The basic function of an exhaust fan is simple: nasty air out, fresh air in. This function is well understood by many yet homeowners tend to overlook the other vital purpose of exhaust fans, which is removing moisture in the air.

Moisture problems resulting from high humidity is one of the biggest contributors to structural rot in Australian homes. Every time you run the water, you create steam to a certain extent. Contrary to popular beliefs, steam is not only produced by hot water. So long as the water is relatively warmer than the current room temperature, steam will be created from that difference in temperature.

That steam will penetrate every opening and crevice, before cooling down and turning back into water from condensation. This creates the perfect breeding environment for mould and mildew, leading to nasty odour and eventually eating into the drywall and timber frames.

Exhaust fans provide a way to expel the humid air before it has the chance to condense, along with removing any unpleasant odour that may linger. In Australia, exhaust fans are required by building codes, especially if there’s no operable window.

What Size and Capacity of Exhaust Fan Do I Need?

Now that we know the importance of this little workhorse, how should we decide which exhaust fan to buy?

An exhaust fan’s ability to move air is rated in meters cubed (m3) per hour, or m3/h. That is, the higher the m3/h, the more air the exhaust fan will move from your room. This means:

  • The larger your bathroom or laundry, the higher the m3/h rating needed, and vice versa.
  • If an exhaust fan is to be installed near a shower or bathtub, look for one with IPX4 Rating. IPX4 rating is the standard for exhaust fans that are suitable for installation above shower or bathtub.

The exhaust fan you need is determined by the type and size of the room it is to be installed in, as different rooms require different rates of air flow. The rate of air flow is represented by the number of complete changes of air within the room per hour.

Please see the following exhaust fan size guidelines to choose an exhaust fan with an extraction value that is suitable for your size of bathroom or laundry.

Exhaust Fan Size Guidelines

To calculate the m3/h required for your own room, simply follow this formula:

Size of room (m3) x Average changes of air per hour (type of room) = m3/h required.

Average Changes of Air Per Hour (Type of Room)

As a general rule of thumb, we will use the following estimations for the ideal changes of air per hour:

= Bathroom without shower, including laundry (10 changes of air per hour)

= Bathroom with shower (15 changes of air per hour)

Once you have calculated the extraction value (m3/h) needed for your room, use the number as the main criteria to narrow down your choices.

  • Bathtub, sauna, or steam room – If you have a bathtub, sauna or steam room, you will need to increase the size and capacity of the exhaust fan to compensate for the additional moisture level

Where is the Ideal Position for an Exhaust Fan?

The ideal position for an exhaust fan is one directly opposite and far from the source of in-flowing air such as a window or door. Putting an exhaust fan in this opposite position will encourage cross-breeze which is essential to absorb moisture and odours throughout the entire room.

It is important not to install an exhaust fan too close to a window or door as this will only provide air circulation around the limited area but have little effect to the rest of the room.

Some Final Important Considerations:

  • Exhaust fan with light is very convenient – Consider installing an exhaust fan with light to replace an existing light fixture for easier wiring and convenience. It is cheaper to do so too.
  • Bigger is not always better – Smaller, less powerful exhaust fans are perfectly suitable for smaller bathrooms.
  • Noise level – Pay attention to the noise level as not all exhaust fans are created equal. Expect to pay more for quieter exhaust fans.
  • Energy efficiency – Take note of the energy efficiency of the exhaust fan. When given a choice between two exhaust fans with comparable m3/h, choose the one with lower wattage.
  • Air ducts – Air ducts may be necessary if the roof space is narrow and limited. Please consult your builder or electrician for more information.