Real price for single-family house, possibly turnkey

That is basically not wrong. Saving energy must be economical and should not be practiced at all costs. However, I consider airtightness of the building envelope necessary and correct, not only in terms of energy saving but also from the perspective of building physics.

Regards

I definitely don't want free advice now, I've long since finished building
but I would be interested in how airtightness is related to building physics. Explained roughly, of course, and as I said, purely out of interest.

I certainly don't want free advice now, as I have long finished building
but I would be interested in how airtightness relates to building physics. Explained roughly, of course, and as I said, purely out of interest.

Very roughly explained, it is partly about moisture protection. Warm, moist air penetrates the unprotected construction, cools down, and condenses.
The result is mold growth and impairment of the insulation effect.
Air drafts can also reduce the effectiveness of thermal insulation. That is why, with an airtight building envelope, it is so important to ensure controlled air exchange.
If the air is filled with and saturated with water vapor, then this excess has to go somewhere.
It is then at the coldest spots where dew condensation occurs (thermal bridges).

Ok, thank you very much. I am aware of the controlled ventilation. But wouldn't it be more sensible to build the houses a bit more "leaky" right from the start, in order to prevent mold formation (assuming one forgets to ventilate when there is no controlled residential ventilation)?

Ok, thank you very much. I am aware of the controlled ventilation. But wouldn't it be more sensible to build the houses a little "less airtight" from the start to prevent mold formation (assuming one sometimes forgets to ventilate without controlled residential ventilation)?

Then uncontrollable heat losses occur again. Of course, you can remove the seals from the windows, but that doesn't really make sense, does it?

In addition, these areas cool down faster and then become thermal bridges, which nobody wants either.

I can see, it is a dilemma...