Dehumidifier vs humidifier: what is the difference?

What each machine does

A dehumidifier draws room air over a cold coil or through a desiccant wheel, takes water out and returns drier air. The water ends up in a tank or down a hose. Its job is to bring relative humidity (RH) down to a target you set, usually somewhere around 50 to 60 %.

A humidifier puts water into the air, either as a fine cool mist from an ultrasonic plate, as vapour from a wick and fan, or as steam from a heated element. Its job is to raise RH in a room that is too dry. You fill a tank with water and the machine releases it into the room over time.

How to tell which your home needs

Use a digital hygrometer in the room for a few days and note readings morning and evening. Most homes are comfortable somewhere between 40 and 60 % RH. Readings that sit above about 60 to 65 %, with streaming windows or a musty smell, point to a dehumidifier. Readings that stay below about 35 to 40 % for long periods point to a dry room.

Look at the signs too. Condensation on windows in the morning, damp corners, mould spots and washing that takes days to dry all mean too much moisture. Static shocks and very dry air near radiators can mean too little. Our guide on what humidity to set a dehumidifier to gives target figures.

Why most UK homes need a dehumidifier

The UK has a mild, damp climate, and homes produce a lot of moisture inside from cooking, showers, breathing and drying washing. In winter, warm moist indoor air meets cold windows and walls and condenses. That is why condensation is a far more common problem here than dry air.

A dehumidifier tackles the moisture directly. A heated, ventilated home helps too, but in many flats and older houses a dehumidifier is the simplest way to bring readings down.

Some rooms need more help than others. Bedrooms collect moisture overnight from breathing, bathrooms and kitchens produce bursts of steam, and north-facing rooms with cold external walls are where condensation shows first. A dehumidifier can be moved to whichever room has the highest readings, and its humidistat stops it from drying the air further than you want. If you are unsure what size to buy, see what size dehumidifier you need.

See dehumidifiers for condensation →

When a humidifier might make sense

Rooms can get dry in a well-sealed, well-heated modern home in cold, clear weather, when outdoor air holds very little moisture. If a hygrometer shows readings in the low thirties or below for long periods, some people use a humidifier to bring them up. Set it to a moderate target and keep an eye on windows: if condensation appears, the setting is too high.

Never run a dehumidifier and a humidifier in the same room at the same time. They will simply work against each other and waste electricity.

Combination machines and air purifiers

Some products combine a dehumidifier with an air purifier filter, and a few climate units claim to do both humidifying and dehumidifying. These are less common in the UK. If you are also weighing an air purifier, our comparison of a dehumidifier vs air purifier explains what each one actually does.

If you have a combined unit, use only the mode your hygrometer readings call for. Remember that the filter in a dehumidifier with a purifier still needs cleaning or replacing on the schedule in the manual, and a humidifier's tank needs regular emptying and cleaning so water does not stand in it for days.

Frequently asked questions

Will a dehumidifier make the air too dry? Not if you set the humidistat to a sensible target, usually 50 to 60 %. The unit stops when it reaches the target and restarts when humidity rises.

Do I need a humidifier in winter in the UK? Most UK homes do not. Winter is when condensation is at its worst. Only consider one if a hygrometer shows consistently very low readings.

Can a humidifier cause mould? Running one at too high a setting can push humidity up enough for condensation and mould on cold surfaces, so keep the target moderate.