Insulation of the top floor ceiling in an old building

Hello everyone,

I am new here in the forum and have spent the last few days wading through numerous threads, but still have not found a clearly suitable answer to my specific problem. Therefore, I am turning directly to you and hope for expert help.
Basically, it is about the fact that with my recently purchased old building, I have to comply with the obligation to insulate the roof or the top floor ceiling. I want to do this as simply and cost-effectively as possible myself. I am currently ruling out insulating the roof because it is largely covered with wood paneling and I do not want to remove and reinstall these panels. The attic should remain walkable, but the height limitation due to the insulation (about 16 cm) would be acceptable.
Regarding the necessary insulation, I have already received information that an insulation material of category WLS 035 with a thickness of 140 mm would be required. My plan so far was to lay a vapor barrier foil on the existing floor covering of the top floor ceiling, place 140 mm thick load-bearing polystyrene on it, and cover this with 22 mm thick OSB3 boards. The material cost would be just under €1,000 – polystyrene €20/m², OSB3 €10/m² for about 30 m² area.
Based on what I have read now, however, I am worried about the construction regarding moisture dissipation. To better assess the scenario, I have taken various pictures and will now try to describe the current construction as best as possible:

[*]The floor ceiling is covered from below with wooden ceilings, suspended ceilings, etc., which I definitely want to leave as is.
[*]The floor ceiling itself consists of wooden beams whose spaces are filled continuously with solid hardened material (no idea what was used back then).
[*]The floor ceiling is partially covered in the attic with old floorboards and particleboard.

Behind the chimney, there is a small cavity which has no floor covering; here I would later only install vapor barrier and polystyrene, and would forego OSB3 boards in this non-walkable area.
How should I best construct the insulation to avoid damage from moisture and keep effort and costs as low as possible?
Thanks in advance for your help and best regards

Current floor/ceiling construction:





Ceilings below the floor ceiling:

1. Are you sure that you need to insulate? Only the minimum thermal protection must already be in place. 2. There is no insulation police that checks this. If anything, you do it for yourself. 3. You can calculate the structure yourself with ubakus. 4. Off the top of my head, I would say that it could fit. The panels are vapor-tight. If necessary, tape the edges.

1) Yes, I am sure and have now confirmed it again with ubakus. Exactly the 140mm WLG 035 are missing to meet the minimum thermal protection. 2) The chimney sweep has already addressed this with me and emphasized my obligation in this regard. I believe - as I understood it - he even reports if I have not fulfilled my obligation after 2 years. 3) see under 1) 4) Unfortunately not, that would have been great.

Yes, I am sure and have now confirmed it again with ubakus. Exactly 140mm WLG 035 are missing to achieve the minimum thermal protection.

I doubt that. Not the minimum thermal protection of today! In old buildings, a concrete ceiling (without any insulation) is often sufficient for that. I often see that in multi-family houses, where nothing needs to be done at all.

Shame on me, you are of course right. I overlooked something regarding the requirement. My old blanket only needs to achieve a thermal resistance of R=0.90 m 2 K/W. But how do I find out what the blanket is filled with and whether this material already meets the requirement? Just looking at it is really difficult for me as a layman.

How do I find out now what the ceiling is filled with and whether this material already meets the requirement? Just by looking at it, it's really hard for me as a layman.

Well. Good question. I guess a professional will have to do it.