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What to check, and in what order

Mould keeps coming back in my bathroom

Mould grows where a surface stays wet long enough, so it returns to the same place because that place is the coldest or the least ventilated. Cleaning removes the growth and changes nothing about the conditions — which is why it comes back on the same timetable.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

Mould needs moisture, a surface and time. A bathroom supplies the first abundantly, so what decides where it grows is which surfaces stay wet longest — and that is a question about temperature and air movement rather than about hygiene.

THE LOCATION IS THE DIAGNOSIS. Growth in the corners of a ceiling, behind a cistern, on an external wall or around a window reveal is following the COLD: those are the coolest surfaces in the room, so moisture condenses there first and stays longest. Growth confined to the silicone around a bath or shower tray is a different thing — that is water sitting in a joint, often because the joint has failed or because the tray moves under load.

VENTILATION is the other half, and the commonest finding is not an absent extractor but an ineffective one: a fan that runs for too short a time, one that is ducted into a loft rather than outside, or one fighting a closed door with no gap under it for replacement air.

The thing most people do first — clean it — is worth doing and is not a fix. It removes the growth and leaves the conditions, so it returns at the same rate. The useful work is on the surface temperature and the moisture, in that order.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. Moisture not being removed fast enough

    Most common

    General growth in the coolest parts of the room, with the mirror staying fogged long after a shower and the room feeling humid an hour later.

    How to check: Time how long the mirror takes to clear after a shower. A room that stays fogged for a long time is not exchanging its air.

  2. An extractor that is not doing its job

    Most common

    The same general pattern, with a fan that is audible but moves little air, runs only while the light is on, or discharges into a loft.

    How to check: Hold a sheet of paper against the running fan. A fan that will not hold it is moving very little air — then trace the duct to where it actually discharges.

  3. A cold surface — thermal bridge or uninsulated wall

    Common

    Growth concentrated on one wall, in ceiling corners, around a window reveal, or in a line following structure — following geometry rather than spread evenly.

    How to check: Whether the affected areas form a pattern. Mould tracing a corner, a lintel or a stud line is following the cold, not the damp.

  4. No replacement air path

    Common

    An adequate fan that still leaves the room humid, with a close-fitting door, carpet or draught excluder blocking the gap beneath it.

    How to check: Whether there is a gap under the door. A fan cannot extract air it has no way of replacing, and the flow simply falls.

  5. A failed sealant joint or a moving tray

    Common

    Growth confined to the silicone line around a bath, tray or basin, often with the joint split, and worse where somebody stands.

    How to check: Press down in the bath or tray and watch the joint. Movement that opens the seal lets water behind it, and no sealant survives that.

  6. Water getting in from elsewhere

    Less common

    A damp patch that does not dry between uses, sometimes with staining spreading beyond the growth, and present regardless of how the room is used.

    How to check: Whether the area stays damp when the bathroom has been unused for a day or two. Damp that survives disuse is not condensation.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    Corners, reveals and one external wall mean cold surfaces; the silicone line means a joint; a patch that ignores both means water from elsewhere. Three different problems.

  2. 2

    It is a free measurement of how fast the room clears its moisture, and it tells you whether ventilation is the limiting factor before you touch anything.

  3. 3

    A fan that runs is not a fan that works, and one ducted into a loft moves the moisture rather than removing it — creating a second problem above the ceiling.

  4. 4

    Extraction needs replacement air. A well-sealed door with no undercut starves the fan, and the flow falls far below its rating whatever the fan is capable of.

  5. 5

    It separates a room-air problem from a joint problem in seconds, and a moving tray is why resealing keeps failing.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

CFM Ventilation Calculator →Vapor Barrier Dew-Point Condensation Risk Calculator →Attic and Loft Ventilation Calculator (Intake, Exhaust and Vent Count) →

When to stop and get somebody in

  • Where mould keeps returning over a substantial area after ventilation has genuinely been improved, particularly where anyone in the building has asthma or another respiratory condition.
  • Where an area stays damp even when the bathroom has been unused, which indicates water arriving rather than condensing.
  • Where an extractor discharges into a loft or a void — that needs re-ducting, and the loft may already have a moisture problem of its own.
  • Where the wall or ceiling behind the growth is soft, stained through, or the plaster has blown, which is damage to the fabric rather than growth on it.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.