ᐅ Installing Additional Insulation in the Attic

Hello, I already have some insulation in the attic, but the energy consultant recommended 22 cm (9 inches) of rigid foam boards EPS-W15 (10 cm (4 inches) and 12 cm (5 inches)).

The attic ladder will be replaced with a new one, insulated at the cover.

Currently, there is about 10 cm (4 inches) of mineral wool insulation and a screed layer in the attic. (Walkable)

The ceiling is made of reinforced concrete.
The rigid foam boards are planned to be laid on top of the existing screed, with OSB boards installed over them to make the surface walkable.

I would like to know if a vapor retarder or an airtight sealing needs to be installed before placing the polystyrene boards.

To make the attic walkable again, I plan to install 18 mm (0.7 inch) OSB boards.

Please advise.
Thank you,
Franz

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

daFraunz schrieb:

It is a reinforced concrete slab

You don’t need anything else. It’s unnecessary