sound proofing

Now I have the headlining out I thought maybe add some insulation. I think 5mm should be possible, such as https://www.ebay.co.uk/itm/185524437723 Any thoughts? What about condensation? Looks like one side is adhesive so that begs the question to fix to the roof or fix to the headline biscuit...

when I had the bonnet off I noticed some loose fabric type material just above the nearside drive shaft. Any idea where it should be and how hard to put back? I guess this is some sound proofing material and that a clip(s) has broken. Cannot see the offside drive shaft as too many...