Roof insulation like this okay?

Hello lars vegas,

I would have a vapor barrier installed, it is somewhat complex from the outside, but it works if the roofer has experience. A roof can also be constructed to be vapor-permeable, but it must be structurally ensured that the vapor diffusion resistance decreases from the inside to the outside - nevertheless, there remains a residual risk because the pressure conditions in the roof area can also reverse due to climatic reasons.

Hello Hertweck,
Thanks for the quick response!
I had already asked the roofer if he was not installing a vapor barrier. He said that this was his diffusion-open construction method. That's how he would do it.
How can I tell if the vapor pressure resistance decreases from the inside to the outside? What needs to be done structurally for this?
Unfortunately, I am a complete layperson when it comes to all this...

"Diffusion open" can mean many things, including faulty roof constructions... There are calculation methods applied to ensure that a roof or wall structure functions correctly in terms of building physics; this also provides information about vapor diffusion and the dew point of the structure.

The roofer should either provide such a dew point calculation or a roof structure that is technically approved, a system structure from a company for roofing products, for example (the roofer can prove this based on technical brochures).

Hello again, yes, that looks correct. However, the work must be carried out airtight so that any warm air potentially penetrating into the insulation structure does not condense and cause moisture damage.

You mean that the LDB foil between the above-rafter insulation and mineral wool must airtight shield the mineral wool from outside? But it would also be problematic if moist air from the interior of the house gets into the wool and then condenses there, for example on a cold foil, wouldn't it?

Exactly right: The whole thing must be airtight from the inside. So the vapor barrier, which is laid between the rafters before the insulation is installed, must be airtight at all connections. This makes the installation of the vapor barrier a bit more complicated than mounting it from the inside, but it is possible.