AS 4254 Ductwork: Sizing, Leakage + Insulation Guide

What You Need to Know

Every air-conditioned building needs ducts. AS 4254 is the Australian Standard that sets how those ducts must be built, sealed, and tested. It comes in two parts: Part 1 covers flexible duct, Part 2 covers rigid duct. The NCC calls up both parts. If your ducts leak, fail fire tests, or fall short on insulation, you will not get a compliance certificate.

• Rigid ductwork must comply with AS 4254.2:2012 for materials, construction, and installation
• Flexible duct must comply with AS 4254.1:2021 and is limited to 6 m maximum length for the final run-out to air terminals (NCC, Class 2-9 buildings)
• Duct systems of 3000 L/s or more must be sealed to AS 4254.1 and AS 4254.2 (NCC 2025, J6D7)
• Those same systems must pass leakage testing with a maximum of 5% leakage at 1.25 times design operating pressure (AS 4254.2, Cl 2.2.4)
• At least 10% of the duct system must be tested, covering all seam types, joint types, and sealing methods
• Duct materials must meet fire hazard properties: Smoke Development of 3 or less and Flame Spread of 0 under AS 1530.3 (AS 4254.2, Section 1.8)
• Duct insulation must comply with AS/NZS 4859.1 with minimum R-values: R1.2 inside conditioned spaces, R2.0 in ceiling voids and risers, R3.0 where exposed to sunlight (NCC 2025, Table J6D6, climate zones 1-7)

Take a typical 5000 L/s supply air system serving two floors of a commercial office. Because the system exceeds 3000 L/s, every duct joint and seam must be sealed, and at least 10% of the system must pass a leakage test. The test pressure will be at least 1.25 times your design static pressure. If your design pressure is 400 Pa, you test at 500 Pa. The system must leak less than 250 L/s (5% of 5000 L/s) to pass.

For duct sizing, most engineers use the equal friction method. You pick a friction rate, typically 1 Pa/m for commercial buildings, and size every duct section to that rate. This keeps pressure drops predictable and simplifies balancing. Main supply ducts in an office typically run at 8 to 10 m/s. Branch ducts drop to 6 to 8 m/s. Return ducts sit lower, around 6 to 7.5 m/s, to keep noise down.

Flexible duct connects the rigid system to diffusers and grilles. AS 4254.1 caps each piece at 6 m. Pull it taut with no kinks or sags. Compressed flex duct can dramatically increase the pressure drop and wreck the airflow balance.

All ductwork outside the conditioned space needs insulation. In most climate zones, that means R2.0 on supply and return ducts in ceiling voids. Ducts in direct sunlight need R3.0. Cooled-air ducts also need a vapour barrier on the outside face to stop condensation.