The duct material in your Melbourne home affects how cleaning is performed, what the long-term maintenance requirements are, and how to interpret signs of duct problems. Two principal duct types are found in Melbourne residential properties: flexible duct (the dominant type in homes built from the 1970s onward) and rigid sheet metal duct (common in older homes and in main trunk lines). Understanding the difference matters for both homeowners and technicians.
Flexible Duct: Construction and Characteristics
Flexible duct — universally used in Melbourne residential ducted systems since the 1970s — is a multi-layer composite assembly designed to be easy to install in confined roof spaces and around obstacles.
Layers of a flexible duct assembly
Wire helix coil: A steel spring coil provides structural support and allows the duct to bend without collapsing the airway. The coil pitch determines the duct’s flexibility and resistance to compression.
Inner liner: A foil or metalized polyester film forms the inner airway surface. This is the surface that comes into contact with the conditioned airflow. It is also the surface where contamination accumulates and where rotary brush cleaning makes contact. The inner liner is the most vulnerable component — tears or perforations in the liner allow conditioned air to leak into the insulation layer and ultimately into the roof space.
Insulation layer: Fibreglass or polyester insulation wraps the inner liner, providing thermal resistance to reduce heat gain or loss as conditioned air travels from the system unit to the supply register. Degraded or absent insulation increases energy loss and can contribute to condensation on the inner liner surface — a mould risk factor.
Outer jacket: A foil or reinforced film outer cover protects the insulation from physical damage and moisture. When the outer jacket deteriorates — from UV exposure in exposed sections, pest damage, or age-related brittleness — the insulation layer becomes exposed and vulnerable.
Vulnerabilities of flexible duct
Flexible duct’s characteristics that make it easy to install also make it prone to specific failure modes in Melbourne’s roof spaces:
Compression and kinking: Unlike rigid duct, flex duct can be compressed by weight (trades standing on it), kinking at sharp bends, or sagging between support points. Compressed or kinked sections dramatically reduce airflow to the register they serve. See our guide on collapsed air duct in Melbourne homes.
Excessive length and sag: Flex duct that is installed too long for a run hangs in curves rather than running relatively straight. Each curve adds resistance to airflow. Over time, the insulation insulation slumps, creating an uneven surface that accelerates debris accumulation and increases resistance further.
Joint failures: Flex duct connects to the trunk line and to supply register boxes via collars and tape or mastic. These connections can separate from thermal cycling, physical disturbance, or original installation that used inadequate tape. A disconnected joint loses the entire conditioned air output of that branch into the roof space. See our guide on disconnected air ducts in Melbourne.
Rigid Sheet Metal Duct: Construction and Characteristics
Rigid sheet metal duct — also called hard duct — is fabricated from galvanised steel sheet formed into rectangular or round sections. It is found in older Melbourne homes (pre-1970s systems), in the main trunk lines of many newer systems, and in commercial buildings.
Advantages of rigid duct
Rigid duct has superior airflow characteristics — a smooth interior surface offers less resistance than the corrugated inner liner of flex duct, reducing pressure drop and fan energy consumption. It is structurally robust — it does not compress, sag, or kink. Well-installed rigid duct in good condition can last 30 to 50 years. It is also easier to clean effectively — the smooth, hard interior surface is ideal for rotary brush cleaning and allows complete debris removal without risk of liner damage.
Disadvantages of rigid duct
Rigid duct is significantly more expensive to install — it requires fabricated sections and fittings, professional installation, and more time to route through a roof space. It is not well-suited to tight spaces or complex routing. Most Melbourne residential installers defaulted to flex duct from the 1970s onward because of the significant installation cost and time advantage.
Rigid trunk lines with flex branches
Many Melbourne ducted systems use a hybrid approach: a rigid sheet metal main trunk line running the length of the roof space, with flexible duct branches tapping off the trunk to each supply register. This provides the structural and airflow efficiency benefits of rigid duct for the high-volume main distribution run, while using the installation flexibility of flex duct for the final run to each register. The trunk line is typically cleaned first in a professional service, followed by each flex duct branch.
Cleaning Implications: Flexible vs Rigid
The duct type directly affects the cleaning approach a professional technician should use:
Brush selection
Rigid sheet metal duct can accommodate stiffer rotary contact brushes that aggressively dislodge compacted debris from the duct walls — the hard metal surface is not damaged by brush contact. Flexible duct requires softer brush types that clean the inner liner without tearing it. Using a rigid-duct brush in a flex duct run tears the inner liner, creating air leaks and potentially introducing insulation fibres into the airstream. See our guide on professional duct cleaning equipment for how brush selection works in practice.
Inspection approach
Rigid duct interiors can be inspected more thoroughly by camera — the smooth walls and consistent cross-section allow the camera to traverse further with better visibility. Flex duct’s corrugated surface and variable cross-section (from sag and compression) makes camera inspection more limited but still valuable for confirming debris levels and identifying join failures.
Post-clean assessment
After cleaning flexible duct runs, the technician can assess the condition of the inner liner — visible tears, delamination, or areas where the liner has collapsed inward. These findings may indicate that section replacement is warranted. Rigid duct post-clean assessment focuses on joint integrity, surface corrosion in older galvanised steel systems, and insulation condition on externally insulated sections.
When to Replace Rather Than Clean
Professional cleaning is appropriate when duct material is in serviceable condition — structurally intact, without significant liner damage, and with joints that are properly connected. Replacement becomes the appropriate action when:
- Flexible duct inner liners are torn, perforated, or collapsed over multiple sections
- The outer jacket is extensively deteriorated, leaving insulation exposed and unprotected
- Multiple joint failures mean a significant proportion of conditioned air is being lost
- The system is over 20 to 25 years old and showing widespread age-related deterioration
- Pest damage has compromised multiple sections across the duct network
A professional cleaning service that includes a duct condition inspection provides the assessment needed to determine whether cleaning, targeted section replacement, or full duct system replacement is the most cost-effective path. See our guide on air duct replacement in Melbourne for the replacement decision framework.