Damp and Mould in the Bathroom: Causes and What to Do

A hot shower can release a large amount of water vapour into the air in minutes. In a small room with poor extraction, that moisture has nowhere to go except onto the coldest surfaces — tiles, grout, the ceiling above the shower, and window frames.

Why it keeps coming back

Mould in bathroom grout and silicone sealant is extremely common and, on its own, isn’t usually a sign of a structural damp problem — it’s a sign the room isn’t drying out between uses. The most common causes are an extractor fan that’s underpowered, not switched on, or not run for long enough after showering, and a lack of any window ventilation as a backup.

What to check

• Does the extractor fan actually pull air, or is it just noise? Hold a piece of tissue near it — it should be pulled toward the fan.
• Is the fan wired to run for a set time after the light switches off (a “run-on timer”), or does it stop the instant you leave?
• Is there mould specifically around a window frame or an external wall, rather than just grout? That can point to condensation from a colder surface rather than just poor extraction.
• Any staining or bubbling paint that follows a pipe run (behind a bath panel, under a sink) rather than a general spread — that’s more likely a leak than condensation.

What to do

• Run the extractor fan during the shower and for at least 15–20 minutes afterwards.
• Wipe down tiles and the shower screen after use — this alone significantly cuts the moisture left to evaporate into the room.
• Leave the bathroom door open (or a window cracked) after bathing to let moisture escape rather than migrate into the hallway or bedroom.
• Re-seal grout and silicone if it’s already black — mould in old sealant is very difficult to fully clean out and usually needs replacing.
• If the fan is weak or has no run-on timer, a qualified electrician can usually upgrade it relatively cheaply — get more than one quote before committing.