Insulation of upper floor ceiling of gable roof

Meal everyone,
I have purchased a mid-terrace house from the 70s.
The currently uninsulated roof is being used as a storage room, no conversion is planned.
The top floor ceiling is concrete.

It is a gable roof. This was apparently redone in the last 5-10 years. For this purpose, the rafters were doubled and new roof tiles were laid. However, nothing was insulated.
The roof is open behind the wall plate between the rafters. That means there is a proper gap of 10-15 centimeters there, and accordingly, there is quite a draft.

Now I want to cover the floor with walkable insulation boards to get the heat losses in the upper floor under control.
I have read a bit about this and it does not seem complicated.
The concrete ceiling takes over the function of the vapor barrier, and then either directly walkable panels or compressive boards and an OSB layer are applied on top.

Should I just do this and leave these gaps open? Do they possibly even have a purpose (ventilation)?
If not, how do I close them? Why didn't the roofer do that?

Thanks for any tips that help me move forward.

I guess the columns have a purpose, namely for ventilation. I don't really believe that the carpenter just left them open because he had -no more motivation-.

If in doubt, I would consult an independent expert. The costs for such an inspection are not that high and then you simply know what to do and in the end there is no botched construction.