Insulation / Thermal insulation old interior doors old single-family house

That's just what a door looks like. Can't be changed. But it was the same a hundred years ago...

That may be. I didn’t find it very user-friendly in the first Google hits anyway. Apparently, it’s intended that way here; it wasn’t different with the bathroom stuff recently. In the end, I have to pay whatever the service provider charges for the entire solution anyway.

You’re only supposed to replace the interior door. That can’t be such a big effort. Leave the outer one as is (due to historic preservation).
That’s not really surprising, is it?

I don’t understand that. There are front doors that shield a house from the outside world.
Then there are apartment entrance doors that form the barrier to a stairwell or similar. They are basically better interior doors.
And finally, there are interior doors that are just decoration or separate rooms inside the house/apartment.

Do you mean your old interior door to the entrance area? Yes, if it’s that old, it’s energetically worthless. That’s why you normally install an energy-efficient front door for the house. Then the rest inside the house is just decoration (see above). Or in your case, you leave the front door in place and replace the inner door (similar to apartment entrance door/interior door).

So according to this terminology, my "smaller" problem child is the interior door. Probably once used as decoration or window dressing. And the easiest thing is probably to replace it now with a thermal protection door. The front door with its history is also crap, but I’ll probably sacrifice the vestibule because I see little chance of improvement even with complete replacement. Ages ago, a custom-made replacement for a neighbor’s damaged door was around 5,000.00 euros—and that’s not a 2022 door with top heat transfer resistance values but a replica of a fossil.

Seriously improving the energy efficiency of a door with little home remedies doesn't work. [...] Either do it right or don’t bother. So either spend money or leave it alone.

That’s the crucial point.

I’d already be quite satisfied if after such a renovation, in the usual conversations, no tape-hero looks down at me pityingly because I naively threw away this top door, whereas even students in general studies learn with chewing gum how to solve such issues.

You’ve helped me great—thanks again.

Maybe. I didn't find it very user-friendly with the first Google hits.

I don't understand. You already have such a door, don't you, or am I mistaken? Please post a photo.

In the end, I have to pay whatever the service provider charges for the entire solution anyway.

I would skip the service provider for an interior door. That really is a DIY task.

So according to this terminology, my "smaller" problem child is the interior door.

Yep. Because an exterior door with historic preservation costs multiples of an interior door. Just for comparison – a fancy front door will cost you starting from €2,000 plus installation. For historic preservation (I assume) only a custom-made, extra expensive solution will do.

The front door with its history on its back is also a pain, but I will probably give up the anteroom because I see little chance for improvement even with a complete replacement. Ages ago, the fabrication of a replacement for a neighbor’s damaged door was around €5,000 – and that is not a 2022 door with top heat transfer resistance values but a replica of a fossil.

I see it the same way as you. Although at least wooden doors nowadays also have top values. For new construction, I would actually just drive to Poland for a wooden door. They are really good at that.

I am quite satisfied if after such a renovation in the usual conversations, not every tape-hero looks down on me pityingly. Because I naively threw away this top door, whereas even schoolchildren in general studies learn with chewing gum how to solve this.

MacGyver probably would have solved the problem with a paperclip and a piece of chewing gum ;)