Mould on Bathroom Walls: Why It Comes Back

You cleaned the mould off the bathroom wall in January. By March it’s back. By May it’s bigger. By August your partner is threatening to call the agent. Brisbane bathrooms grow mould faster than almost any room in the country — but the reason isn’t what most people think, and the fix isn’t the paint.

Here’s the honest, painter’s-eye view of why bathroom mould comes back and what actually stops it for good.

Why Brisbane bathrooms are worse than most

Three things combine to make Brisbane bathroom mould particularly stubborn: average annual humidity over 65%, a long summer wet season where humidity routinely passes 85%, and the fact that most Australian bathrooms are built with the bare minimum ventilation legally required, which is rarely enough for our climate.

Older Brisbane homes (pre-1990s) often have no ventilation at all in the bathroom beyond a small window. Newer homes typically have an exhaust fan but it’s frequently underpowered, ducted into the ceiling cavity rather than outside, or simply never turned on.

The four causes of recurring bathroom mould

Almost every recurring mould problem in a Brisbane bathroom traces back to one (or more) of these four causes:

1. Poor ventilation

This is the most common cause by a long margin. A hot shower puts roughly 1 litre of water vapour into the air every 10 minutes. If that vapour can’t escape — through an effective exhaust fan running for 20+ minutes after the shower, or through an open window — it condenses on cool surfaces (walls, ceilings, mirrors) and creates the constant damp environment mould needs to grow.

2. Paint with no mould-resistant additive

Standard interior wall paint isn’t designed for bathrooms. Painted with regular acrylic, a bathroom wall provides a damp, organic surface that mould happily colonises. Paints specifically formulated for kitchens and bathrooms contain additives that inhibit mould growth — they don’t prevent it entirely, but they slow it dramatically.

3. Untreated existing spores

When you scrub mould off a wall and immediately repaint, you’re not removing the mould — you’re trapping it under a new layer of paint where it continues to grow. Within months it bleeds back through the new finish.

4. Building moisture from behind the wall

Less common but the most serious cause. Water entering from a leaky shower screen, a failed waterproof membrane, a plumbing leak, or rising damp can saturate the back of bathroom walls. Mould on the surface is the symptom, not the disease. No amount of repainting will fix this — only finding and fixing the source of water.

Diagnosis — which one do you actually have?

Before you spend money on solutions, identify which cause you’re dealing with. Quick diagnostic guide:

Solution 1 — Improve ventilation

If your bathroom doesn’t have an exhaust fan, install one. If it has a weak one, upgrade it. A good bathroom exhaust fan should move at least 90 litres per second and should be ducted outside (not into the ceiling cavity). Run it for the entire shower and for 20–30 minutes after. Many modern fans have built-in timers that handle this automatically.

Cost-wise, this is usually the cheapest fix — $300–$800 for a quality fan plus electrician installation, and it eliminates the root cause of 60–70% of recurring bathroom mould.

Solution 2 — Use the right paint

If you’re repainting the bathroom (and you should, to clear the existing visible mould), use a paint specifically formulated for wet areas. Recommended products:

• Dulux Wash&Wear +Plus Kitchen & Bathroom Low Sheen: Mould-resistant additives, scrubbable, the most common premium choice.
• Haymes Bathroom & Kitchen Low Sheen: Comparable Australian-made alternative, similar performance.
• Resene Sonyx 101 Semi-Gloss: Premium option for serious mould problems, harder finish.

Standard interior wall paint should not be used in a bathroom. The price difference between standard and bathroom-grade paint is roughly $20 per 4L can — trivial against the cost of repainting again in 12 months.

Solution 3 — Treat the spores before painting

Painting over visible mould guarantees regrowth. Before any new paint goes on:

• Scrub the affected area with a dedicated mould treatment (Concrobium, Selleys Rapid Mould Killer, or a 1:4 bleach-to-water solution)
• Allow the treatment to dwell — most products require 10–30 minutes of contact time
• Rinse thoroughly and let the wall dry completely (at least 24 hours)
• Apply a mould-blocking primer (Zinsser Mould Killing Primer is the standard) before the bathroom paint

This sequence — kill, clean, dry, prime, paint — is what separates a temporary fix from a long-term one.

Solution 4 — Investigate behind the wall

If you’ve improved ventilation, used the right paint, and properly treated the spores — and mould still comes back — the cause is almost certainly moisture entering from somewhere you can’t see.

Common sources:

• Failed shower screen seal allowing water behind tiles
• Cracked grout or failed waterproof membrane in the shower base
• Slow plumbing leak in the wall cavity
• Inadequate flashing around windows or vents
• Rising damp through the slab (rare but possible in older homes)

This is where you call a plumber, builder or waterproofing specialist before the painter. No paint, however good, can fix a structural moisture problem.

The full bathroom repaint approach

When we repaint a Brisbane bathroom, the process looks like this: identify and address any ventilation issue (often we’ll recommend a fan upgrade before we start), thoroughly treat all visible mould and let the surface dry fully, apply mould-killing primer, then apply two coats of bathroom-grade paint. The whole job typically takes 1–2 days and lasts 5–8 years before any visible mould returns — provided the ventilation has actually been addressed.

What doesn’t work — popular but useless fixes

• Bleach-only solutions without addressing ventilation — mould comes back within months
• Painting straight over visible mould — guaranteed regrowth
• White vinegar instead of bleach — kills less than 50% of mould species
• Ventless air purifiers — they don’t remove the moisture that’s causing the mould
• Dehumidifiers as a primary solution — useful in extreme cases, but they don’t address the source

Frequently asked questions

What causes mould on bathroom ceilings specifically?

Almost always condensation from steam rising during showers. The ceiling above a shower is the coolest surface in the room and condenses the most water. Ventilation is the only real fix — paint quality helps but doesn’t solve the root cause.

Will the paint smell harm me?

Modern bathroom paints are low-VOC and the smell dissipates within hours. Ventilate well during application and for 24 hours after. If you have asthma or are pregnant, we can spec a zero-VOC bathroom paint at no extra charge.

Should I seal the grout when repainting?

If the grout is more than 5 years old and looks porous or discoloured, yes — sealing the grout before painting walls reduces moisture migration through the tiled areas. We can include this in a bathroom repaint quote or you can DIY with a grout sealer from any hardware store.

How often does a Brisbane bathroom need repainting?

With proper ventilation, bathroom-grade paint and reasonable care, every 6–10 years. Without good ventilation, every 2–4 years. The ventilation factor is enormous.

Can ducted air conditioning help?

Partially. Reverse-cycle ducted aircon does dehumidify, but it doesn’t move air out of the bathroom — and steam from a hot shower will still condense on cool surfaces faster than the aircon can compensate. You still need a properly sized, properly ducted exhaust fan.

If your bathroom is overdue for a proper repaint — with ventilation upgrade if needed — call Ben on 0489 903 846 or request a free quote. We’ll diagnose the cause, recommend the fix, and quote the work in writing.