Preventing Mould in Wet Areas: A Complete Guide

Australia's climate makes bathroom mould a near-universal challenge. High humidity levels, particularly in coastal and tropical regions, create ideal conditions for mould growth. Add in daily showers, poor ventilation, and warm temperatures, and you've created a perfect storm for those black spots to appear on your caulk, grout, and silicone seals.

The good news is that mould prevention is largely about consistent habits and proper product choices. Understanding why mould grows helps you target your prevention efforts effectively.

Understanding Mould Growth

Mould spores are everywhere—floating in the air, settling on surfaces, waiting for the right conditions to germinate and grow. Those conditions are:

You can't eliminate mould spores or avoid normal bathroom temperatures. But you can control moisture and minimise food sources—the two variables within your power.

Ventilation: Your Primary Defence

Inadequate ventilation is the leading cause of bathroom mould. Steam from showers increases humidity dramatically, and without proper air exchange, that moisture lingers on surfaces for hours.

Exhaust Fans

Every bathroom should have a properly sized exhaust fan. The fan should run during showers and for at least 20 minutes afterward—many people switch it off too soon. Consider installing a timer switch that runs the fan automatically for a set period after the light is turned off.

Check that your exhaust fan actually moves air effectively. Hold a tissue near the vent while it's running—it should be sucked firmly against the grill. If there's no strong airflow, the fan may need cleaning, repair, or replacement.

Windows and Passive Ventilation

Where possible, open windows after showering to create cross-ventilation. Even a small gap allows humid air to escape and dry air to enter. In winter, when you might hesitate to open windows, run the exhaust fan longer to compensate.

Consider leaving the bathroom door open after use (when practical) to allow moisture to dissipate into the rest of the house, where it's diluted and easier to manage.

Reducing Moisture at the Source

Beyond ventilation, several practices reduce the moisture available for mould growth:

Wipe Down Surfaces

A quick wipe of shower walls and screens after use removes most of the water that would otherwise evaporate slowly and raise humidity. Keep a squeegee in the shower and make it a habit. This simple practice dramatically reduces mould on caulk and grout.

Fix Leaks Promptly

Dripping taps, leaking showerheads, or weeping pipes maintain constant moisture that feeds mould growth. Repair any leaks immediately—they waste water and encourage mould.

Avoid Leaving Wet Items

Wet towels draped over doors, bath mats left damp on floors, and soggy loofahs all contribute moisture. Hang towels to dry properly, wash bath mats regularly, and replace shower accessories before they become mould havens.

Choosing Mould-Resistant Products

The products you use in wet areas can either resist or encourage mould growth. Make informed choices:

Sealants and Caulk

Always choose bathroom-specific silicone with built-in mould-resistant properties. These products contain fungicides that inhibit mould growth on the sealant surface. The protection isn't permanent—it diminishes over years—but it significantly extends the time before mould becomes visible.

Standard silicone without mould resistance will show mould growth within months in a steamy Australian bathroom. The modest additional cost of mould-resistant formulations is well worth it.

Paint

Bathroom paint should be specifically formulated for wet areas, with both mould-resistant additives and a finish that discourages moisture absorption. Avoid flat/matt finishes in bathrooms—semi-gloss or gloss surfaces are easier to clean and less hospitable to mould.

Grout

When grouting or regrouting, choose grout with antimicrobial properties, or apply a grout sealer after installation. Sealed grout resists moisture penetration, reducing the conditions that allow mould to establish deep in grout lines.

Regular Cleaning Routines

Even with the best prevention, regular cleaning is essential. Mould is easier to manage when addressed early and often:

Weekly

• Wipe down tiles, screens, and sealant lines with bathroom cleaner
• Clean the shower base and floor waste
• Rinse and clean bath/shower accessories

Monthly

• Deep clean grout lines with a grout cleaner or paste of baking soda and water
• Inspect sealant for any mould penetration or deterioration
• Clean exhaust fan grilles (dust accumulation reduces airflow)

Quarterly

• Clean inside exhaust fan housings
• Check under sinks and behind toilets for any moisture or mould
• Assess whether any sealant needs replacement

Treating Existing Mould

If mould has already established itself, address it promptly before it spreads:

Surface Mould on Caulk

Mould on the surface of sealant can often be cleaned. Apply mould remover, let it work, then scrub with an old toothbrush. If the mould returns quickly or has grown into the caulk (appearing as spots that don't clean off), replacement is necessary.

Mould on Grout

Grout mould often penetrates deeper than surface cleaning reaches. For persistent grout mould, consider a poultice treatment (mould remover covered with plastic wrap and left overnight) or professional grout cleaning. Severely affected grout may need raking out and replacement.

Mould Behind Caulk or Tiles

If you see mould at tile edges or behind deteriorated caulk, moisture is getting behind the waterproofing. This requires removal of the affected sealant, treatment of the underlying area, thorough drying, and careful resealing. In serious cases, professional assessment of the waterproofing membrane may be needed.

Climate-Specific Considerations

Australia's diverse climates create different mould challenges:

Tropical and Subtropical Regions

Queensland, Northern Territory, and northern NSW face the biggest mould challenges. Year-round humidity means bathrooms never fully dry without mechanical assistance. Air conditioning and dehumidifiers can help, and exhaust fans should run longer than in drier climates.

Coastal Areas

Salt-laden air compounds moisture problems. Ensure excellent ventilation and consider more frequent caulk replacement, as salt air can accelerate sealant degradation.

Temperate and Cooler Regions

Condensation is the main issue—warm bathroom air hitting cold surfaces creates water. This is particularly common in winter. Running heating in bathrooms, ensuring good insulation, and maintaining ventilation helps reduce condensation-related mould.

Design Considerations

If you're renovating or building, design choices can reduce future mould problems:

• Install generous exhaust fan capacity
• Position showers near exhaust points
• Choose frameless shower screens (less hardware for mould to hide on)
• Minimise horizontal surfaces where water pools
• Ensure adequate natural light—UV inhibits mould growth
• Consider floor-to-ceiling tiles to reduce painted surfaces

Prevention is always easier than cure. By controlling moisture, maintaining good ventilation, choosing appropriate products, and cleaning regularly, you can keep your wet areas looking fresh and mould-free for years.