How do I stop bathroom mould coming back?
Mould is a ventilation and cold-surface problem, not a cleaning problem: fix extraction (a fan that actually ducts outside, run long enough), warm the cold surfaces (insulate external walls during renovation, heat the room), and use mould-resistant silicone and paint. Bleach treats the symptom for a fortnight.
Mould needs moisture sitting on cool surfaces. South London's back-addition bathrooms are the perfect host: two single-skin external walls (cold), a gravity-era heating system (cool room), and an extractor venting into a void or nothing at all (moisture stays). No amount of scrubbing changes that physics — renovation is the moment that can.
The renovation fixes, in order of effect: extraction that genuinely moves 15 l/s to outside air, ideally a continuous-running humidity-sensing fan; insulated plasterboard on the external walls (25–50mm transforms surface temperatures in these rooms); heating sized for the room — a towel rail alone under-heats most back additions; and mould-inhibiting sanitary silicone at every joint, because standard silicone is mould's favourite food.
Post-renovation habits matter less than people think once the fabric is right, but two help: run the fan through and 20 minutes beyond showers (or let the humidistat do it), and leave shower glass wiped or the door open. If a renovated bathroom still moulds, something in the chain above was skipped — usually the ducting.
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