Decision on Insulation Thickness (Economic Calculation)

His building loses energy not only through walls but also through floors, roof, windows and doors, thermal bridges, and ventilation.

That is correct. That is why the change in energy demand is always so small when expressed in absolute numbers. Nevertheless, there is an error in the calculation.

I understood that the U-value curve is exponential. Therefore, the "simple rule of three" would not make sense. Let's see what the final negotiation regarding the additional cost will result in and then we will decide. Regards Markus

Yes, the real curve resembles that of an exponential decay. Whether this can be distinguished from a proportional function in the small interval is debatable.

But the numbers from the first post do not support this. I will try again differently. Every time I increase the insulation by 2cm, there is a reduction in energy consumption. Each increase has a similar but smaller effect than the previous one. If I am not completely blind, however, the numbers in the initial post contradict this relationship.

When I look at it, the 14-value would contradict that.

Without having detailed information about the concrete walls, without plaster and without guarantee of correct input on the U-value calculator portal, the following example U-values result
(according to the Energy Saving Ordinance, exterior walls facing outside air may have a maximum U-value of 0.28)

24cm concrete: U-value 3.65
24cm concrete + 12cm EPS insulation 035: U-value 0.270
24cm concrete + 14cm EPS insulation 035: U-value 0.233
24cm concrete + 16cm EPS insulation 035: U-value 0.206
24cm concrete + 18cm EPS insulation 035: U-value 0.184

If poorer insulation is chosen (EPS 040 or EPS 045), the U-values are correspondingly higher.

Whether it ever makes economic sense mathematically depends in particular on future electricity costs for heating, I think, since these can also be (significantly? crystal ball!) higher than €0.30/kWh. The insulation costs, on the other hand, are a "one-time expense."

From a purely intuitive point of view, I would choose 16cm insulation.

I also see the economic viability as difficult mathematically. Whether it could ever play a role when selling the house, I rather think not.