Vapor barrier/vapor retarder or rather leave it be?

Hello, I am new and looking for some help with my project.

I would like to completely seal my roof from the inside. The most important thought for me is to make it more windproof, since at the moment there are drafts everywhere.
Attached are some pictures of the roof as it looks now,
in my defense, I bought the house like this and now just want to do something useful if I can, little by little.
It is insulated all the way behind the knee wall down to the bottom and up to the peak.
and this is how it looks in reality.

Currently, there is a double wooden ceiling in the upper floor living area (sloped).
Can I now "simply" install the vapor barrier and seal the entire roof with it?
The wooden ceiling (sloped) and knee wall will be removed so I can work better. Then only a wooden ceiling will be installed on top, either directly on the foil, or if necessary with a battens for leveling.

What do the experts say or think about this?
Thanks in advance.

Hello "Maddin", presumably the main insulation for the living space(s) is located below the walkable crawl space. The necessary vapor barrier is also there. The side surfaces of the sloping roof are – as can be seen – either not equipped with a vapor barrier, or the aluminum coating is not verifiable for airtightness of the adhesive seams in terms of functionality. My suggestion: If the roof area visible in the photos is used only sporadically and not for regular living purposes and (this would have to be checked) an insulation layer including a vapor barrier is installed under the crawl space, then a PE foil on the sloping roof surfaces would help to support wind tightness and prevent the insulation material from falling into the room at some point if it is not properly secured. The main load of the vapor barrier would then lie in the area of the crawl space (or the ceiling of the room below), so for this reason no additional vapor barrier would be necessary. -------------------------------- (these notes were made without knowledge of the location, therefore this does not constitute a construction recommendation) Regards: KlaRa

Thank you for your initial information

there is no foil installed either in the ceiling of the upper floor to the attic nor in the slope, unfortunately.
The area in the pictures is the attic; no one is up there all year round, below is the living area of the upper floor, but usually only heated to 16-17 degrees, rarely anyone upstairs.
Sometimes the guest room is visited.

In this case, it is not about heat for me, but only about wind tightness
even if it sounds strange.

I just don’t want to install a foil from bottom to top now and then have damage at some point because no air circulation takes place anymore.
Only when the wind blows and I am standing upstairs, I could imagine having my hair “blown dry.”

Here is an old picture from the beginning of the renovation

The other pictures above are from above the ceiling of the attic.

Hello "Maddin77". What is (unconsciously) planned by you here is a restricted and adapted use, so that the defective installation of the interior insulation (as far as it actually exists between the upper floor ceiling and the insulation) did not include a vapor barrier. Now, if the insulation layers on the sloping roof surfaces do not allow thermal bridges, thus the insulation fits tightly everywhere, then the vapor barrier can be installed on the room side without any danger. However, the PE film should be thick enough (at least 0.15mm) and the joint seams must also be tightly overlapped and glued together with strong adhesive tape. It becomes difficult at the transition to the floor (somehow it has to be connected there) and also at any existing window openings. As soon as the construction, consisting of insulation material + vapor barrier, allows moist and warm air from the underlying floor to penetrate the insulation plane, condensation can occur in winter due to dew point undershooting. Therefore, it applies: if interior insulation is done, then you must know what you are doing! ------------------------------- Regards: KlaRa

Yes, I am already diligently reading and watching videos about the adhesive tape and cartridges for the flexible connection with movement flap and primer for pre-treatment for the screed connection. Known questions from those who have already done something similar.

There is definitely no foil installed in the interior. In the entire upper floor. I am now planning to completely remove the wooden ceiling of the slope and the knee wall in the upper floor. Then pull the foil from the very bottom connection to the screed behind the knee wall all the way to the top. And of course the same on the other side. I wanted to leave the aluminum-coated glass wool in place, it does not seem to be damaged. Just install the foil. Barrier or resistance? The vapor barrier would be with these data that I found

[*]Tensile strength longitudinal: 27 N/mm²
[*]Tensile strength transverse: 22 N/mm²
[*]Elongation at break longitudinal: > 800%
[*]Elongation at break transverse: > 900%
[*]Water vapor permeability: sd > 100 m
[*]Dart Drop: > 175 g
[*]Color: blue
[*]Fire class E according to DIN EN 13501-1
[*]Sd-value > 100m

Does anyone know if the PE foil can also be well attached to metal beams? Or are there special products/adhesives for that? Since my gable consists of steel beams, I have bare metal beams in the knee wall and the attic, and the foil has to be attached there somehow as well. Stapling is not possible.