Additional insulation for the ceiling of the upper floor with a warm attic: useful/necessary?

That's exactly how we did it. We have two low-temperature radiators integrated into the underfloor heating circuit. So far, it works great.

May I ask which brand the radiators are? I'm not very familiar. But are the radiators "somewhat noisy" because of the fans?

That means I now have several options: a) Attic insulated - intermediate ceiling not insulated -> as currently planned, but probably not the best solution?

a) is not quite as critical regarding mold as c), but it will be about 18 to 19 degrees at the top in the deepest winter. At least that is the case for me; I implemented variant a). Should the attic be ventilated by controlled residential ventilation or not? Is an enthalpy exchanger planned for the controlled residential ventilation at some point in the future? Or ventilation by window? I placed a wireless thermometer with dew point display in the attic and ventilate very regularly during the cold season. The air up there must be drier than in the living area, especially if there is still construction moisture at the beginning.

b) Attic insulated - intermediate ceiling additionally insulated -> causes extra costs, but probably a sensible solution to balance the temperature difference

Beware of mold risk! This is possible and doable, but you have to think carefully beforehand and know what you are doing. In the attic, there may be 8-13 degrees cold air with comparatively high relative humidity, which rises through leaks from the living area (or initially due to construction moisture). A dew point of 10 degrees is very quickly exceeded when it gets cold outside. You must either ventilate through controlled residential ventilation or very regularly ventilate manually through windows. However, ventilation via central controlled residential ventilation is somehow contradictory, as it blows warm air back in at the top. And an enthalpy exchanger would be a huge mold risk!

Alternatively, install a decentralized ventilation system in the masonry on the attic (e.g., Bayernlüfter). In addition, airtightness towards the living area must be ensured very well.

I might upgrade from a) to b) sometime soon. Purely as a hobby project, it will not save much heating cost. But it was important to me that the construction moisture goes out of the house over two winters first.

c) Attic insulated - intermediate ceiling not insulated, but radiator in the attic -> I would have to clarify with the plumbing company to what extent this is still feasible. Does it make sense to continuously heat the attic if it is only used for storage?

Best for mold prevention and more efficient in heating costs than a), because you can run a lower supply temperature in the whole system. And you don't have cold ceilings on the upper floor.

The easiest is variant d), exclusively insulation of the top floor ceiling and cold roof. Proven thousands of times, no mold risk, by far the cheapest. But deep frost in winter and high peak temperatures in summer for the storage room.

That would not be worth the storage space to me, I mean the effort with insulating/heating in the attic. As already recommended, I would also make it a cold roof. It initially saves you the effort and cost of insulation, the effort and cost of heating, and lastly no one even guarantees that despite all the effort you won’t still get mold problems.

Warning of mold risk! …However, ventilation via central controlled residential ventilation is somehow contradictory, as it blows warm air back in at the top. And an enthalpy exchanger would be a huge mold risk!

I can confirm that from my own experience. In the first winter with construction moisture, it can only be managed at the moment with a drying device and no further measures.

In summer, I am considering a ventilation concept up there and sealing a few spots.

how exactly was it executed for you? Which building materials and U-values for the top floor ceiling, what insulation for the roof? Concrete ceiling as the top floor ceiling or wooden beam ceiling?

For me, the top floor ceiling is a concrete ceiling. I am still unsure whether I really want to add insulation. The downside of my previous uninsulated concrete ceiling is that some living rooms on the first floor are a bit cool because the concrete ceiling is somewhat cold.