It's not the between decks spaces it's the between hull and living spaces.
When I was here last time I was contemplating does the WIP represent where the Structural support beams, girders an Keels would be.
I guess I was contemplating doing someithing like this with my own starship.
This is really just a fascinating endeavor. I need to by the TNG tech manual. So many projects though...it's nice just to watch someone accomplish there goals for a change.