Attic Expansion - Insulating the Pitched Roof from Inside - Totally Confused

Your craft business is absolutely right in my opinion.... I also built my new house (KFW55) with ventilation behind the cladding. That might not be the latest trend, but the latest trend doesn't have to be the best. Without ventilation, you run the risk that, in case of an execution error or if moisture somehow forms for whatever reason, your roof will decompose.. This topic was also briefly discussed with us. The building materials dealer recommended a warm roof without ventilation. The planning structural engineer and construction engineer strongly recommended ventilation. A well-known building expert of mine then said after appropriate argumentation, better to ventilate, but he basically says both options are possible. The safer one, however, is ventilation.. with the other option you have zero tolerance for mistakes in execution.

What you are saying reflects my current level of knowledge. But that is just "internet knowledge." You always read that the vapor barrier must be done perfectly. I can't watch the craftsmen all the time, nor can I judge whether the execution is correct. If I lose some space in the room but have the security that I won't have any problems later, that's absolutely okay with me. I think craftsmen should be able to explain and argue something like this professionally. I can't do much with a statement like "that's just how it's done."

As I explained, my decision regarding execution was not based on internet knowledge.

I don't understand why plans of the house help to assess the installation of the insulation.

Well then I must have asked out of senile boredom *rolleyes*

Another opinion or report of experience, with which I do not want to contradict the other opinion:
When I built my new house in 1990, there were no possibilities to get such extensive information due to the lack of the internet.
At that time, we also insulated between the rafters, and I additionally clad the roof from the outside with a layer of boards on the rafters before the tiles were finally laid.
The rafter depth was 14cm or 16cm, and even back then the question existed whether to fully insulate the space in between or to leave an air layer.
At that time, KFW paid for a simple "expert report" or a factual evaluation by an institute for wood processing or a similar institution. I had this done (the simplest form of a current KFW calculation) and they gave me advice regarding wall thickness, window orientation, insulation, etc.
From there, I was explicitly told that I could and even should fully insulate the space in between. Even if this actually led to the often feared formation of isolated trapped moisture or similar, in the overall assessment this was absolutely negligible (from memory the wording…). Such was found in various investigations.
Contrary to my then carpenters, the carpenter uncle etc., I then fully insulated it and lived there for 30 years without any problems arising from it.
I think that there is no absolute right or wrong here either and would do it the same way again based on this experience. At that time, only a regular PE foil was applied on the inside against it, airtightness was not such an issue back then… o_O

So the structure is as follows: as tight as possible on the inside, open to the outside. If moisture gets into the insulation from the inside, it must be able to escape at the back. If there were a climate membrane under your roof tiles, the insulation could be placed flush. But since you have wood there, the moisture would accumulate and start to cause mold. Your craftsmen are absolutely right in this case. By the way, I also really like the entire structure very much. This gives you a good windproof layer and also relatively good heat protection in the summer.