Natural Ventilation NCC 2025: 5% Openable Area + DTS Rules
What You Need to Know
The NCC gives you three ways to ventilate a building: natural ventilation, mechanical ventilation, or a mix of both. Natural ventilation is the simplest on paper. Open windows or louvres equal to 5% of the floor area, and the Deemed-to-Satisfy (DtS) path is met (NCC 2025, F6D7). But “simple on paper” does not mean “simple to get right.” The 5% rule ignores how many people are in the room, how deep the floor plate is, and whether anyone will actually open the windows. Get the design wrong, and the building will fail either at certification or in practice.
The Rules
- Openable windows, doors, or vents must provide at least 5% of the floor area as ventilating area (NCC 2025, F6D7(1)(a))
- Openings must face a court, open sky, verandah, or an adjoining room that itself has openings to outside (NCC 2025, F6D7(1)(b))
- For Class 5–9 buildings using borrowed ventilation, the opening between rooms must be at least 10% of the floor area, measured no higher than 3.6 m above the floor (NCC 2025, F6D8(b))
- If an AC system does not supply outdoor air per AS 1668.2, the building must still meet the natural ventilation rules in F6D7 (NCC 2025, F6D6)
- Performance Requirement F6P3: occupied spaces must have ventilation with outdoor air that maintains adequate air quality (NCC 2025, F6P3)
- Verification Method F6V1 lets you prove compliance by showing contaminants stay below Table F6V1 limits, including CO2 at 850 ppm over 8 hours (NCC 2025, F6V1)
What This Means in Practice
Take a Class 5 office with a 200 m² open-plan floor. The DtS path requires 10 m² of openable window area (200 m² × 5%). That is a lot of glass that opens. Awning windows restricted to 125 mm opening on upper floors will struggle to reach that target. Louvre windows provide a larger free area per unit and may be a better fit.
Room depth matters just as much. CIBSE AM10 (an international design guide widely used in Australia) limits single-sided natural ventilation to about 2 times the ceiling height, or 2.5 times if the window has both high-level and low-level openings (such as a sash window). For a 2.7 m ceiling, that caps effective ventilation depth at about 5.4–6.8 m. Cross ventilation (openings on opposite walls) extends that limit to 4–5 times the ceiling height, or roughly 11–13.5 m. A 15 m deep floor plate with windows on one side only will not ventilate properly, regardless of how much openable area you provide.
The biggest trap in current practice is the split-system loophole. A building with split-system air conditioning and 5% openable area technically satisfies the DtS provisions. But once the AC runs, the windows close, and outdoor air drops to zero. The NCC Performance Requirement F6P3 still demands adequate outdoor air. Industry groups including AIRAH have pushed to close this gap in NCC 2025 by requiring AS 1668.2 compliance when any AC system is installed.
Key Design Decisions
DtS Natural Ventilation vs. Performance Solution
The DtS path (F6D7, 5% openable area) works for simple buildings with shallow floor plates and low occupancy. For anything more complex, a Performance Solution using F6V1 gives you flexibility. You model contaminant levels and prove they stay within limits. This path suits mixed-mode buildings where natural and mechanical systems share the load.
Simple Procedure vs. Detailed Procedure (AS 1668.4)
AS 1668.4:2024 offers two calculation methods. The simple procedure matches the NCC’s 5% rule. The detailed procedure adjusts openable area based on occupancy and space type. A warehouse with sparse occupancy might need only 2.5%. A packed school classroom might need close to 20%. The detailed procedure lets you right-size openings instead of applying a flat 5% everywhere.
Single-Sided vs. Cross Ventilation
Single-sided ventilation (windows on one wall only) limits room depth to about 2 times the ceiling height, or 2.5 times with high-low openings. Cross ventilation (openings on opposite walls) extends the limit to 4–5 times the ceiling height. Your floor plate width and building form determine which strategy works. If the building is deeper than 7 m from the facade, single-sided ventilation alone will not deliver adequate air movement.
Natural Ventilation with Air Conditioning (Mixed-Mode)
Mixed-mode buildings switch between natural and mechanical ventilation depending on outdoor conditions. When it is mild, windows open and the AC stays off. When it is hot or humid, windows close and the mechanical system takes over. This approach typically needs a Performance Solution, automated window actuators, and a BMS (building management system) to manage the changeover.
Who Needs to Know What
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References
- National Construction Code 2022, Volume One, Part F6 — Light and ventilation (F6D6, F6D7, F6D8, F6V1)
- AS 1668.4:2024, The use of ventilation and airconditioning in buildings — Part 4: Natural ventilation of buildings
- AS 1668.4-2012, The use of ventilation and airconditioning in buildings — Part 4: Natural ventilation of buildings (superseded)
- AS 1668.2-2012, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings
- CIBSE AM10, Natural ventilation in non-domestic buildings (2005, reprinted with corrections 2014) - international reference
- AIRAH, NCC 2025 Public Comment Draft response - industry position on natural ventilation loophole