Insulation of sloping roofs. Interior foil?

Hello,

we want to insulate the sloping ceilings in the office and KZ of our settlement house from 1959. The previous owner had stuffed Styrofoam from above on both sides, but very provisionally. They don’t fit even remotely. The wall itself is covered with straw/plaster. We have already removed this. The roof is now exposed. The wooden beams and Styrofoam are dry, the roof is still from '59 but looks good from both outside and inside. The beam allows for 12 cm of mineral wool. I now want to attach an underlay membrane from top to bottom, put in wool, and now we come to the point:

Vapor barrier foil from the inside yes or no and why?

Father-in-law, internet, and brother say yes; a good buddy who is a project manager at a large construction company says no. He argues that you shouldn’t completely seal a house like this, that it doesn’t bring much benefit in a house from this construction year, everything is dry, and the indoor humidity can escape better. Windows are new. Attic will be insulated. I am torn and would like to start the project next weekend, what do you recommend to me?

Thanks and regards

Hello! Is there a photo/sketch?

Well, both are sort of right. If you have something like that done, it’s botched in 2 out of 3 cases, which would make it even worse; better to have evenly bad without a brake. But of course, the vapor barrier would be better – if you think you can do it properly, then go with it. 12 cm is actually too little... if the effort is justifiable, I would install twice as much.

Is there currently no roofing underlay beneath the roof covering? The problem with old bitumen membranes is their diffusion tightness. They do not allow moisture to pass outward. If you now also install a vapor barrier (also diffusion tight) on the inside and leave small leaks, condensation will form there and cannot escape.

Vapor retarders (diffusion open) allow some moisture to enter but also let it escape again. Vapor barriers retain moisture better, but no DIYer can make them truly airtight. Therefore, vapor retarders are the better choice.

Not using any foil at all would cause condensation to form on the outside. Provided there is no waterproof underlay there either, the condensation can dry out again.

My choice would be a diffusion open roofing underlay and a variable vapor retarder such as [Isover KM Duplex].

We definitely want to insulate the top floor ceiling. The surface in the attic is very smooth and easy to walk on. The previous owner poured something onto the top floor ceiling here, could it possibly be expanded clay or is expanded clay generally directly walkable? It could also be concrete, but it has larger pellets in the material, color gray. I actually want to insulate the attic on top of that, do you recommend it, even if it is expanded clay?