1970s Timber Frame Refurb with no Tony trays

[Pete T]

I'm planing on extending our 1970s timber-framed house and, prior to that, refurbishing the existing structure as there are a number of draughts, the UPVC windows have (as far as I understand) been fitted incorrectly, the VCL has been punctured in numerous places over the years etc...

 

So with that context, I'm wondering how to approach the lack of "Tony trays" in the house. I had to replace some first-floor flooring a few years ago and when I lifted the floor (in the winter) the cold draught flowing between the floors was impressive. My concern with attempting to eliminate these draughts is that I will "upset the balance" so to speak, and start creating damp where there was none before.

 

My initial thought was to perhaps stuff some rockwool or other breathable insulation between the joists, to the thickness of the internal walls (i.e. not separating the upstairs and downstairs by insulating the whole floor) as this could allow the floor void to "breathe", maybe?

However, this post has had a couple of replies suggesting to either run VCL between the joists or use some spray foam to fill the gaps. Am I just overthinking it all?