Install additional insulation in the attic

Hello, I already have insulation in the attic, but the energy consultant has prescribed 22cm of rigid foam boards EPS-W15 (10cm and 12cm).

The attic ladder will be replaced by a new one, which is insulated at the lid.

Currently, there are about 10cm of insulation wool and screed in the attic. (Walkable)

It is a reinforced concrete ceiling.
The rigid foam boards are to be laid on the existing screed, and then OSB boards will be placed on top to make the whole thing walkable.

I want to know if a vapor barrier or draft sealing must be installed before the Styrofoam is laid.

In order to make the attic walkable again, I want to lay OSB boards (18mm).

Please advise.
Thank you
Franz

This is roughly how the top floor ceiling is constructed. I am not sure if a vapor barrier membrane was installed between the insulation wool and the screed or between the floor slab and the insulation wool.

It is a reinforced concrete ceiling

You don't need anything else. It is unnecessary