Newly built apartment (KfW-55) cannot be cooled, always at least 25 °

The electricity consumption is largely covered by photovoltaic energy, which is increasingly abundant when the air conditioning is running and is even increasingly self-generated.

The "waste heat" is for the most part not generated heat by conversion, but simply the exchanged energy that is then outside instead of inside. According to this logic, you shouldn’t ventilate at night in summer either, because then your waste heat from the house would accelerate climate change outside ;)

Sure, after X days even an old building is properly heated through. But the whole crux of the discussion is that nowadays "highly efficient" houses have to be actively cooled, otherwise you implode.

I believe there are entirely different factors causing the problem than insulation. Because it also keeps the heat out in summer. Here's a spontaneous and certainly incomplete list - oversized windows let a lot more sun into the house - venetian blinds are sensitive to wind and on some days shading is not possible - sprawling terraces collect heat right in front of the house - new buildings lack a mature garden with large trees, leading to a frequently worse microclimate - the large amount of installed building technology also generates waste heat

I think there are completely different factors causing the problem than the insulation. It actually keeps the heat out in summer as well. Here's a spontaneous and certainly not complete list - oversized windows let a lot more sun into the house - blinds are sensitive to wind and don't allow shading on some days - expansive terraces collect heat right in front of the house - new buildings don't have established gardens with large trees, so the microclimate is often worse - all the built-in building technology also generates residual heat

But I don't have big windows because of the shutters... With those, I consistently shade the west side, the south side has a covered roof... what I do think is noticeable is that you can ventilate heat out of a poorly insulated house even in the last 2 weeks, but eventually you have to close up and the walls have nicely stored everything... The heat pump at our place is also an external unit, so no heat inside the house... However, the thing with the trees is true and is unfortunately due to time and will remain that way for a bit... until the trees grow big... in addition, the house has almost no roof overhang, which used to have a reason, I realize why that was done. And of course, you can’t afford any mistakes... Neighbors have the same house as us but with a different orientation... the only large window area is the stairwell with two long narrow windows... those are on our north side, theirs on the unshaded west side—inside, it’s unbearable at their place... Whoever planned that should be killed (-;

what I do think is clear, however, is that you could also ventilate heat out of a poorly insulated house in the last 2 weeks, in my house you eventually close up again and the walls have nicely stored everything

What kind of wall construction do you have? My theory on this is as follows: in old buildings you usually have monolithic masonry. In new buildings you often have ETICS. In both cases, the actual wall heats up (in old buildings from outside and inside, with ETICS mostly from inside). With ETICS, however, the wall can only radiate heat inward; the monolith radiates inward and outward and accordingly loses more heat. But it doesn't always work. With about 20 cm of reinforced concrete, I have here the perfect example of an uninsulated monolith, yet it doesn't radiate enough at night. Ventilating out requires, besides a cool environment (microclimate again), also sufficient wind, which is often a problem here in the calm spot. There simply is no draft.

I already know why I didn’t build a KfW55 or 40... so a plastic house with a ventilation system.

You can also achieve KfW40 with sand-lime brick ;)

What kind of wall construction do you have? My theory on this is as follows: in old buildings you usually have monolithic masonry. In new buildings, you often have ETICS. In both cases, the actual wall heats up (in old buildings from outside and inside, with ETICS mainly from inside). With ETICS, the wall can only radiate heat inward, while the monolith radiates heat inward and outward and consequently loses more heat. But it doesn't always work. With about 20cm of reinforced concrete, I have the perfect example here of an uninsulated monolith, yet it still doesn't radiate enough heat at night. Ventilating requires a cool environment (microclimate again) and sufficient wind, which is often a problem here in this windless spot. There's simply no draft.

I built monolithically. With unfilled brick. The neighbor next to us with ETICS had it noticeably cooler than us, at least he claimed so.