Mechanical Ventilation for Basement Levels

What You Need to Know

Basement levels cannot rely on open windows for fresh air. The NCC requires mechanical ventilation for every occupied underground space, from car parks to plant rooms to retail tenancies. AS 1668.2 sets the airflow rates. Get the system wrong, and the building fails its occupation certificate. This memo covers the ventilation, smoke control, and ductwork rules that apply below ground.

• Every occupied room below ground needs mechanical ventilation complying with AS 1668.2, unless natural ventilation can be proven (NCC 2025, F6D6)
• Enclosed car park storeys need mechanical exhaust. The exhaust rate is the greatest of three calculations: contaminant generation, staff exposure, or 2.5 L/s per m² of floor area (AS 1668.2-2012, Section 4)
• Car park CO must stay below 60 ppm (1-hour average), 30 ppm (8-hour TWA), and never exceed 100 ppm at any point (NCC 2025, Table F6V2)
• Supply air to basement car parks must equal 75–90% of the exhaust rate to maintain slight negative pressure (AS 1668.2-2012, Section 4)
• Fire-isolated stairways serving more than 2 below-ground storeys need automatic air pressurisation per AS 1668.1 (NCC 2025, E2D4)
• Zone pressurisation systems must hold 20–80 Pa between fire compartments to stop smoke spread (AS/NZS 1668.1:2015, Section 4)
• Exhaust discharge must sit at least 3 m above ground and 6 m from walkways, openable windows, or property boundaries (AS 1668.2-2012, Section 4)
• Air-handling systems that recycle air between fire compartments must shut down on smoke detection or operate as smoke control systems (NCC 2025, E2D3)

Take a three-level basement with 80 car spaces per level and a total floor area of 6,000 m². Using the area-based method alone, each level needs at least 5,000 L/s of exhaust air (2,000 m² × 2.5 L/s/m²). That is roughly two large axial fans per level. Multiply by three levels, and you have six fans plus the ductwork risers to move that air up and out of the building.

The deeper the basement, the harder the ductwork routing. Exhaust ducts must travel vertically from each level to a discharge point at least 3 m above ground. That vertical run needs dedicated riser shafts through every floor slab, fireproofed at each compartment boundary. Supply air ducts run the same path in reverse, drawing outdoor air down into each level. On a three-level basement, these risers can consume 2–4 m² of floor area per shaft.

Smoke control is separate from everyday ventilation. If the basement has more than two levels below ground, the NCC requires pressurised fire stairs. The pressurisation system pushes fresh air into the stairwell during a fire, holding a positive pressure of 20–80 Pa to keep smoke out. This needs its own fan, ductwork, and connection to the fire alarm panel, separate from the everyday ventilation system. Every air-handling unit serving basement levels must also be wired to shut down on smoke detection, so it does not spread smoke between compartments.