Buy a wood stove prophylactically

It certainly depends on the season and the duration of the heating-free period. Assuming the worst: a power outage lasting several days. You have to ventilate then, since controlled residential ventilation also doesn't work. If no sun shines, you would eventually approach outdoor temperatures.

I would also doubt that. A few years ago, we had a heating failure in November. Not really cold outside, but already in the low single digits. Apartment built around 1980, so concrete more or less uninsulated and a series of thermal bridges due to cantilevered parts for the balcony. The orientation of the windows here is WNW, so little sun. Even there, after 2 days, we were still at an estimated 15 degrees. With a KfW40 house, you are already close to a passive house. Without electricity, cooking is essentially out, probably also showering (?), and moisture input should be limited. I would also be quite pragmatic and not ventilate much for 2 days.

Da lüftest du auch nicht, oder?

I usually close the window in December. Occasionally I air out the room when there are people playing upstairs. For that, the waste heat from people is missing.

And the heated living space is also right below...

Not really. The floor is also insulated. Passive house. Heat loss to an unheated attic - insulated or not - is not desired.

I am rather pessimistic about that: If we have a multi-day power outage, or gas outage, or oil outage, then nothing works anymore anyway and we probably have completely different problems. Either I am a real prepper, or I leave such individual ideas alone... Without electricity, you really don't get far.

Power generator and supplies according to the disaster protection list. Then you are pretty much secure for about 2 weeks. In addition, 2 gas cylinders, gas stove, and gas heater. Then all that will last for quite a while.

I already thought that I am not the only one thinking about this, I am pleased about the response. I have an unused fireplace, I will ask the chimney sweep how to reactivate and use it.

Regarding firewood: if you don't only use split firewood but also thicker branches and pallets, there is still a lot to give away at the moment. Or I'll use the "human heater" ;).