Everyday condensation, or something worse?
Almost every bathroom shows some growth. What separates a ventilation problem from a building problem is where it sits and whether the room dries out when nobody uses it.
| Sign | Condensation | Failed waterproofing |
|---|---|---|
| Where | Ceiling, silicone, window reveals | Low on walls, base of the shower |
| Grout | Surface staining, wipes off | Stays dark, never dries out |
| Tiles | Sound | Drummy, loose or lifting |
| Next room | Unaffected | Damp patch on the other side of the wall |
| Room below | Unaffected | Ceiling stain or growth |
| Smell | Mild | Persistent musty smell |
| After a week unused | Dries out | Still damp |
Anything in the right-hand column means water is getting past the tiles into the structure, and the fix is shower waterproofing — strip out, bonded membrane, corrected falls, re-tile. Re-grouting does not touch it.
Fixing the ordinary kind
- Extract at source. A fan that actually moves air, vented outside and not into the roof space, running during the shower and for 15–20 minutes after. An over-run timer is the single most effective bathroom upgrade available.
- Close the door, open the window. Keep the moisture in the bathroom and push it outside rather than letting it spread through the house.
- Squeegee the walls after showering. Unglamorous and very effective — it removes most of the water before it can evaporate.
- Warm the room. A cold bathroom condenses far more.
- Replace failed silicone rather than cleaning it repeatedly. Once growth is inside the sealant it does not come out.
- Leave a gap behind bath panels and vanity units where you can.
The silicone question
Mouldy silicone around a bath or shower tray is usually just mould, and it is replaced rather than cleaned — cut it out, dry the joint thoroughly, and re-apply a sanitary grade with fungicide.
But silicone at the wall–tray junction is also a working waterproofing joint. If it has failed and water has been running behind it for a while, replacing the bead seals the visible gap over a substrate that is already wet. If the tiles nearby sound hollow or the room next door has a damp patch, the problem is behind the tiles and new silicone will hide it rather than fix it.
Related reading
Start with Mould on Walls for the overview, or:
Common questions
Why does mould keep coming back in my bathroom?
Almost always because the extraction is inadequate — no fan, an underpowered one, one that vents into the roof space, or one that runs only while the light is on. The moisture has to leave the building, not just the room.
Is bathroom mould a sign of a leak?
Not usually. Most is condensation. It becomes a leak question when growth is low on the walls rather than high, when tiles are drummy, or when there is a damp patch on the other side of the wall or on the ceiling below.
Should my extractor fan vent into the roof?
No. Venting into a roof space dumps warm moist air into a cold void where it condenses on the underside of the roof and on the timbers. It must terminate outside. This is one of the most common faults we find.