Extractor fans, PIV and dehumidifiers: which ventilation fix stops mould?

Updated 6 November 2026 · By the Mould Cleaning team

Short answer: For most UK homes, the most effective fix for condensation mould is a properly sized extractor fan in the bathroom and kitchen, running during and after use, plus open trickle vents for background ventilation. Positive input ventilation can help whole houses with widespread condensation, and dehumidifiers are a useful targeted or short-term fix. None of these solve leaks or rising damp.

Start with the moisture sources

Ventilation works best when the biggest sources of moisture are dealt with first. Drying clothes indoors on radiators or airers puts a large volume of water into the air, unvented tumble dryers do the same, and cooking without lids or extraction adds more. Moving laundry to a vented dryer, outside or a room with a dehumidifier running often makes a bigger difference than any equipment upgrade.

Extractor fans: the first line of defence

Approved Document F sets extract rates for new and replacement fans in England. For intermittent extraction it gives 15 litres per second for a bathroom, 30 litres per second for a kitchen cooker hood that extracts to outside, 60 litres per second for a kitchen fan elsewhere, 30 litres per second for a utility room and 6 litres per second for a WC. Wales and Scotland have their own equivalent guidance.

A fan with a run-on timer or humidistat keeps clearing moisture after the shower stops
Long, kinked or flexible ducting can cut real airflow well below the fan rating
Fans need a gap under the door, typically around 10mm, so replacement air can get in
A clogged grille or failed backdraught shutter reduces performance
Recirculating cooker hoods filter grease but do not remove moisture

A fan that is noisy, slow to clear mirrors or only runs while the light is on is a common cause of recurring bathroom mould.

Background ventilation: trickle vents and air bricks

Trickle vents in window frames provide a small, constant supply of fresh air and are designed to be left open. Air bricks and wall vents do the same in some homes. They are often closed or painted shut, which leaves rooms with little or no fresh air, especially bedrooms with doors and windows shut overnight. Opening them is free and usually has only a small effect on heating costs.

Comparing the main options

Option	Best for	Typical UK cost guide	Limitations
Bathroom or kitchen extractor fan	Steam at source	£200 to £450 supplied and fitted	Only works when running and properly ducted
Continuous running fan with humidity sensor	Bathrooms without windows, flats	£300 to £600 supplied and fitted	Needs suitable ducting and power
Positive input ventilation	Whole houses with widespread condensation	£500 to £1,200 fitted	Needs loft space, less suited to some flats
Dehumidifier	Laundry drying rooms, problem bedrooms	£150 to £300 to buy	Only treats one area, running cost, needs emptying
Mechanical ventilation with heat recovery	Airtight homes and major refurbishments	Several thousand pounds	Disruptive to retrofit, needs servicing

These are rough guide figures. Prices depend on the product, ducting route and electrical work required.

Heating and insulation also matter

Ventilation removes moisture, but cold surfaces are where it condenses. Homes heated in short bursts, or with rooms left unheated, get more condensation than homes kept at a steady, moderate temperature. Missing loft insulation above a bathroom or bedroom ceiling, uninsulated solid walls and thermal bridges around window reveals and lintels create cold spots where mould keeps returning. Topping up loft insulation over the problem area is often a cheap and effective fix.

Heat the home steadily rather than on and off
Do not leave rooms completely unheated in winter
Check loft insulation over ceilings where mould appears
Leave a gap between wardrobes and cold external walls

Simple checks on an existing fan

Before replacing a fan, check whether the one you have is doing its job. Run it while the room is steamy and see whether the mirror clears within a few minutes of finishing a shower. Hold a sheet of tissue up to the grille: if it is not pulled firmly against it, airflow is weak. Clean the grille and check outside that the external vent is not blocked by nests, paint or a stuck flap. A fan that only runs while the light is on, with no run-on timer, switches off just as moisture levels peak.

These are rough checks. An electrician or ventilation installer can measure actual airflow with an anemometer and compare it with the Approved Document F figures, which shows whether the fan, the ducting or both are the problem.

When equipment will not help

If mould appears in one defined patch, after rain, low on a wall or near a known leak, the cause is likely to be damp rather than condensation. More ventilation may dry the surface slightly but will not fix a leaking gutter, defective render, a bridged damp-proof course or a plumbing leak. In those cases get the building repair done first, then clean and treat the mould. BS 5250, the British Standard code of practice for managing moisture in buildings, sets out this whole-building approach to moisture.