insulation above tenement ceiling

[Sandra1953]

This is a different take on previous questions.

The upstairs flat had a very small but persistent leak onto mine. It has now been cured but could possibly occur again I imagine. In repairing the ceiling much of the between-floors deadening fell out. What I want is to reinsert deadening that will also absorb future drips. The ceiling finish on my ceiling will be plasterboard, and I plan also to create a false ceiling below that as the natural ceiling is very high, plus I will put in that gap further insulation and a vapour membrane. Belt and braces but necessary. Views?

 

[Leejames]

I have some leftover loft installation. Would if be alright to use that to fill the gaps, or would that run the risk of getting wet/going mouldy? Would spray installation be better?