Roofer wants to do without a vapor barrier

I unfortunately have to watch my money closely and build as cheaply as possible. I have now obtained an offer from a roofer for roof insulation. He suggested putting 12 cm of mineral wool between the rafters and applying 6 cm thick wood fiber insulation on the roof. He considers a vapor barrier unnecessary, as the attic is already fully finished and the elements made of drywall, Fermacell, and Heraklith would already act like a vapor retarder. If water nevertheless condenses on the insulation, it would dry quickly again. Is that correct, can I forgo a vapor retarder?

It is possible, if well planned and properly executed, to do without a vapor barrier, but it is absolutely essential to ensure that the vapor resistance decreases outward. This means that vapor should be directed outward if it enters the insulation layer. Dry plaster (gypsum boards, etc.) does have a vapor-retarding effect, but that is not the only issue; it is also very important that warm air does not enter the insulation layer through joints and condense there (causing wet insulation). The gypsum boards must therefore have airtight joints – especially at connections to the wall, beams, or the chimney, etc. Perhaps you should have this checked.

We had the same problem and I sought various opinions. These were different and another roofer said he would not do without the vapor barrier. Now I am somewhat conflicted and maybe there are further experiences here.

Hello,

I think the roofer has to decide here. He is also liable for it. As far as I understand, the roof must be completely watertight. Then a vapor barrier can be omitted.

At this point, I have a different opinion and I would not do without a vapor barrier. I also do not believe that if moisture penetrates there, it dries immediately.

On the contrary, this is the case, especially when condensation occurs.

This is quite a lively discussion with different opinions. However, I can gladly do without a vapor barrier. However, the joints of the gypsum plasterboards must be sealed airtight. If that is the case, I can do without it.