Why is shading particularly important in newly built houses?

Grym, just believe people who have built - the "problem" is consistent for everyone...

I do believe it... I would also understand if someone said: Despite good insulation and triple-glazed windows, the heat gain in summer from the sun REMAINS a problem. Compared to poorly insulated old buildings, it is not as bad, but measures should be taken. What I don't quite understand is why one should PAY PARTICULAR ATTENTION to this especially in new construction. Why shouldn't one PAY PARTICULAR ATTENTION to it especially in old buildings, but in new construction it's also important?

How do you want to pay attention to that in the old building? You're not building new...

And it has been explained to you x times now: More windows and above all higher thermal insulation that keeps the heat out.

You know the u-value calculator, it says enough about summer thermal protection. And there you can see very well when the heat has penetrated the material.

above all higher thermal insulation, which retains the heat.

I would not agree with that. It makes no physical sense to me. Heat loss (or gain, if you want) is achieved by air exchange orders of magnitude faster. Just compare the heat dissipation through the special thermal conductivity of the facade with the material-bound energy transport through air movement.

I don't need to do any calculations there, I live in practice.

You can twist and turn it however you want... The fact is: Houses nowadays are built so that the heat stays inside... and always...

What I don't fully understand is why you should especially pay attention to this in new buildings. Why shouldn't you pay special attention to it in old buildings, but in new buildings it's also important?

- Passive houses (new houses) have large south-facing windows to capture heat in winter. Even in normal new houses, the trend is towards large window surfaces (that's my impression). This was not the case to such an extent in the past.
- Thermal insulation in terms of the U-value practically only considers convective heat transfer, but not radiation. Regarding heat loss, radiation doesn't matter (the surface temperature is too low). With solar radiation, it's just the opposite; here the problem reduces to radiation, meaning the modern window practically does not "insulate" against this.
- To block radiation, windows could be coated/tinted, but ... see above "passive house" and the light transmission would also suffer.

Therefore, shading is necessary in order not to suffer in summer.

By the way: old building attic apartments are and have been very warm in summer (> 30 °C, at 10 p.m.). Ground floor apartments are significantly more protected (thick walls, trees in front of the house, etc.). And in the backyard ground floor apartment it never gets warmer than about 20 °C.