Firstly, in May 2012, the building industry adopted the new Building Code Australia (BCA). This code clearly details what thermal rating duct is to be used in what installations and in most cases the bare minimum is R1.0.
Secondly, increasing the thermal rating on your duct significantly reduces the amount of temperature loss in the air running through the duct. This in turn reduces the load on your air conditioner which contributes to the reduction in running costs of the unit. Similar to ceiling batts, the insulation in air conditioning duct varies in thickness and density the higher the thermal rating
Remember, The right choice of flexible duct can reduce the load on your air conditioner and save you money on running costs.