Perimeter insulation 12 or 16 cm XPS

What cannot be assessed at all based on the information provided. If the calculation is correct, 12 cm are probably enough.

I doubt that these 4 cm are worth it, the effect of additional insulation decreases with increasing thickness.

Whether it is worth it is always a question of what you are referring to.

Heating costs will certainly be lower with 16 cm XPS. The question is how big the difference is.

I can only quantify the difference from 12 to 16 cm XPS and it is about 1,500 EURO. It will probably take several decades to amortize. But then the heat pump runs less intensively and therefore a bit longer? Additionally, maybe not as much piping needs to be laid in the underfloor heating? Hmm, I am very undecided.

Take the advice here: If it is not necessary, it is not worth it. Of course, if you want to ease your green conscience, that is something different.

I don't need to calm my green conscience. I would be more interested in a more economical installation of the underfloor heating in the basement or a more durable WP. But whether the extra 4 cm make much of a difference, I am starting to doubt as well.

If you feel more comfortable with it and [euch das Geld nicht wehtut] do it, it won't do any harm.

You don't install underfloor heating sparingly but according to the heating load of the rooms and designed for a low flow temperature.

You can always throttle the heating, but later you can never open up additional areas.

Well, then I'd rather save the 1 1/2 thousand and have it available elsewhere.

Isn't Ht (or also the insulation of the outer shell of the thermal envelope) relevant in floor heating planning/installation as well?