What type of attic insulation is sensible?

Is a vapor barrier actually necessary here? It's not a conventional concrete ceiling but a straw-dry reed ceiling that is plastered (about 1-2cm thick). A vapor barrier would be very tedious and complicated to install over the cross beams.

Otherwise, new insulation wool about 20cm thick directly on the ceiling (in every cross beam space). Then completely cover on top with chipboard? It should at least be walkable.

So..I'll try again..even if there isn’t much reaction.

In the meantime, I have thrown out and disposed of all the old rock wool. At the moment, I’m cleaning as much as possible with an industrial vacuum cleaner so it looks somewhat decent. I’ve also disposed of all the junk and trash that had been "forgotten" there.

Now, once again the question whether a vapor barrier is necessary at all for such a ceiling. As mentioned, it is a very thin ceiling (approx. 1.5 cm plaster or concrete with reed as the supporting material). So the ceiling is basically dry as straw. I can’t imagine that such a ceiling still emits moisture. It’s not a solid screed ceiling. In addition, in the roof slope area, the previous insulation wool was laid directly on the ceiling (without foil). You can’t even get in there to lay a foil.

Once the insulation wool is laid in the attic (whether with or without a vapor barrier), should there then be a foil completely over it from above (to keep it from getting dusty again)? Or should it be completely covered with chipboard? I’m throwing out those small boards.

Best regards

Hello everyone,
I have finished renovating my apartment for the most part. Now the only thing left is actually the insulation of the attic above me. In spring, I disposed of all the old dirty stuff and cleaned everything (removed all the dirt and dust with an industrial vacuum cleaner).
I also cleaned everything between the beams. Now the question is how do I properly insulate here so that it really makes a difference? One of the problems is the gaps between the roof slope and the roof, as shown very well in the sketch above.
I have taken a few pictures of that.

Should I just stuff in some friction-fit insulation? Installing a vapor barrier here is out of the question. You can’t really get anywhere there, and you definitely won’t get it sealed.

For the cross beams, just remove the small boards, put in a vapor barrier (over the beams and down into the gaps) and seal it as tightly as possible.
Then there is also the question of how to seal the vapor barrier at the connection to the roof slope. Also, the substrate is quite rough and partly sharp-edged. How is it with the foil there?
Then glass wool in the gaps between the cross beams.
Probably best up to the top edge of the cross beams, right? If I were to put the small boards back, then only a 100-120 mm friction-fit insulation would fit. All the way up would be 180 mm.

I would be very grateful for any help. I just don’t know what makes sense here. I actually can’t get it sealed because of the gaps. Difficult.

Here are the gaps of the roof slope:


Here is a view from above (small boards removed):

As you can see, remote diagnoses are rather difficult here... I would help you, but I am a layperson when it comes to roof insulation... I would simply recommend that you bring in someone who knows about it, as you can make quite a few mistakes with the roof.

could work if you just lay the stone wool on top. BUT: your problem: through the drillings (here spots) you have created lots of holes through which the warm, moist air from the living space can flow unhindered into the cold attic and condense there! Here I would say, if no vapor barrier is installed (please absolutely do not lay a vapor seal over the beams! Then after one year you have high moisture and mold in the ceiling beams!!!) - and if vapor barrier, then it should always be on the warm side - meaning best under the beams, i.e. from the room side (there are also vapor retardant wallpapers that could be useful). If at all, seal the spots reasonably well so that there is no draught, lay stone wool on top and above that at most a few tongue and groove boards (they can absorb and release moisture) (but with air / spacing between them). Under no circumstances install chipboard or make it airtight! OSB boards are practically vapor tight --> rising moisture still moves through the stone wool (which is vapor open), but condenses on the OSB boards as it cannot escape. --> boards get wet, start to mold, it drips onto the stone wool --> becomes damp, sags, etc.... So: always design to be vapor open! Is the attic actually ventilated? If yes, a construction without vapor barrier should be possible, if no, moisture problems/damage can occur again. You should have an expert look at this!!! Cellulose flakes could be applied without problems - also in the sloped areas! Advantage: even distribution, better in case of any moisture problems, quick to apply (pumped in via hose) and relatively inexpensive for that. Don’t make a mistake here!! Otherwise you will have a big problem after 5 years at the latest when the roof truss rots away....

Hello, thanks for the reply. So a vapor barrier would not be suitable here, which would be unfavorable anyway with the roof construction (even without spots), because I can’t get the foil under the beams.
Batting or glass wool without a vapor barrier between the beams (cutting out the spot areas) probably doesn’t make much sense either, right? The spots are quite close together as far as I can tell.
I already know that I shouldn’t close it off at the top with OSB boards. Tongue and groove boards would be a good alternative to make it ventilated.
The attic has ventilation openings at the gable ends.. so it is ventilated.

Maybe you could also cut styrofoam boards and place them between the beams? I could have a larger amount cheaply. The question is whether it’s effective or better than nothing..
Or would it help to simply lay styrofoam boards over the current beams and boards?

I will definitely try to find someone who knows what they’re doing.