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Welcome to the Bunnings Workshop community @smiley13. It's fantastic to have you join us, and many thanks for your question about insulating a ceiling.
If possible, it would really help if you could upload a photo of the underside of the roof showing the roof structure and the space available above the ceiling. The depth of the ceiling joists will largely determine what type of insulation will work best in your situation. If you need a hand uploading photos, just let us know and we can help you through the process.
Given Darwin’s hot and very humid climate, the goal is to install the highest R-value insulation that will comfortably fit within the available space. If the roof cavity depth is quite limited, rigid foam board insulation can be a good option as it provides a higher thermal performance in a thinner profile. If you have around 90mm or more available between the ceiling joists, then installing R2.5 insulation batts is usually the most effective approach, followed by lining the ceiling with plasterboard.
Once we can see what you are working with, members will be able to give more accurate advice tailored to your home.
Please let me know if you have any questions.
Mitchell
