Condensation and ventilation control in London
Condensation forms when warm, moist air meets a cold surface — a cold bathroom wall, a window, a cold bedroom corner with poor airflow. Bleaching off the black mould that follows treats what you can see, not what is causing it. The fix is getting the moisture out of the building before it condenses, and stopping the cold spots that give it somewhere to land.
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What this includes
- Extractor fan installation and repair, bathrooms and kitchens
- Humidity-triggered and timer-run fan upgrades
- Positive input ventilation (PIV) systems
- Trickle vents and passive ventilation checks
- Identifying cold-bridging and cold-spot causes
- Mould treatment following the ventilation fix, not instead of it
Worth knowing first
Some of this may save you a callout entirely. We would rather tell you than not.
Black mould is a symptom, the moisture is the cause
Cleaning mould off without dealing with the humidity behind it means it returns within weeks. Every job here starts with why the air is that damp before anything gets treated on the surface.
An extractor fan that switches off with the light is often the problem
Steam and moisture keep circulating for a good while after a shower ends. A humidity-triggered fan, or one on a timer that runs on after the light goes off, clears it properly instead of stopping the moment the switch does.
Drying laundry indoors is a major, avoidable source
A single load of washing drying indoors releases a genuinely large amount of water into the air. Where there is no outside space or tumble dryer, that moisture needs somewhere to go — proper extraction or a dehumidifier, not a closed window.
Trickle vents get painted shut more often than people realise
Passive vents built into window frames are routinely painted or sealed over during redecoration, quietly removing the background ventilation the room was designed around. Checking they still open is a two-minute job worth doing.
Common questions
How do I know if it is condensation rather than penetrating or rising damp?
Condensation typically appears on cold surfaces — windows, north-facing walls, corners with poor airflow — and worsens in cold weather with cooking, showering or drying laundry. A damp survey settles it where it is not obvious.
Will a dehumidifier fix it on its own?
It helps in the short term but does not address the underlying ventilation, and running one constantly costs more over time than fixing the extraction properly.
Do I need an electrician as well for a new extractor fan?
Only if new wiring or a new switched supply is needed. Many replacements use the existing wiring and are a straightforward swap.
Can you treat the mould as well as fix the ventilation?
Yes, but always alongside the ventilation fix — treating mould without it is a temporary result, and we will say so rather than sell it as a standalone job.
If it turns out to be plumbing
Where the cause is a leak or a pipe rather than damp itself, these are the pages that cover it.
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General Plumbing
Taps, toilets, radiators and pipework. The jobs that never quite get done.
More on General Plumbing -
Leak Detection
Hidden leaks traced with acoustic and thermal equipment, before floors come up.
More on Leak Detection
Damp and condensation, the other pages
Related advice
Need condensation & ventilation control in London?
Tell us what’s happening and where you are, and we’ll tell you what it costs before anyone starts work.