Is a vapor barrier necessary under insulation on a floor slab?

Feedback from the roofer verbatim: "You don't need a vapor barrier in that area because you have airtightness under the roof". Now I'm completely confused. I think he didn't understand something correctly. Let's see what the energy consultant says.

hmm yes, he probably didn't understand your problem. So if you install the vapor barrier, it is important that you become more diffusion-open towards the attic space (this is probably given with the floorboards, unless they are varnished) and that you regularly ventilate up there.

Feedback from the energy consultant just came in as well, a vapor barrier is indispensable, especially since these are bedrooms and a bathroom. He would have used wood fiber material instead of mineral wool - too late, it has unfortunately already been installed under the ceiling. Well, then with a vapor barrier. It should be done properly. Then I will start cleaning the walls. Diffusion openness to the attic is given. I think I will work with decentralized ventilation there.

, but used wood fiber material

good man!

too late, unfortunately it’s already installed under the ceiling

Ah, I unfortunately overlooked that.

Yes, mineral wool isn’t much worse, it’s just not as ecological, sound- and heat-insulating, and not quite diffusion-open. But it’s really non-combustible and cheaper...

Next time there will be no more glass wool, definitely. That stuff is so nasty to work with, even with protective gear. Horrible. And the ecological aspect also plays a role - basically it's no longer appropriate to work with something like that. Unfortunately, I was a bit hasty with the order.

One more question about the vapor barrier. It really has to be 100% airtight, that's what you read. That makes sense. Are the effects really that devastating with the slightest holes or improper sealing? I mean, the goal is certainly to do it as precisely as possible and we do take the time for that - but mistakes can always happen, even with professionals, if you are one. If there is a small opening through which moisture penetrates, but there is diffusion openness above, then the moisture that has penetrated can also evaporate / dry out again, or am I mistaken?

I'm not entirely sure about that. Normally, I would have said that since the top is already insulated, the temperature difference isn't that big and therefore, as you say, not too bad. But by insulating the warm side now as well, the temperature difference actually becomes bigger again. That's also why I said you should preferably heat there as well. If you ventilate regularly, it's probably less risky. But it remains a weak point. Alternatively, really use a vapor retarder and not a vapor barrier. Then it is a bit open everywhere and only slows down diffusion so that with regulated ventilation there is always enough time for it to dry out. For that, it's best to install two decentralized fans on opposite exterior walls of the house and have them cross-wired (one in, one out, alternating). This way a proper airflow can develop.