How to pick a dehumidifier that actually stops mould

The mould trick

Mould needs three things to grow: moisture, something to grow on, and time. Cut the moisture, kill the mould. Wiping walls with bleach feels productive but doesn't stop it coming back. A dehumidifier does.

What humidity level you want

Mould stops growing under 60% relative humidity. A healthy home sits between 40 and 55%. Anything over 65% and you're inviting trouble.

Buy a £10 hygrometer to spot-check. If your readings are over 60% most evenings, you've got a problem to solve.

How to size the unit

Dehumidifiers are rated in litres of water removed per day. Rough guide:

• Small room (bathroom, laundry): 6 to 8 L/day
• Medium bedroom: 10 to 12 L/day
• Living room or basement: 16 to 20 L/day
• Whole apartment: 25 L/day

Buy a notch bigger than you think you need. The unit runs less often, uses less energy, and lasts longer.

Compressor vs desiccant

Compressor: Standard, cheaper, struggles below 15°C. Best for most warm-climate homes.

Desiccant: Works at any temperature, including cold rooms. Good for unheated garages and basements, or anywhere genuinely cold. Uses a bit more electricity.

Where to put it

Centre of the room ideally, away from walls and curtains, with at least 15cm of clearance around the air intake. If you can see where the damp is coming from (a sweaty wall, the bathroom door), put the dehumidifier between that source and the rest of the room.

Quick wins while it's running

• Keep extractor fans on for 20 minutes after every shower
• Cook with the rangehood on, lids on the pots
• Don't dry clothes inside without ventilation
• Open windows for 10 minutes in the morning to flush overnight humidity

What humidity level actually stops mould

Mould needs moisture to grow, and the magic number is 60% relative humidity. Below that, mould spores struggle to take hold. The sweet spot for a comfortable, mould-resistant home is between 45% and 55%. A cheap hygrometer, the little gadget that reads humidity, costs very little and takes the guesswork out of the whole problem. Buy one before you buy the dehumidifier, because it tells you whether you even have a problem and later tells you whether the dehumidifier is winning.

Sizing a dehumidifier without overspending

Dehumidifiers are rated by how many litres of water they pull from the air per day. For a slightly damp room you want roughly 10 litres a day. For a noticeably damp space with visible condensation or a musty smell, step up to 20 litres or more. A unit that is too small simply runs constantly and never gets ahead of the moisture, so erring slightly large is the safer mistake here.

There are two technologies worth knowing. Compressor models are cheaper to run and work best in warm, humid conditions, which makes them the default choice for most homes. Desiccant models keep working in cold rooms like garages and unheated spare rooms where compressor units struggle, but they use more electricity. Match the type to where the damp actually is.

Fixing the cause, not just the symptom

A dehumidifier treats the air, but persistent mould usually has a source. Look for the obvious culprits first: drying laundry indoors without ventilation, a bathroom with no extractor fan, a kitchen where pot lids are never used, or a cold external wall where warm moist air condenses. Addressing the source means your dehumidifier does far less work, and sometimes you find you barely need it at all. Open windows for a short, sharp airing each day, even in winter, and use extractor fans every single time you cook or shower.

Frequently asked questions

Will a dehumidifier remove mould that is already there?

No. It stops new mould by drying the air, but existing mould has to be physically cleaned off. Once the surface is clean and the humidity stays below 60%, it should not come back.

Where should I put the dehumidifier?

In the dampest room, with some clearance around it for airflow, and with the doors to that room closed so it is not trying to dry the whole house at once. Move it to whichever room currently has the problem.

How much does it cost to run?

Modern compressor dehumidifiers are reasonably efficient, but running one around the clock does add up. Using a humidity reading to switch it off once you hit 50% rather than leaving it on permanently keeps the cost sensible.

Related reads