Worried about water/condensation when insulating a deck roof

[helenmay]

Thanks in advance for any help with this. I have read a lot of the very helpful existing conversations about insulating deck/patio roofs and think I have a plan. However, I am new to all of this and just want to check if it works, and what I have to do/ be concerned about with water/condensation. 

So we are in a Queenslander, with a wood framed deck and simple corrugated metal roof. It gets incredibly hot out there and we want to insulate it so we can make good use of the space. Photos of the roof here

 and here 

From previous posts I know that some condensation is unavoidable, but I also don't want to cause problems with poor choices now! We do get some condensation and drips from time to time out there. 

We have good depth with the beams to be able to do something tidy. My plan currently is to use the following: 90mm earthwool batts and then plasterboard for a ceiling that can be fixed to the wooden beams.

Do I need to (or should I) coat the metal with something or add a layer that is water resistant--something like ametalin silverwrap between the metal and the batts? Or is there a better choice that I am missing?

Thanks for your help! I know there are a lot of posts asking about deck roofing, and I did try and see if there was an answer before I made a new post!