Mechanical Ventilation in Australian Homes Under the NCC

What it is

Mechanical ventilation moves air in and out of a building using fans rather than relying on open windows. In Australian residential builds it covers bathroom and laundry exhaust fans, kitchen rangehoods and (less commonly in detached houses, more often in apartments) whole-of-house balanced ventilation. The rules sit in NCC Volume One Part F6 for Class 2 to 9 buildings, NCC Volume Two for Class 1 houses and AS 1668.2 for the mechanical design.

NCC ventilation requirements

NCC 2022 carries forward the ventilation requirement that was in 2019 under Part F4.5 into Part F6 Light and ventilation. The default route for habitable rooms is natural ventilation through openings to outdoor air with a total openable area of at least 5% of the room floor area. Borrowed ventilation through an adjoining room is allowed if both rooms together meet the 5% figure.

If the room cannot meet that openable area (typical of single-aspect apartments and deep-plan layouts) the alternative is a mechanical ventilation system designed to AS 1668.2.

Wet areas have their own rule. Bathrooms, laundries and sanitary compartments without an openable window need mechanical exhaust to outside. Recirculating to the roof space is not compliant. NCC 2022 made this explicit because moisture dumped into the roof space is one of the main drivers of the condensation problems Part F8 now addresses.

AS 1668.2 design flow rates

AS 1668.2 sets the design air-flow rates the mechanical system has to deliver. For a residential bathroom or toilet the typical figure is 25 L/s when the fan is running, with the fan ducted to outside through a sealed duct. For laundries the figure is similar. For kitchens with an exhaust hood the flow depends on the hood type and the cooktop size. AS 1668.2 also sets outdoor air rates for habitable rooms when the room is being ventilated mechanically because natural ventilation cannot be met.

The duct from the fan to the outside discharge has to be sized so the fan operating point still meets the design flow with the duct losses included. A 100 mm flex duct running 6 m to a roof tile vent will not deliver the rated airflow on most cheap bathroom fans.

Balanced vs unbalanced systems

A balanced ventilation system has matched supply and exhaust fans, usually paired through a heat exchanger (sometimes called a heat recovery ventilator or HRV). It maintains room pressure close to outside. A balanced system is the right answer for tight, well-insulated, high-performance builds because it gives controlled outside-air without the energy penalty of dumping conditioned air.

An unbalanced system has only an exhaust (or only a supply) fan. Exhaust-only systems pull replacement air in through gaps in the building fabric. That is fine for a leaky 1980s brick veneer but in a sealed 7-star NatHERS build it can pull combustion gases out of a flued appliance or pull damp air out of a wall cavity into the room.

For Class 1 residential builds with 7-star NatHERS performance the planning conversation often lands on a balanced system with heat recovery in cooler climate zones (5, 6, 7, 8) and exhaust-only with passive vents in warmer zones.

Condensation interface with Part F8

NCC 2022 introduced Part F8 Condensation management. The two key requirements that touch ventilation are: exhaust fans in wet areas must discharge to outdoor air (not into a roof space), and a sarking installed on the cold side of insulation in cooler climate zones has to be a vapour-permeable membrane. Part F8 does not on its own require a mechanical ventilation system but it removes the workarounds that used to let cheap installs pass.

What to check on site

Look for: bathroom and laundry fans ducted with rigid or sealed flex straight to a roof-mounted cowl or wall vent (no roof-space dump); duct runs as short as practical; an air-flow commissioning sheet showing the measured flow at each grille; sarking specified to the right vapour-permeance class for the climate zone; and where AS 1668.2 design has been used for a deep-plan or single-aspect room, the consultant's design report on file with the as-builts.