ᐅ Is Additional Insulation of the Attic Useful in New Construction?

Hello,

we are building a house with two full stories and a pitched roof above, constructed according to the Energy Saving Ordinance standard. The construction company includes an insulated attic space (insulation between rafters with 200 mm (8 inches) thick mineral wool insulation (thermal conductivity 0.035 W/mK), with a vapor retarder foil, sealed airtight). The concrete ceiling above the upper floor (i.e., the attic floor) remains uninsulated.

Now I am considering whether it would be beneficial for summer heat protection and winter heating costs to additionally lay insulation boards on the attic floor, for example, EPS insulation boards. Is this sensible or just a waste of money? Can this be done well as a DIY project, and what should I pay attention to? Do the boards need to be placed tightly together/glued, or is a little gap acceptable?

I am somewhat concerned about the ventilation of the attic during the first year or two after construction, as the attic will not be connected to the central mechanical ventilation system (I will probably have to leave the attic windows open occasionally in weather with low dew points). Should I wait some time before insulating the attic floor for this reason? Or can I do it right away without any issues?

Or would you recommend a thicker insulation between the rafters instead of working on the attic floor? The latter would have the advantage that I could avoid the very high prices charged by the general contractor by doing it myself.