
| Keep the room heated to 18 °C minimum | Ongoing | Raises the surface temperature |
| Keep furniture 5 cm off external walls | Free | Allows air circulation |
| Wipe condensation daily in winter | Free | Removes water before mould starts |
| Dehumidifier | $100–200 | Removes the moisture from the air |
| Insulate the wall | $200+ | Solves it properly, rarely possible in a rental |
The free one that matters: wiping the window reveal and the wall each morning in winter. It takes 30 seconds and it is the difference between a clean reveal and a black one.
What does not work
| Method | Why it fails |
|---|---|
| Bleach spray | Kills surface only; embedded hyphae regrow |
| Painting over mould | Mould grows through the paint |
| Vinegar alone | Effective on some species, not all; weak on grout |
| "Mould-resistant" paint on an untreated wall | The mould is beneath the paint |
| Scrubbing without fixing ventilation | Returns in 2–3 weeks |
| Sealing the room and heating it | Increases humidity, makes it worse |
The bleach point is worth repeating because it is the most common mistake: bleach is inactivated by organic matter (soap scum, body oils), so on a bathroom surface it is much less effective than its reputation suggests. It whitens; it does not eradicate.
The ventilation calculation
The single most effective thing you can do is ventilate properly, and the numbers are specific.
An extractor fan should move at least 15 litres per second for a small bathroom (under 5 m²), 25 l/s for larger. Most cheap builders' fans move 8–10 l/s, which is why they do not work.
The check:
- Hold a piece of toilet paper against the grille with the fan running. It should stay.
- If it falls, the fan is underpowered, blocked, or the duct is disconnected.
Run time: during the shower and 20 minutes after. A fan that runs only during the shower leaves most of the moisture in the room.
The timer switch: a fan with a 15–20 minute overrun timer costs $15–30 more than a plain switch and is the single best upgrade in a bathroom, because it removes the need to remember.
The duct check: if the duct runs through a cold loft, moisture can condense inside it and run back. An insulated duct ($20–30) fixes this.
The routine that keeps mould away
| Frequency | Action |
|---|---|
| Every shower | Fan on, door open 30 min after |
| Daily (winter) | Wipe window reveals and cold walls |
| Weekly | Squeegee shower walls; wipe sealant dry |
| Monthly | Clean grout with alkaline cleaner |
| Every 3 months | Check fan, clean grille |
| Yearly | Inspect sealant, replace if mouldy inside |
| Every 1–2 years | Re-seal grout |
The squeegee point: a $5 squeegee used for 30 seconds after each shower removes most of the water before it can evaporate. It is more effective than any cleaning product, and it costs almost nothing.
The honest summary
Mould returns because moisture and food remain. Cleaning removes neither.
The two interventions that actually work: replace mouldy silicone (the only treatment that removes the growth medium) and fix ventilation (the only treatment that removes the water).
Everything else — bleach, sprays, anti-mould paint — is suppression, and suppression buys weeks rather than years.