AS 4254 Explained: Australia's Ductwork Construction Standard
What is AS 4254?
AS 4254 is the Australian Standard for ductwork construction in air-handling systems. It tells the contractor how thick the sheet metal must be, how to seal joints, what leakage is acceptable, and how to support the duct in the ceiling. It tells the engineer what to specify to make all of that buildable and certifiable.
The standard is published in two parts. AS 4254.1:2021 covers flexible duct. AS 4254.2:2012 covers rigid ductwork. Most commercial mechanical installations use both: rigid duct for the main supply and return spine, flexible duct for short final connections to diffusers.
NCC 2022 calls AS 4254 up directly in Section J (J6D5 and J6D7, duct construction and insulation), and AS 1668.1 also refers to it. Failing AS 4254 on site means rework, retesting, and delayed handover. Confirming the right pressure class, sealing class, and leakage limit at design stage is the cheapest place to fix this.
Where AS 4254 Fits in the HVAC Standards Stack
AS 4254 sits alongside several adjacent standards that engineers and contractors hit on a typical project. Use this map to find the correct document.
| Standard | What it covers | When you need it |
|---|---|---|
| AS 4254.1 | Flexible duct construction and installation | Final diffuser connections, ceiling void runs |
| AS 4254.2 | Rigid sheet metal ductwork construction | Most commercial supply, return, exhaust |
| AS 1668.1 | Fire-rated duct, smoke control | Stair pressurisation, smoke exhaust ducts |
| AS 1668.2 | Air quantities and ventilation rates | Sets the airflow that AS 4254 ducts carry |
| NCC Section J | Duct insulation R-values | Energy efficiency mandates on conditioned ducts |
AS 4254 is the construction-and-build document. AS 1668.2 is the ventilation-rate document. AS 1668.1 is the fire-and-smoke document. Section J is the insulation-R-value document. Specifications normally call up multiple of these in parallel.
What AS 4254 Covers
Pressure and Velocity Classification
AS 4254.2 classifies rigid ducts by pressure class, from 125 Pa up to 2500 Pa, positive or negative. The classification fixes sheet thickness, reinforcement spacing, and joint type. Office HVAC at low pressure / low velocity uses lighter gauge than a high-pressure VAV main; documentation must specify the class so the contractor builds the right thickness.
Sealing Class and Leakage Testing
Three seal classes, A through C, set by pressure class. Class A seals all joints, seams and wall penetrations at 1000 Pa and above. Class B applies at 501 to 999 Pa, and Class C at 500 Pa and below. Systems of 3000 L/s or more are leakage tested at 1.25 times design pressure, with leakage no more than 5% of design airflow. The class must be specified in the documents.
Joint Type and Reinforcement
Slip-joint, drive-cleat, and TDC flange joints all appear in AS 4254.2, each with their own pressure and width limits. Reinforcement (tie rods, angle frames) must match the duct dimensions so the duct does not flex under pressure or thermal load. The standard tabulates these limits explicitly to remove guesswork.
Hangers, Brackets and Supports
AS 4254.2 prescribes maximum hanger spacing by duct size and orientation. Wider ducts need closer support. The spacing table is the single most-checked clause on a site inspection because under-supporting a long duct run causes sag, joint stress, and leakage.
Insulation and Vapour Barriers
NCC Section J sets the R-value targets for ductwork insulation. AS 4254 overlays the construction requirements: how to fix the insulation, how to seal joints in the vapour barrier, and how to handle bends and access doors. Conditioned air ducts in unconditioned ceiling voids commonly need R1.5 to R2.0 with an unbroken vapour barrier.
Most-Asked Questions
- Does flexible duct have a maximum length? Yes. In Class 4 to 9 buildings, AS 4254.1:2021 allows it only as the final run-out to a terminal, 6 m maximum. Many consultants hold runs much shorter, fully extended with one bend at most. Long flexible runs are the most common cause of low diffuser airflow on site.
- Can I use a heavier gauge than AS 4254 specifies? Yes, the standard sets minimums. Heavier gauge is acceptable but adds cost and weight. The reverse is not acceptable.
- Does AS 4254 require leakage testing of every duct run? No. Systems of 3000 L/s or more are tested at 1.25 times design pressure. At least 10% of each system type is tested as it is built, with records kept. The documentation can call for more.
- What about phenolic and pre-insulated panel duct? Pre-insulated panel duct is treated under separate technical statements; it is not covered by AS 4254 directly. For conventional sheet metal duct, AS 4254 applies.
- Is fibreglass-faced duct allowed? Yes under AS 4254.2, with specific construction and joint requirements. Less commonly used in current commercial projects than metal duct, but still valid.
Three Compliance Traps We See Most
- Pressure class not specified in the documentation. The contractor builds to a default low-pressure assumption, then the duct fails under VAV start-up. Specify pressure and sealing class on the schematic and on the spec.
- Flexible duct used to bridge a 4-metre gap. Common shortcut to avoid a coordination clash. The pressure loss starves the diffuser. Re-route in rigid duct or move the branch.
- Vapour barrier broken at access doors and turning vanes. Sectional insulation completed by the contractor without continuous vapour barrier sealing causes condensation in the ceiling. Spec an explicit detail at every break.
For deeper detail on duct sizing and AS 4254 design rules, see our AS 4254 ductwork sizing memo and AS 4254 ductwork construction guide.
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References
- AS 4254.1:2021, Ductwork for air-handling systems in buildings, Part 1: Flexible duct, Standards Australia.
- AS 4254.2:2012, Ductwork for air-handling systems in buildings, Part 2: Rigid duct, Standards Australia.
- AS 1668.1:2015, The use of ventilation and airconditioning in buildings, Part 1: Fire and smoke control in buildings, Standards Australia.
- AS 1668.2:2024, The use of ventilation and airconditioning in buildings, Part 2: Mechanical ventilation in buildings, Standards Australia.
- National Construction Code 2022, Volume One, Section J Energy efficiency, ABCB.
- AIRAH DA9, Air conditioning load estimation and design.