ᐅ DIY Insulation of Roof Space Between Rafters – What Materials to Use?

Hello everyone,

We want to insulate the attic of some acquaintances.
It is a 1.5-story house with an unfinished attic.

Currently, there is fiberglass insulation between the rafters, and that’s all.

We would like to replace this and then seal it with breathable membranes (underlay membranes).
The floor consists of wooden boards, and underneath should be the suspended ceiling of the bedrooms.

Questions:
- Is the effort worth it?
- What material is recommended for insulation between the rafters?
- Is it worthwhile to install drywall (plasterboard) after the underlay membrane, or is that only for aesthetic reasons?
- Is it worth insulating the attic floor as well? If yes, which material should be used?

Any additional tips?

The attic will only be used for storage (Christmas decorations, Easter decorations, ski clothes, etc.).