How Do You Stop Mould Coming Back After a Refit?

Mould returns after a refit when the moisture source has not been removed, which in a bathroom almost always means ventilation or a cold surface rather than cleaning. Replacing tile and grout without fixing airflow produces the same mould on a new surface within one heating season.

There are only three causes worth checking: insufficient extraction, thermal bridges that stay cold, and water getting in from behind the finish. Each has a different fix, and only the first is cheap.

How do you tell which cause you have?

Look at where the mould sits. On the ceiling above the shower means extraction. In corners and along the external wall means a cold bridge. Along the base of the wall or at the grout lines means water is behind the tile.

Wipe a patch with a cloth and smell it. A musty, earthy smell suggests a leak; a damp, flat smell with no source suggests condensation.

How much extraction does a bathroom actually need?

Fifteen litres per second, running for 15 to 20 minutes after the shower, with the door closed and the window shut. An open window with the fan on short-circuits the airflow and extracts nothing.

The habit matters as much as the fan. A fan switched off with the light leaves the room above 80 percent humidity, which is enough for mould to germinate overnight.

Why does mould grow on silicone and grout first?

Because both are porous, slightly flexible and stay damp longest. Silicone also collects soap residue, which feeds the spores.

Anti-mould silicone is not a permanent answer. It delays colonisation by a year or two, and the only real fix is keeping the joint dry.

What is a thermal bridge in a bathroom?

A point where the internal surface is colder than the dew point of the room air. An uninsulated external wall, a steel lintel, or a pipe chase running through an outside wall all qualify.

Condensation forms there first, which is why mould appears in a vertical stripe on one wall and nowhere else. Insulating the surface, not cleaning it, is the answer.

How do you fix a cold external wall in a bathroom?

Line it with insulated plasterboard or a tile backer board with an insulated core, if the room can afford 25 to 50mm. Where it cannot, use an anti-condensation coating and accept more heating.

Raising the surface temperature above the dew point is the mechanism. A warmer room does the same thing more expensively.

Does underfloor heating help with mould?

Yes, indirectly. A warm floor raises the surface temperature of everything in the room and speeds drying after use. It does not remove the need for extraction.

The combination of heating and ventilation is what keeps a bathroom dry. One without the other moves the problem rather than solving it.

What about a dehumidifier?

Useful as a temporary measure during a build, and useful permanently in a bathroom with no external wall. It is not a substitute for a ducted fan, because it treats the air rather than removing it.

A desiccant dehumidifier works better in a cold bathroom than a compressor unit, which stops working efficiently below 15 degrees.

Does sealing the grout help?

Slightly. A penetrating sealer reduces water absorption into the grout, which reduces how long the joint stays damp. It does not stop condensation on the tile surface.

Reapply every one to two years in a shower. A sealer that has worn off is worse than none, because it fails unevenly.

Why does mould come back after re-grouting?

Because the moisture is behind the tile, not in the grout. Re-grouting seals it in, and the mould returns through the new grout within months.

If the grout is dark underneath, the tile has to come off. That is a waterproofing failure, not a cleaning failure.

How do you check for a hidden leak?

Look for efflorescence, a white powdery deposit on the grout, and for grout that stays dark days after use. Both mean water is entering from behind.

Test by taping polythene over a suspect area for 48 hours. Condensation on the room side means the room; moisture under the polythene means the wall.

What paint works in a bathroom?

Water-based eggshell with a fungicidal additive, on a mist coat over the plaster. Standard emulsion in a bathroom absorbs moisture and grows mould within a year.

Vinyl matt is better than contract matt but still porous. Eggshell or satin is the practical minimum for a wet room.

Should the bathroom door be left open?

No. Leaving it open spreads moisture into the landing and cools the bathroom surfaces. Closed door plus running fan is the correct routine.

The exception is a bathroom with no fan, where the door open is the only ventilation there is. That is an argument for fitting a fan.

How long should the fan run after a shower?

Fifteen to twenty minutes, or until the mirror clears and the walls feel dry to the back of the hand. A humidity sensor automates this better than a timer.

If the walls still feel damp after twenty minutes, the fan is underperforming or the duct is restricted. Both are measurable.

Does a bathroom need a window to avoid mould?

No. A windowless bathroom with good mechanical extraction is drier than a bathroom with a window that is never opened. The fan is the control, not the glazing.

Where there is no external wall, a ducted fan run through a ceiling void to a roof terminal is the only compliant option.

How much does fixing a mould problem cost?

200 to 600 for a fan upgrade and duct work, 400 to 1,500 for insulated lining on a cold wall, and 2,000 to 6,000 if the waterproofing has to be redone.

The order matters. Fix ventilation first, because it is cheapest and resolves the majority of cases on its own.

Can mould be cleaned rather than removed?

On a hard surface, yes, with a fungicidal wash and a stiff brush. On silicone and porous grout, the growth is inside the material and cleaning only removes the surface.

Cut out and replace silicone that has gone black. No amount of cleaning reverses a colonised joint.

What is the long-term routine that works?

Squeegee the walls after a shower, run the fan for twenty minutes, keep the door closed, and leave the window on trickle if there is one. That routine beats any coating.

Once a month, clean the fan grille. A dusty grille cuts airflow by a third, and the mould returns without any obvious change.

When is mould a building problem rather than a habit problem?

When it appears on walls away from the bathroom, when it returns within weeks of treatment, or when there is a musty smell in an adjoining room. All three point to water ingress.

At that stage the fix is diagnostic, not decorative. The guide on tanking a walk-in shower sets out where a waterproofing failure usually begins.

Moisture control comes down to three systems working together: extraction, waterproofing and surface temperature. The guides on extractor ducting and on tanking cover the first two in detail.