A bathroom without good ventilation gets mould on the ceiling, peeling paint and swollen doors. A fan that is badly placed or badly ducted can be almost as bad as no fan at all. This guide covers how to choose, place and duct an extractor fan so it does the job.
Why bathroom ventilation matters
A hot shower puts a lot of water vapour into the air. If it isn’t removed, it settles on cold surfaces like ceilings, windows and outside walls. Over time that causes:
- Black mould on ceilings and in corners
- Paint and plaster that bubbles or peels
- Damp in the ceiling space and framing
- Doors and cabinets that swell and stick
In New Zealand, the Building Code has rules for ventilating wet areas, and rental homes must also meet the Healthy Homes standards, which include an extractor fan in bathrooms with a shower or bath. Australia has its own code requirements. A good fan, installed well, protects your work and the client’s home.
Choosing the right fan
Fans come in a few main types:
- Ceiling fans sit in the ceiling and connect to a duct. These are the most common.
- Wall fans vent straight through an outside wall. Simple, but only work on an outside wall.
- Inline fans sit in the ceiling space, partway along the duct. They are quieter in the room and can be more powerful.
- Combination units include a heater or light with the fan.
When comparing fans, look at the airflow rating. This is usually given in litres per second or cubic metres per hour. Check the code requirement for your job and pick a fan that meets it with room to spare. Remember that the rated airflow is measured in ideal conditions. A long duct with bends will reduce the real airflow, sometimes by a lot.
Also look at the noise level. A noisy fan often gets switched off early, which defeats the purpose.
Where to put the fan
Placement makes a big difference.
- Put the fan close to the shower or bath, where the steam comes from
- Place it on the opposite side of the room from where fresh air comes in, usually the door
- Keep it away from the door itself, or it will just pull in air from the hallway and leave the steam behind
- Make sure there is a way for replacement air to get in, such as a gap under the door
Check the manufacturer’s instructions and the electrical rules for how close a fan can be to the shower. These zones are there for safety.
Ducting it properly
The duct is where many installs go wrong.
- Vent to the outside. Never let a fan blow into the ceiling space. That just moves the moisture to where you can’t see it, and it can rot the framing.
- Keep the run short and straight. Every metre and every bend reduces airflow.
- Use the right duct size. Don’t reduce the size from the fan outlet.
- Pull flexible duct tight. Sagging flexible duct holds water and slows the air.
- Slope the duct slightly down towards the outlet where you can, so condensation drains out, not back into the fan.
- Insulate the duct in cold ceiling spaces to reduce condensation inside it.
- Use a proper outlet through a wall, soffit or roof, with a backdraught flap and weather sealing.
Controls that help
How the fan is switched affects how well it works.
- A fan wired to the light is simple, but often turns off when the person leaves
- A run-on timer keeps the fan going for a set time after the light goes off
- A humidity sensor turns the fan on when the room gets steamy and off when it has cleared
For most homes, a run-on timer or humidity control works far better than a basic switch.
A simple example
A client has mould on the ceiling above their shower. The existing fan is near the door and blows into the ceiling space. The fix is to move the fan to above the shower, run a short insulated duct to a soffit vent with a backdraught flap, and fit a run-on timer. Clean and treat the mould, then repaint with a bathroom paint once everything is dry.
Common mistakes
- Fan blowing into the ceiling space
- Long, sagging flexible duct with lots of bends
- Fan placed right next to the door
- No gap for replacement air
- Fan too small for the room, or too noisy to be used
Quick checklist
- Check the code and any rental rules for the job
- Choose a fan with enough real airflow and low noise
- Place it near the shower, away from the door
- Duct it outside with a short, tight, insulated run
- Fit a proper outlet with a backdraught flap
- Use a run-on timer or humidity sensor
- Leave a gap for replacement air
This guide is general information only. Ventilation, electrical and building rules depend on where you work, and some work must be done by a licensed electrician. Check the current Building Code or your local building rules, any rental standards that apply, and the manufacturer’s installation instructions.