Mould doesn’t appear out of nowhere. It needs three things to grow: moisture, warmth, and something organic to feed on. Your home has all three. The good news is that if you control moisture, you control mould.
We’ve completed over 100,000 mould removal jobs across Queensland, NSW, and Victoria since 2011. In that time, we’ve seen the same preventable problems come up again and again. This guide covers what actually works to keep mould out of your home in the first place.
These tips apply whether you’re dealing with Queensland humidity, coastal NSW conditions, or Victorian winter condensation.
The 60% Rule: Understanding Humidity
Mould spores are always in the air. You’re breathing them right now. But they only start growing when humidity stays above 60% for extended periods.
Below 60% humidity: Mould spores stay dormant. They can’t grow or spread.
60-70% humidity: This is the growth zone. Mould can start developing within 24-48 hours.
Above 70% humidity: Rapid growth. Existing mould doubles in size every 5-7 days.
Every tip in this guide comes back to one thing: keeping indoor humidity below 60%.
Track your local conditions: The Australian Mould Index shows daily mould risk levels for your suburb based on temperature, humidity, and rainfall.
Room-by-Room Prevention Guide
Bathroom: Ground Zero for Mould
Bathrooms are where most mould problems start. Every shower pumps litres of moisture into the air. Without decent ventilation, that moisture just sits there.
Bathroom Prevention Tips:
- Run exhaust fans for 15-20 minutes after every shower. Not during the shower, but after. The fan needs to clear out the steam that’s already built up in the room.
- Squeegee shower walls and glass after use. It takes 30 seconds and removes about 75% of the surface moisture that would otherwise evaporate into the room.
- Leave the bathroom door open after showering. This lets the humid air spread out instead of staying concentrated in one small space.
- Check grout and silicone seals every few months. Cracked grout and failed seals let water get behind tiles, where mould grows without you seeing it.
- Keep exhaust fan vents clear. Dust and grime on the vent can cut exhaust effectiveness by half.
Quick Test: If your bathroom mirror is still fogged up 20 minutes after your shower, your ventilation isn’t good enough.
Bedroom: The Hidden Moisture Trap
You produce about 200ml of moisture every night just from breathing and sweating. In a closed bedroom, that adds up to more than 1.4 litres per week going into the air.
Bedroom Prevention Tips:
- Keep wardrobes 10cm away from external walls. External walls are colder and create condensation. Furniture pushed hard against them traps moisture and blocks airflow.
- Open bedroom windows for 10-15 minutes each day. Even in winter. A quick burst of fresh air clears out the overnight moisture buildup.
- Don’t make the bed straight away. Let the sheets air for 20-30 minutes first. Pulling the covers up while they’re still warm traps heat and moisture, which is exactly what mould and dust mites love.
- Check behind furniture a couple of times a year. Pull wardrobes and bed heads away from the wall and have a look for any hidden mould.
Warning Sign: If you get a musty smell when you open the wardrobe, that usually means hidden mould or too much humidity.
Kitchen: Cooking Creates Humidity
Boiling water, steaming vegetables, running the dishwasher. All of it adds moisture to the air. One family dinner can put 2-3 litres of moisture into your home.
Kitchen Prevention Tips:
- Use the rangehood on high when cooking. Keep it running for another 10 minutes after you finish too. Steam keeps rising even after you turn off the stove.
- Put lids on pots when boiling. An uncovered pot releases steam the whole time. A lid cuts moisture release by about 90%.
- Check under the sink every month. Slow leaks under the sink are one of the most common causes of kitchen mould. Catching them early saves a lot of trouble.
- Crack the dishwasher door open after the cycle finishes. Let the dishes air dry instead of releasing all that hot steam into a closed kitchen.
Laundry: The Forgotten Moisture Source
A single load of washing releases 2-5 litres of water as it dries. If that’s happening inside your house, all that moisture has to go somewhere.
Laundry Prevention Tips:
- Make sure your dryer vents outside. If it’s not vented externally, it’s pumping hot, wet air straight into your laundry. Check that the vent hose is connected and goes outside or into the roof cavity.
- Don’t dry clothes inside when it’s humid. Drying one load indoors can push humidity up by 30%. Use the clothesline or a properly vented dryer.
- If you have to dry inside, pick the right spot. Use the bathroom with the exhaust fan going, or a room with a dehumidifier. Never dry clothes in living areas or bedrooms.
- Fix leaking washing machine hoses straight away. Slow drips behind the machine create perfect conditions for mould to take hold.
Whole-Home Prevention Strategies
Ventilation: Your Best Defence
Good airflow is the single most effective thing you can do to prevent mould. It pushes out humid air and brings in drier air from outside.
- Open windows on opposite sides of the house to get cross-ventilation going. Even 10-15 minutes a day makes a real difference.
- Ventilate after anything that creates moisture. Showers, cooking, mopping, exercising indoors.
- Leave trickle vents open if your windows have them. They’re designed for steady background airflow.
- Think about mechanical ventilation if your home is hard to air out naturally. This includes apartments, terraces, and homes where security concerns mean you can’t leave windows open.
Humidity Control Tools
Hygrometer: A basic humidity meter costs under $20 from any hardware store. Put one in the bathroom and one in the bedroom. If readings regularly go above 60%, you need better ventilation or a dehumidifier.
Dehumidifier: A must-have for coastal properties, harbour suburbs, and any home that struggles to stay below 60% humidity. Set it to maintain 50-55% humidity using the timer or humidistat.
Air Conditioner Dry Mode: Most split systems have a dry mode, usually shown as a water droplet symbol. It pulls moisture out of the air without cooling the place down too much. Run it for 2-3 hours a day during humid weather.
Maintenance: Stop Problems Before They Start
- Clear gutters before storm season. Blocked gutters overflow into roof cavities and wall spaces.
- Check roof tiles after storms. Cracked or shifted tiles let water in.
- Look at window and door seals once a year. Failed seals let moisture in when it rains.
- Get exhaust fans serviced yearly. Clean the filters and make sure the motors are still working properly.
- Fix leaks immediately. Don’t put it off. Every day a leak continues is another day for mould to get established.
- Check subfloor ventilation. If your home has a subfloor, make sure the vents aren’t blocked by dirt, plants, or rubbish.
Seasonal Prevention Tips by State
Queensland: La Niña and Humid Summers
Queensland’s subtropical climate means mould risk peaks in summer (December to February) when La Niña brings weeks of high humidity.
- Run the aircon on dry mode for 2-3 hours a day, even when it’s not that hot
- Keep bathroom and laundry exhausts running longer than usual
- Check holiday homes before and after guest stays
- Keep an eye on the Australian Mould Index for extreme risk days
Service areas: Brisbane, Gold Coast, Sunshine Coast, Ipswich
Related: Queensland Mould Removal Guide
NSW: Year-Round Vigilance
NSW doesn’t really get a mould-free season. You’ve got harbour fog in winter, coastal humidity in summer, and everything in between. It means you need to stay on top of it all year.
- Homes near the harbour need dehumidifiers running year-round
- Beachfront properties cop extra wear on window seals from salt air
- In heritage buildings, keep furniture away from sandstone walls
- Wipe window condensation every morning in winter
Service areas: Sydney, Newcastle, Northern NSW, Sutherland Shire
Related: NSW Mould Removal Guide
Victoria: Winter Condensation
Victoria’s mould season is winter (June to August). Cold external walls meet warm indoor air and you get condensation. Heating dries the air out but creates temperature differences that encourage moisture buildup.
- Wipe window condensation every morning
- Keep heating at a moderate, steady level rather than cranking it up and down
- Open windows briefly every day, even when it’s cold outside
- Pay extra attention to south-facing rooms that don’t get direct sun
Service areas: Melbourne, Geelong, Ballarat, Gippsland
Related: Victoria Mould Removal Guide
Early Warning Signs: Catch Problems Early
Prevention is the goal, but early detection is the next best thing. These signs tell you conditions are heading towards mould if you don’t act:
Visual Warning Signs
- Condensation on windows: Especially if it happens every day. Indoor humidity is too high.
- Water stains on ceilings or walls: There’s a leak above or behind. Mould will follow.
- Peeling paint or wallpaper: Moisture is getting behind the surface.
- Discoloured grout: Early mould often shows up as pink or orange before turning black.
- Warped timber: Wood soaks up moisture. Warping means it’s been wet for a while.
Smell Warning Signs
- Musty odour: The classic mould smell. If you can smell it but can’t see it, it’s probably hidden somewhere.
- Damp smell after rain: Water is getting in somewhere.
- Smell that’s worse in certain rooms: That points you to where the problem is.
Health Warning Signs
- Respiratory symptoms that get better when you leave the house: Sneezing, coughing, blocked nose.
- Allergy symptoms with no obvious cause: Itchy eyes, skin irritation.
- Asthma getting worse: Mould spores are a common trigger.
When Prevention Isn’t Enough
Sometimes mould shows up no matter what you do. Here’s when you can handle it yourself versus when to call in the professionals.
DIY Cleaning Works When:
- The mould patch is smaller than your hand
- It’s on hard, non-porous surfaces like tiles, glass, or metal
- It’s the first time it’s appeared (not a repeat problem)
- There’s no underlying moisture issue
Call the Professionals When:
- Mould covers more than 1 square metre
- It’s on porous surfaces like plasterboard, carpet, timber, or fabric
- It keeps coming back after you clean it
- It appeared after water damage or flooding
- You can smell mould but can’t find it
- Anyone in the house is having health symptoms
- It’s in the aircon or HVAC system
Important: Don’t bother with bleach. It takes the colour out but leaves the root structure behind. The mould comes back, usually worse than before. Professional treatment kills spores at the source using antimicrobial misting that gets rid of airborne spores as well.
Frequently Asked Questions
How long does it take for mould to grow?
Mould can start growing within 24-48 hours if conditions are right (humidity above 60%, warmth, something organic to feed on). You’ll usually see visible patches within 1-2 weeks. When humidity is really high, mould colonies can double in size every 5-7 days.
Does air conditioning prevent mould?
It helps by lowering humidity, but it can also cause problems if you don’t maintain it properly. Use dry mode, make sure drainage is working, and clean the filters regularly. Cold air hitting warm surfaces creates condensation, which is where mould likes to grow.
Can opening windows make mould worse?
On really humid days (above 70% outdoor humidity), opening windows can let more moisture in. But most of the time, outdoor air is drier than indoor air that’s been loaded up with moisture from showers, cooking, and breathing. A quick daily airing almost always helps.
Why does mould keep coming back after I clean it?
Surface cleaning gets rid of what you can see, but it doesn’t touch the root structure growing underneath. It also doesn’t deal with the spores floating in the air or fix whatever moisture problem caused it in the first place. Until you sort out the moisture source and get proper treatment that kills spores at the root, it’ll keep coming back.
Is a dehumidifier worth the money?
If you live near the coast, in a harbour suburb, or your home regularly sits above 60% humidity, then yes. A decent dehumidifier costs $200-500 and doesn’t use much power. Compare that to professional mould remediation at $800-3000+ and it’s a pretty easy decision.
How do I know if my exhaust fan is doing its job?
Easy test: Run the exhaust for 20 minutes after a shower, then check if the mirror has cleared. If it’s still foggy, your exhaust is either too weak, blocked, or not venting properly. A lot of older systems vent into the roof cavity instead of outside, which just moves the moisture problem somewhere else.
Quick Reference: Prevention Checklist
Daily Habits
- Run exhaust for 15-20 minutes after showers
- Squeegee shower walls
- Air out the house for 10-15 minutes
- Use rangehood when cooking
- Wipe window condensation
Weekly/Monthly Checks
- Look under sinks for leaks
- Check bathroom grout and seals
- Clean exhaust fan filters
- Look behind furniture
- Empty dehumidifier tank
Seasonal Tasks
- Clear gutters before storm season
- Check roof after bad weather
- Pull furniture out and check walls behind
- Adjust ventilation habits for the season
- Service the air conditioning
Annual Maintenance
- Get exhaust fans professionally serviced
- Inspect window and door seals
- Check subfloor ventilation
- Look over roof tiles
- Consider a preventative mould inspection
Need Help? We’re Here
If mould has already turned up despite your best efforts, don’t sit on it. Mould spreads fast, especially when humidity is high.
Request a free quote – Send us some photos and we’ll get back to you with a detailed assessment within 24 hours. No site visit needed for the initial quote.
Call us: 1300 60 59 60
We’ve treated over 100,000 Australian homes since 2011 across Queensland, New South Wales, and Victoria. Our technicians use a safe, non-toxic misting treatment that kills mould spores and helps stop regrowth.