KfW - construction: is it necessary or not?

A nice summary can be found, for example, by googling "Baugorilla Ökobilanz Energiebedarf". (And no, the conclusion is not that insulation is bad.)

At some point a few posts ago, the focus was also more on the eligibility for funding as an eco-indicator. For that, I find a brilliant example.

Of course, I am in favor of building ecologically. It is just not so black-and-white and simple to recognize WHAT is really ecological.

Why so aggressive right away?

That was not meant to be aggressive. But I think a lot of things are being presented here that are simply baseless or hearsay. I don't think anyone here is an "energy expert," whether in construction, production, or disposal of data materials. Yet statements are made that, if you research a little, are quickly relativized. Unfortunately, linking is not allowed here, so it is difficult to have a factual discussion with facts. But I find it hard to accept when statements are made without somehow substantiating or putting them into perspective.

But first tell something about grey energy

There is a Swiss study that deals with the proportion of grey energy in house construction. In the shell construction alone, approximately 2,000 kWh per m² are involved; only slightly less comes together for building services, interior finishing, and facade. So in the end, you land somewhere around 3,500 - 4,000 kWh per m². That means ~500,000 kWh for a finished 160 m² house. Last year we consumed ~9,000 kWh of gas and ~3,000 kWh of electricity. How long I would have to live in the house until my ongoing energy consumption is a bigger environmental mess than the new building itself is something anyone can easily calculate for themselves. The background of the issue with grey energy is: Anyone who wants to live consistently ecologically must not build new, but must use existing buildings. Burning the energy used during new construction in the operational phase is difficult. Measures that reduce the consumption of operational energy are basically commendable and welcome, but with regard to the grey energy of the building structure, they are just a drop in the bucket.

All well and good, MayrCh. But we're not talking about the total energy required for house construction, but about the surcharge that insulation or other heating systems need, which reduces energy consumption. The question from the thread starter was whether KFW 55 yes or no. The question of new construction yes or no is not involved at all. So you completely argued off-topic.

Thanks to the generous subsidy, KFW55 is very economical and definitely sensible. The developer

So you are completely off-topic in your argument.

Measures that reduce the operational energy consumption are basically commendable and to be welcomed, but with regard to the embodied energy of the building structure, they are just a drop in the ocean.

Here lies the answer to the question whether KfW 55 is worthwhile from an ecological perspective in an end-of-life consideration. Whether energy saving ordinance, KfW 55 or other: the energy savings, taking into account the energy used during construction, only pay off after decades, often after the end of the service life. I actually thought that could be inferred.
Economically, one has to consider separately. If it's worthwhile – why not?