Short answer: yes, a dehumidifier can stop condensation mould from spreading by pulling moisture out of the air, and it's one of the quickest things you can do. But it treats the symptom. If mould is growing because a room is too cold, badly ventilated or wet from a leak or penetrating damp, the dehumidifier is managing the problem rather than solving it, and it costs money to run every day. Use it as a tool, not a fix.
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Get free quotesHow a dehumidifier helps
Condensation mould grows when moist indoor air meets a cold surface and leaves water behind. Take some of that moisture out of the air and less of it condenses. A dehumidifier does exactly that, collecting the water in a tank or sending it to a drain.
Most guidance suggests keeping indoor relative humidity roughly between 40% and 60%. Above that range for long periods, especially in cold rooms, condensation and mould become much more likely. A cheap digital hygrometer (a few pounds) will tell you where your rooms sit, and many dehumidifiers have one built in.
Compressor or desiccant?
There are two main types, and the right one depends on how warm the room is.
Compressor (refrigerant) dehumidifiers
These work like a small fridge: air passes over a cold coil, water condenses out and drips into the tank. They're usually the cheaper to run, and they work well in heated rooms. Their performance drops off as rooms get colder, because the coil struggles to get below the dew point of cold air and can ice up.
Best for: lived-in, heated homes, flats and rooms used every day.
Desiccant dehumidifiers
These pull air through a wheel of moisture-absorbing material, then heat it to drive the water off. They work well in cold spaces, are lighter and quieter, and give off a little warmth. The trade-off is higher power use.
Best for: unheated bedrooms, garages, conservatories, boats and caravans, or homes kept cool.
What size do you need?
Dehumidifiers are rated by how many litres of water they can remove in 24 hours. Two things to know:
- The headline figure is measured in test conditions that are often much warmer and more humid than a typical British home in winter. In real use you'll collect less.
- Bigger isn't always better. A larger machine reaches your target humidity faster and then cycles off; a small one may run flat out all day and never get there.
As a rough guide, a small unit in the 10 to 12 litre range suits a bedroom or small flat with mild condensation. A 20 litre class machine suits a family home, a room used for drying washing, or a property drying out after a leak. After a flood or major leak, use professional drying equipment.
What does a dehumidifier cost to run?
Running cost depends on the machine's power and how long it runs. Under Ofgem's price cap for October to December 2026, the average electricity unit rate is around 26p per kWh. (Ofgem announces the January to March 2027 rates on 25 November, so check your own tariff.)
The sum is simple: watts ÷ 1,000 × hours × your unit rate.
- A compressor unit drawing around 200 watts for 8 hours uses 1.6 kWh, which is roughly 42p a day at 26p/kWh.
- A desiccant unit drawing around 600 watts for 8 hours uses 4.8 kWh, which is roughly £1.25 a day.
Over a winter, that adds up. It's one reason a dehumidifier is not a sensible long-term answer to a building that can't manage normal moisture.
Getting the most from it
- Close doors and windows in the room you're treating, or it's trying to dry the whole street.
- Put it near the source: the room you dry washing in, or the bedroom with streaming windows.
- Keep it away from walls and curtains so air can circulate.
- Use the humidistat. Set a target around 50% rather than running it constantly.
- Empty and clean it. A full tank stops the machine; a dirty filter cuts its performance.
- Keep heating on. A dehumidifier and steady low heat work better together than either alone.
When a dehumidifier isn't enough
Call in a specialist if:
- mould keeps returning even with the dehumidifier running
- the tank fills quickly every day, which suggests a large or unusual moisture source
- damp patches appear after rain, or are at the bottom of walls, which points to penetrating or rising damp rather than condensation
- the room is still cold and you can feel draughts or cold spots on walls
A condensation specialist can measure humidity and wall temperatures and recommend a permanent fix, such as better extract fans, positive input ventilation or MVHR, insulation, or a repair. If you can't tell what kind of damp you have, start with condensation or rising damp?
For landlords
Handing a tenant a dehumidifier is a common response to a mould complaint. It can be a reasonable stopgap while you investigate, but it moves the running cost onto the tenant and doesn't fix a ventilation, heating or insulation problem. Government guidance warns against blaming damp and mould on tenants' lifestyles, and the updated Housing Health and Safety Rating System in England assesses the building, not the occupants. If a property needs a dehumidifier running every day to stay mould-free, it needs work. See condensation control for landlords.
FAQs
Should I run a dehumidifier all the time?
Run it with the humidistat set to your target (around 50%) and let it switch itself on and off. Constant running wastes electricity once the air is dry enough.
Can a dehumidifier kill mould?
No. It reduces moisture so mould is less likely to grow, but existing mould still needs cleaning, and the cause still needs fixing.
Is a dehumidifier or an extractor fan better for mould?
They do different jobs. An extractor fan removes moist air at the source in kitchens and bathrooms; a dehumidifier dries air in a room. For a permanent fix, ventilation usually matters more.
Where's the best place to put a dehumidifier?
In the dampest room, with doors and windows closed, away from walls and furniture. If you dry washing indoors, put it in that room.