ᐅ The attic heats up significantly.

We have an unoccupied attic with insulation made of mineral wool installed between the rafters. There is also an insulated ceiling between the attic and the upper floor. Nevertheless, the attic heats up extremely in the summer.

Is this normal, and how can I prevent it?

This is normal under intense sunlight (which is almost always the case in summer). Insulation can only delay the heating up, but once the heat is inside, it naturally cools down more slowly when it gets cooler (usually at night). So, you either need to ventilate at night (beware that this can also lead to condensation issues) or just ignore it.
I assume the space is unoccupied? Are you storing any sensitive items there?

Double insulation has its pitfalls – the issue with heat that you mentioned is one of them.

Therefore, I would replace the "Nevertheless" with a "Therefore."

Our problem is the central ventilation system. It is located in the attic and at an outside temperature of 20°C (68°F) it already supplies air at 30°C (86°F). This is probably due to the heat in the attic.

Does anyone have a solution for this?

Enclose the area and insulate it with vacuum insulation panels (this can be expensive). Alternatively, use much thicker insulation with other materials. Install an air conditioning system—either just for the enclosed space or for the entire attic. Then run it continuously at a low level of cooling (instead of turning it on full blast only during extreme heat).

Jimy3435 schrieb:

Is this normal, and how can I fix it?

Yes. Heat will eventually penetrate even the best insulation.
Air circulation can help reduce the problem, but insulation works both ways—from outside to inside and vice versa. Once the attic has "heated through," it will retain the heat well.
An air conditioner is one option—though it might be a bit excessive for an unoccupied attic.
My approach would be to simply leave it as is and just be mindful of what you store up there.