KfW - construction: is it necessary or not?

Yes okay, although the whole house and the energy savings from insulation are not really the topic, but okay. Then it would be most energy efficient to live in a 2m² cave, since it is small enough that your body heat provides sufficient heating. Eating berries and the occasional fish. But you’re not serious, we live nowadays the way we live and we have to discuss at this level.

And then old building versus new building. If the new building consumes 5,000 kWh/a heating energy and the uninsulated old building 20,000 kWh/a. Then the 15,000 kWh/a difference already adds up after just over 30 years to the 500,000 kWh grey energy you mentioned.

Or calculated over 40 years:
Old building: 0 kWh (existing) + 40*20,000 kWh = 800,000 kWh
New building: 500,000 kWh ("grey energy") + 40*5,000 kWh = 700,000 kWh

I don’t feel like calculating that exactly but the breakeven point at which the old building overtakes the new building in the overall energy balance should be at about ~35 years. Where is the benefit of existing properties now so much better?

*The values 5,000 kWh for a low-energy new building and 20,000 kWh for an old building are assumed averages from a brief internet search for a 160m² house.
*The value 500,000 kWh for the construction of the new building is adopted from MayrCh.

Regardless of the fact that it was not about the question of existing building or new construction: As already wrote: An old building could be renovated in an energy-saving way with significantly less energy input, which shifts the calculation somewhat. That the subsidies do not make this even more attractive is probably a failure of politics, or a success of lobbying. On the other hand, new construction (at least of multi-family houses) is certainly desired in metropolitan areas with a housing shortage. For such buildings, longer lifetimes than those discussed here are also intended, so I consider it sensible to build objects as energy-saving as possible.

No, the house is actually very efficient. We just didn't build it according to KfW standards - for example, the heating.

How did you do it?

And Super Gasoline comes directly out of the ground next to the gas station and therefore produces zero CO2 during its production and is better because of that?

If you criticize the environmental costs for battery production, then please also include in your calculation the environmental damage caused by crude oil extraction and transport to the refinery. I mean, oil is known for coming practically from the tap and being absolutely clean, and nothing ever spills when you "pump" it. Then please also calculate that the refining process is very energy-intensive to actually make gasoline from crude oil. Please also include what happens with the waste products of production (keyword heavy oil). Oh yes, and then please don't forget the transport by truck from the refinery to the dealer or gas station. And then remember that the gasoline is finally also burned.

So if I consider the complete environmental balance of gasoline, from the time of crude oil extraction to the time it is burned in the engine, then honestly I don't find gasoline or heating oil really more environmentally friendly than batteries.

That's how it is!
It is always a complete consideration, especially including personal lifestyle. That is also the predominant consensus here.
My parents, for example, didn’t even know the word ecology. Back then, they sometimes used substances in the garden that are now forbidden poisons among other things, and heated quasi uninsulated houses with oil/wood. All of this wasn’t really long ago.
And yet, in their overall ecological balance, they achieved values that none of us today would even remotely reach, and certainly wouldn’t want to.
No car, no travel, vegetables and fruits only from the garden, forest or neighbors, hardly any meat and if so, self-slaughtered, pasta made from scratch, and no unnecessary consumption and energy-consuming technology and so on...
This is not meant to be the old familiar praise “Everything was better in the past,” because it really wasn’t, and I like innovation, a comfortable life and nice things.
But that generation could have really called themselves super-ecologists in their overall balance, given what they had as a normal, possible life.
When I compare that to our (including my own) current consumption, we can never see ourselves as acting ecologically, regardless of certificates.
For this reason, I view all these certificates critically since our own, today's usual lifestyle simply cannot be ecological.
Then we would have to give up so many comforts (which I would reluctantly do), and many people couldn’t do that at all.
We should be honest with ourselves, with or without KFW or whatever.
The best is for everyone to start alone...
I am happy about every nature-preserving action, for example, I will hardly develop my property at all and just leave it as a meadow, as it is. But I am aware that even the digger that carries out the excavation causes an almost irreparable damage to the ecosystem of this beautiful, large meadow, no matter if I build a parking lot, playground or house on it.

Yes okay, although the whole house is not the topic and the energy should not be compared to the energy savings from insulation, but okay. So it would be most energy-saving to live in a 2m² cave, as it is small enough for your body heat to provide enough warmth. Eating berries and occasionally a fish. But you can’t be serious, we live nowadays as we do and we have to discuss on that level.

And then old building versus new building. If the new building consumes 5,000 kWh/year of heating energy and the uninsulated old building 20,000 kWh/year. Then the 15,000 kWh/year difference already adds up after a little over 30 years to the 500,000 kWh of gray energy you mentioned.

Or calculated over 40 years:
Old building: 0 kWh (existing stock) + 40*20,000 kWh = 800,000 kWh
New building: 500,000 kWh ("gray energy") + 40*5,000 kWh = 700,000 kWh

I’m not keen on calculating precisely but the breakeven point at which the old building surpasses the new building in the total energy balance should be around ~35 years. So where is the benefit of existing properties now that much better?

*The values 5,000 kWh for a low-energy new build and 20,000 kWh for an old building were assumed from a brief internet search as averages for a 160m² house.
*The value 500,000 kWh for the construction of the new building was taken from MayrCh.

Please really leave out the undertone from the statements, it is supposed to be an open discussion.
Of course, it is a glib killer argument when you talk about a 2 m² cave and berries or body heat as the sole heating.
But if you then write "we simply live as we do," then you could justify or excuse everything with that. We heat, drive cars, fly on vacation, indulge... that’s just how it is nowadays. To consider today’s lifestyle (which I partly live myself) as immutable and a permissible right, as you write, I do not see it that way. Perhaps the way of living, housing, driving, consuming really has to change fundamentally and drastically.
I have no solution ready, certainly not, but to regard our present life as a justifiably lived "level" does not seem appropriate to me. It is fine that it is possible, but if it is no longer possible, e.g. from an ecological perspective, then it simply will not work anymore.

Everything is fine and well, MayrCh. But we are not talking about the total energy required for house construction, but about the surcharge that insulation or other heating systems require, which reduces energy consumption. The question of the thread creator was KfW 55 yes or no. The question of new construction yes or no is not included there at all. So you are completely off-topic in your argument.

No, I don’t believe that, because the statement from can be included by the questioner in their overall consideration of new construction. When I consider the many aspects presented here, I can come to a different conclusion on the individual decision regarding KfW than without these aspects. Therefore, the questioner will decide for themselves whether this is relevant to their decision or "off-topic." None of us knows what seems important to them, so we should not devalue contrary contributions here. I am interested in the different contributions, even if I then decide differently.