Why Your Bathroom Fan Isn’t Cutting It (And What to Do About It)
Let’s get straight to it: your bathroom extract fan isn’t doing nearly as much as you think. You might hear it hum after your shower and assume everything's fine. But if your walls are sweating, mirrors fog up, or you’re spotting mould, then no, everything is not fine.
After 15+ years testing homes that look perfect until the fan goes on, we can say with confidence: most bathrooms in New Zealand are under-ventilated. And we’re not talking about old villas with dodgy wiring. We mean new builds, too.
What Your Bathroom Fan Is Actually Supposed to Do
The moisture problem
Every person in a household contributes to indoor humidity. Between breathing, cooking, showering, drying laundry, and just existing, each person adds up to 4 litres of moisture per day into the air.
A family of four? That’s 16 litres a day – more than a bucket of water hanging around in your walls, carpets, and ceiling cavities.
What does the code say?
New Zealand’s Building Code Clause G4 (Ventilation) gives you two basic options:
Natural ventilation (i.e. windows you open daily)
Mechanical extract fans in rooms with moisture or pollutants (kitchen, bathroom, toilet)
Most new homes go with a code-minimum extract fan – often with an on/off switch or a short-timed run-on. That might technically be compliant, but it often doesn’t do the job.
Why Most Bathroom Fans Fall Short
1. Not enough runtime
Your fan moves 25 L/s (which is generous). If you’re generating 16 litres of moisture per day, you need to replace over 1,800 m³ of air to remove it. That means your bathroom fan should be running 24-32 hours a day to keep up. So you could be running out of day to extract…
Instead, most people flick it on for 5 minutes, or let the run-on timer go for 20. Not even close.
2. Poor installation
Fans often discharge into the ceiling cavity instead of outside. That just moves moisture from the bathroom to the roof space, where it can condense and create mould or rot. We see this constantly during blower door testing.
3. Wrong type of fan
Ceiling-mounted fans that rely on ducting need to be sized properly. Long duct runs, bends, or cheap flexi-duct reduce performance drastically.
How to Know If You Have a Moisture Problem
Your bathroom mirror fogs up for more than a few minutes
There’s condensation on windows or walls
Paint is peeling or bubbling
You smell that musty "old towel" scent
Mould is growing in corners or behind furniture
Even better: install a cheap hygrometer and check your indoor RH (relative humidity). If it’s consistently above 60%, your ventilation is underperforming.
What You Can Do About It
1. Increase fan runtime
Install a humidity sensor or a longer run-on timer (60+ minutes). Or better yet, run it continuously at low speed, and boost on humidity or light.
2. Vent it outside
Ensure all exhaust fans are ducted to the exterior. No exceptions.
3. Size it right
How Long Per Person Per Day?
Want a per-person estimate?
25 L/s fan: ~5.15 hours/person/day
50 L/s fan: ~2.58 hours/person/day
Multiply by your household size and you’ve got your real runtime.
What the Code Says (and Where It Falls Short)
NZ’s Acceptable Solution for Clause G4 just says:
Minimum 25 L/s extract in bathrooms (via mechanical fan)
No mention of runtime or actual household moisture load
In theory, it assumes intermittent use. In reality? People generate moisture all day, not just when they shower.
4. Consider whole-home ventilation
In airtight or well-insulated homes, spot ventilation often isn’t enough. Consider balanced mechanical ventilation with heat recovery (MVHR) to control humidity, temperature, and air quality throughout the whole house.
FAQs About Bathroom Ventilation
Do I have to use a mechanical fan?
Yes, if there’s no openable window. But in practice, relying on open windows for moisture removal is hit-and-miss.
Is a positive pressure system good enough?
Not for moisture control. Those systems push dry air in, but don't actively extract the wet air from source areas like bathrooms and kitchens.
How can I check if my fan is doing its job?
Hold a piece of toilet paper near the grille. If it barely moves, you’ve got a weak fan or a blocked duct. Or get a flow hood test done to measure it.
Final Thoughts
Ventilation isn’t about ticking a compliance box. It’s about removing litres of moisture your family adds every day — before it soaks into your home.
Your bathroom fan might be working. But is it working enough?
A builder told me last week, “We always put extract fans in… but no one ever turns them on.”
That’s like buying a fridge and never plugging it in.
Want help getting it right? Book a quick call. We can test, assess, and recommend real fixes — not just tick boxes.
(And no, your house does not need to "breathe". It needs to ventilate. There’s a difference.)
