ᐅ Is roof insulation like this acceptable?

Hello lars vegas,

I would recommend installing a vapor retarder. Although it can be somewhat complex from the outside, it is manageable if the roofer has experience.

It is also possible to build a vapor-permeable roof, but this requires a construction design that ensures the vapor resistance decreases from the interior to the exterior. However, there remains a residual risk because, due to climatic conditions, the pressure conditions in the roof area can sometimes reverse.

Hello Hertweck,
Thank you for the quick response!
I already asked the roofer whether he would not install a vapor barrier. He said that this is his vapor-permeable construction method. That is how he would do it.
How can I tell if the vapor resistance decreases from the inside to the outside? What needs to be done from a construction perspective to achieve this?
Unfortunately, I am a complete layperson when it comes to all of this...

“Breathable” can mean many things, including faulty roof constructions...
There are calculation methods used to ensure that a roof or wall assembly functions correctly in terms of building physics; these calculations also provide information about vapor diffusion and the dew point within the assembly.

The roofer should either provide such a dew point calculation or use a roof assembly that is technically approved, such as a system assembly from a company specializing in roofing products (which the roofer can verify using technical datasheets).

Hello again,
yes, that looks correct. The work must be carried out airtight to prevent warm air potentially entering the insulation structure from condensing and causing moisture damage.

You mean that the LDB membrane between the exterior insulation above the rafters and the mineral wool must make the mineral wool airtight from the outside?

However, it would also be problematic if moist air from inside the house reaches the wool and then condenses there, for example, on a cold membrane, wouldn’t it?

Exactly right: The entire assembly must be airtight from the inside. Therefore, the vapor barrier, which is installed between the rafters before adding the insulation, must be airtight at all connections. This makes the installation of the vapor barrier somewhat more complicated than fitting it from the inside, but it is doable.