Proposal for insulation construction in a ventilated under-roof needed

For builders and homeowners, the question arises whether a roof insulation without a vapor barrier is still possible based on the new standards. The answer depends on the insulation materials used: roof insulation without an explicit vapor barrier is an option whenever capillary-active insulation materials are used that are able to independently regulate the "moisture balance" of the roof.

Roof insulation without a vapor barrier – not yet a general standard
That a vapor barrier cannot be omitted in roof insulation today is technically necessary. In the ventilated/back-ventilated pitched roofs of past decades, it was usually integrated into the roof construction by a water-, air- and vapor-tight bitumen roofing membrane above the roof sheathing. Through the ventilation on the outside and the insulated inside, moisture or water vapor that had penetrated was reliably transported outwards. An interior vapor barrier was only necessary in case of insufficient ventilation and/or as a prerequisite for a greater drying reserve.

Mineral wool is not capillary active.

The section is for new constructions. The bolded part is what the OP most likely encounters.

For me, this still doesn't make sense, or rather, the bold part does not apply in connection with mineral wool. Ventilation cannot ensure that mineral wool dries safely. It would be different with wood fiber. That is why I would only insulate with mineral wool during renovation if a [DampfSPERRE] is used on the inside and the installation is verified by measurements (Blower-Door).

That was the state of the art in the past. Today, one would no longer build it that way; but with older pitched roofs, they were part of the roof structure.

Therefore, the question to the OP is who said that the current roof is not professionally done and must now be replaced.

: Many thanks in advance for your response. An energy consultant (unfortunately nowadays you have to let them into your house during a renovation) and two carpenters have taken a look at it. What I unfortunately forgot to mention is that the roof has only a 28° pitch. It was not professionally done in the sense that Styrofoam was partially alternated with Rollisol, partially stapled inside the rafters.

Unfortunately, however, you have gone down the wrong path. It is not about preserving the glass wool, which is no longer available, but:

Which materials and construction would you recommend for an insulation between the rafters? WARNING: Full rafter insulation is not possible due to the vapor-tight underroof with a bitumen roofing membrane (Buzi AK)!