Providing roof insulation will reduce your ceiling insulation requirements.
Be sure to select the applicable roof insulation for each roof type that you have nominated. You must select roof insulation if you have a medium- or dark-coloured roof.
If you have a roof with a low solar absorptance it may not require roof insulation, however you can still choose to install roof insulation and reduce your ceiling insulation requirements.
You will also need to check that the selected (or required) roof insulation fits within the available roof space. Any foil product under the roof will require an air gap of at least 20mm between the under-side of the foil and the top of the ceiling insulation, as shown in the diagram below. The insulation must be installed in accordance with the relevant provisions and Australian Standards specified in the NSW additions to the Building Code of Australia (BCA).