Air conditioner vs dehumidifier: controlling humidity and mould at home

Does an air conditioner dehumidify?

Yes. A reverse cycle split or ducted system pulls moisture out of the air every time it runs in cool mode. Daikin describes it plainly: warm, humid air passes over cold coils, the moisture condenses, and the air comes out drier.

Cool mode dehumidifies as a by-product of dropping the temperature. Dry mode, the water droplet on the remote, runs the coil cold with the fan slow, so more water comes out while the room temperature stays roughly where it is. It suits the sticky 24 degree day. Our guide to air conditioner modes and symbols covers the rest of the remote.

Heat mode is different. Warming the air lowers its relative humidity, so the room feels drier, but the water is still there. The coil is hot, so nothing condenses on it. That's why a house heated all winter with the windows shut can still grow mould on cold glass and in ceiling corners.

Dehumidifier vs air conditioner: which one do you need?

It comes down to the size of the space and whether you also want the temperature changed.

If the damp room is one you live in and it also gets hot or cold, a correctly sized reverse cycle split does all three jobs from one wall. If nobody sits in it, a portable unit is the cheaper fix.

When a portable dehumidifier still makes sense

There are jobs an air conditioner can't do well, and it would be daft to pretend otherwise.

• A single closed room with no aircon, such as a spare room or a study that only gets used on weekends.
• Wardrobes, linen cupboards and storerooms. Air from a split across the room rarely gets into a closed cupboard.
• Drying washing indoors in winter. Heat mode won't pull the water out, and dry mode cools the room you're trying to warm.
• Short-term drying after a leak, while the cause is fixed. The WA Department of Health treats water damage and ongoing dampness as signs to find and fix the source first.

We don't sell or rank portable dehumidifiers. If you buy one, check the rated input power on its spec sheet so you can price it against the air conditioner using the method below.

How to reduce humidity in a house: what level to aim for

The National Asthma Council Australia recommends keeping indoor relative humidity between 30 and 50 per cent where possible, because dust mites and mould both love damp air.

A cheap digital hygrometer tells you where you stand, and the National Asthma Council suggests one. Take a week of readings before spending money.

The cheap fixes come first. Run bathroom, kitchen and laundry exhaust fans that vent outside. Fix leaks. Vent the dryer outdoors. NSW Health and the Better Health Channel both list these before any appliance. Opening windows helps, except when it's more humid outside than in, which the National Asthma Council points out is common. After that, the Better Health Channel's advice is to use reverse cycle air conditioning or mechanical dehumidification if you have it.

Mould in the house: when it's a humidity problem and when it isn't

Mould needs moisture, and where the moisture comes from decides the fix. The WA Department of Health's 2024 guidance separates dampness from water getting in (leaks, rising damp, flooding, gaps in the building) and humidity in the air itself.

One patch under a window or around a pipe is usually a leak or a cold spot, and no machine fixes it. Mould across ceilings and wardrobes in several rooms is a humidity problem.

Condensation in the house in winter is the cold-climate version. YourHome, the federal government's sustainable house guide, lists reducing indoor humidity and adding ventilation as the fixes. In warmer climates it says to size the air conditioner so it contributes to dehumidification, or use a purpose-made dehumidifier.

Mould inside the air conditioner is its own issue. The WA guidance notes poorly maintained systems can spread mould, and treats growth in a ducted system as a job for professionals. Clean the filters, keep the drain clear, and get the unit serviced if it smells musty on start-up.

Humid climates: Brisbane, Sydney, Cairns and Darwin

In the tropics and subtropics, humidity is the main event for half the year. The WA Department of Health notes northern Australia's relative humidity is usually above 70 per cent over summer, and that Darwin's average ran between 66 and 77 per cent across the year in a ten-year analysis.

In Darwin and Cairns, expect bedrooms to run dry or cool for long stretches, and ask the installer about the condensate drain, because the unit will produce a lot of water. In Brisbane and coastal Sydney, the damp stretch is late summer and wet autumns, when a unit sized for the peak January afternoon can short-cycle on a humid 25 degree day and leave the room clammy.

Evaporative coolers are the wrong tool here. They add water to the air by design, and the National Asthma Council advises avoiding them in most Australian areas. Our reverse cycle vs evaporative guide covers the trade-off.

Whole house dehumidifier: ducted dehumidification explained

If the whole house is damp, the simplest whole-house dehumidifier is one many homes already have: a ducted reverse cycle system with a dry mode across every zone. Check your controller or manual before buying anything else.

Dedicated in-line dehumidifiers can also be fitted to the ductwork. They need roof space, power and a drain, so ask for them as a separate line item on the quote.

In new, well-sealed homes, moisture overlaps with ventilation. A heat or energy recovery ventilator swaps stale, damp indoor air for filtered outdoor air. See ducted air purification and fresh-air ventilation for that, and the ducted system guide for the air conditioning side. For a whole-house price with a zone plan, get up to three local quotes.

Dry mode vs dehumidifier running costs: how to compare

Electricity drawn in kW, times hours, times your rate. Our running cost guide uses 2026 default-offer rates of about 28 to 42 cents per kWh.

A 2.5 kW split draws about 0.41 kW in a typical running hour and a 3.5 kW split about 0.57 kW, which is 11 to 17 cents and 16 to 24 cents an hour at those rates. Dry mode runs the compressor at low output, so it tends to sit at or below those cool-mode figures.

For a portable dehumidifier, take the rated input power from its spec sheet. As an example of the method, a unit rated at 250 W draws 0.25 kW, about 8 cents an hour at 33 cents per kWh, or 80 cents for a 10 hour day.

So a small portable can cost less per hour, while the split covers a bigger room, drains itself, and cools or heats as well. Try your numbers in the running cost calculator, then get up to three local quotes for the room.

What size air conditioner do I need for a humid room?

Installers call the moisture part of the job the latent load, and the temperature part the sensible load. A unit sized only for the hottest afternoon can be too big for humid, mild days, and YourHome warns that oversized systems can switch off before removing enough moisture.

Size for how the room is used. A modern inverter that runs slowly for long periods dries the air better than an oversized unit that blasts and stops. Our sizing guide and size calculator are a starting point.

• Tell each installer the room is damp, and where the mould is.
• Ask what kW they chose and how they allowed for humidity as well as floor area.
• Ask where the condensate drain goes. A blocked drain in a humid town means water down the wall.
• For ducted, ask whether the controller runs dry mode across all zones.

Three quotes that describe the same room differently tell you who actually measured it. Get up to three local quotes and compare how each installer handled the humidity.