Cold roof - Insulate floor ceiling

Hello,

I have an approx. 100 sqm attic (cold roof), the roof itself remains ventilated. There are also ventilation inlets present. I now want to insulate this (attic insulation).
So far, I have seen many nice videos about this and it is quite clear to me, but all videos and instructions referred to a smooth floor. In my case, it looks like I can see the beams and from below there are the ceiling boards of the story. I think the picture explains it.
My idea was to put insulation in two layers in between and staggered. But what about a vapor barrier? All videos and instructions say that I should put a vapor barrier under the insulation. That is difficult to implement (or just lay it over the beams and then put insulation on top)?
Furthermore, electrical cables run over the beams and then distribute downwards into the rooms. Therefore, I’m not quite sure whether or not to use a vapor barrier and whether insulating only between the beams is sufficient.
I could also lay a membrane over the beams, would this be an option?

I would use as material - Isover Topdeck DF 2-035 ceiling felt thickness 80mm (160 mm through both layers) ---- Would this material be suitable or do you have a better material.

Attached is a sketch and also the ceiling structure.

Hand installation ceiling FB190, execution according to ceiling plan. The ceiling construction under the wooden nailed truss consists of 25mm counter battens, 25mm HWL board as plaster carrier and insulation boards with wire mesh and 15mm ceiling plaster as well as 100mm mineral wool mats on one layer of 350-type nBT paper as wind protection. For enclosing walls of 240mm aerated concrete, the thermal insulation should be reduced from 100mm to 60mm mineral wool mats.

Hello,

to my knowledge, the general recommendation for a vapor barrier/vapor retarder is based on the fact that heated rooms adjoin unheated areas (="cold roof"). When the air cools down, condensation occurs. The moisture is either stopped before the insulation (approx. 100%) or slowed down (< 100%, allowing some trapped moisture to escape), to prevent damage to the insulation or the wooden structure.

As far as I know, there are also calculation models where floor insulation can be installed without a vapor barrier/vapor retarder and the dew point is reached only very far outside, where any occurring water can evaporate and not penetrate into the wooden structure. For this, a specific arrangement (insulation material, layer thickness, and structural design) must be considered, which a specialist can calculate for you. In the worst case, if you do it yourself "roughly," the wooden beam ceiling could rot.