Insulated flexible ductwork

An important part of any effective air conditioning system

When choosing a heating or cooling system for a house, regardless of whether it is gas, evaporative or refrigerated air conditioning, these days we have access to increasingly more efficient units, in terms of both energy consumption and running costs. Gary Bath, Master Plumbers technical advisor explains.

Whilst many plumbers and refrigerated air conditioning specialists advise on the type of system most appropriate to the consumer’s requirements, consumers usually rely on the Energy Rating Label. More stars mean more savings and greater efficiency and guides them to choose a unit once the type of system has been established.

Where the system uses ductwork to distribute cooled or heated air throughout the building, the choice is between rigid (usually metal) or flexible.

The latter has become increasingly popular because of the difference in both installation and unit costs of the solid duct, particularly in residential installations.

Flexible duct has made the heating and cooling of the whole house rather than one or two rooms a viable alternative for many consumers, particularly those in newly constructed residential housing, where increased insulation requirements are standard.

The Building Code of Australia (BCA) sets out the requirements for the insulation (R value) of flexible duct, taking into account both the intended climate zone and type of system to be installed (AS 4254.1 2012- Flexible duct). This R value relates to the insulation used in the manufacture of the flexible duct and is labelled, along with the manufacturer’s name and generally the size of duct, as a mandatory requirement.

Most heating or refrigerated cooling systems across Victoria require a minimum of an R value of one for the flexible duct. This will be dependant on insulation used under the roof cover in those systems installed in a ceiling space and whether the underfloor is enclosed, if the duct is installed under the floor.

Flexible duct is essentially manufactured by wrapping an inner core, usually reinforced to ensure the duct retains its circumference with an insulating material like fibreglass or polyester material. Over the top of this is a vapour proof outer covering of plastic to prevent outside moisture coming into contact with the insulating material. This also ensures that the air contained within the duct is sealed within the insulated area.

The R value of the insulation blanket provided for this product essentially becomes the R value of the flexible duct. Considering that this type of insulation relies on the volume of air that can be trapped within, it’s not hard to see where some of this insulation property (R value) can be lost. Its also important to consider that when we purchase our flexible duct, it is compressed in shorter packages than its six metre length would suggest.

Recent proposed changes to Standards may have a significant impact on these R values. Previously, flexible duct manufacturers were required to provide details of the R values of the insulation used in the flexible ducts manufacture. A review of the current AS 4859.1 2002
Thermal Insulating materials for buildings has included a new test criterion for insulated flexible duct. It proposes that three 6 metre sections (as purchased) be provided for the external testing body. These will be removed from their packaging and ‘conditioned’ at a set
temperature for a minimum period of 24 hours. The duct will then be placed in a test chamber, 4.5 metres long and a temperature differential shall be measured along this length. Any change in temperature would be attributed to how effective the insulation is in the manufactured duct, rather than testing the insulation as an isolated component of the duct as is procedure in the current requirement.

Regardless of the efficiency of the heating or cooling unit, poorly installed ductwork results in losses in efficiency and in extreme cases failures of the system.

Assuming that the duct has been sized correctly and has the appropriate R value, there are a number of installation requirements to ensure that the system operates to its optimum.

• Make sure that the flexible duct you intend using has been correctly labelled as per the Standard As4254.1. It should have the R value, size and manufacturer clearly indicated

• Ensure that the flexible duct is ‘seasoned’ before installation. Flexible duct should be removed from the packaging, pulled out to its full extension and left to ensure that the compressed insulation can resume its volume

• Flexible ductwork should be supported (at intervals of no more than 1.5 metres) using hangers, which are a minimum of 25mm and support the duct for at least a quarter of its circumference with a semi rigid, fire resistant load support of no less than 75mm wide

• Flexible ductwork should not be in contact with the ground

• Avoid running ductwork where it will be exposed to direct sunlight, as the outer vapour barrier can degrade

• Avoid tight bends in the ductwork at termination points

• Sealing the inner core of the flexible duct, independently to the outer sleeve will ensure no leakage or possible condensation can occur when joining ductwork to junctions, diffusers, etc

• Protect flexible ductwork from any moisture before or during the installation, once wet, the insulation can remain so for extremely long periods of time, promoting fungal growth and contamination.

The above are but a few considerations to ensure that flexible ductwork is installed to comply with Australian Standards and importantly ensures efficient running of the air conditioning system it forms part of. Anyone wishing to update their knowledge and skills in this area might wish to access the Master Plumbers Flexible Duct installation course, an online program available at