ᐅ Insulation of the Attic Floor in a Pitched Roof

Hello everyone,
I recently purchased a terraced house from the 1970s.
The currently uninsulated roof is being used as storage space, and there are no plans to convert it into living space.
The top floor ceiling is made of concrete.

It is a gable roof. It was apparently renovated in the last 5 to 10 years. For this purpose, the rafters were doubled up and new roof tiles were installed. However, no insulation was added.
The roof is open behind the wall plate between the rafters. That means there is a gap of about 10–15 centimeters (4–6 inches) at each point, and as a result, there is a noticeable draft.

I would now like to cover the floor with walkable insulation boards in order to reduce heat loss from the upper floor.
I have done some research, and it doesn’t seem complicated.
The concrete ceiling acts as a vapor barrier. On top of that, either walkable insulation boards or load-bearing boards plus an OSB layer will be installed.

Should I just do this and leave these gaps open? Do they perhaps serve a purpose (ventilation)?
If not, how should I close them? Why might the roofer not have done this?

Thanks in advance for any advice that will help me move forward.

I assume the gaps have a purpose, most likely for ventilation. I find it hard to believe that the carpenter just left them open because he was too lazy to finish.

If in doubt, I would consult an independent expert. The cost for such an inspection is usually not very high, and then you clearly know what needs to be done, ensuring there are no construction defects in the end.