Hi @traceryan,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Check out How to insulate a metal roof? from @bowmatty for a fantastic example of how to insulate a ceiling like this where you don't have beams to work with.
He has used Foilboard 10mm x 2400mm x 1200mm Standard 10 Insulation stuck to the ceiling with Sikaflex 11FC Plus Polyurethane Adhesive Sealant.
It's tough when you don't have beams to work with as insulation manufacturers recommend mechanical fixings to attach their products. You can have some success using adhesives to install insulation, but they may need to be reglued in future as you'll see in Bowmatty's update.
If you are aware that this is not a perfect way to install insulation and are fine to do some maintenance in the future, I think this is a good option for you.
Let me know what you think.
Jacob
Note: For our New Zealand readers, please note that the use of foil-covered insulation products is banned in New Zealand.
