Intermediate rafter - under-rafter insulation = double vapor barrier foil?

In a construction with 220-240mm cavity insulation between rafters and 40-60mm insulation below the rafters, the vapor barrier is usually installed between these two insulation layers, but why is that? I understand that it cannot be omitted, but why isn’t an additional vapor barrier installed on the room side?

Common variant:
1) Roof
2) Underlay membrane
3) Rafters + cavity insulation
4) Vapor barrier
5) Substructure + insulation
6) Plasterboard

Why not?
1) Roof
2) Underlay membrane
3) Rafters + cavity insulation
4) Vapor barrier
5) Substructure + insulation
6) Vapor barrier
7) Plasterboard

Vapour barriers/vapour retarders are a physically necessary evil due to the modern airtight construction standard. It makes sense to limit them to a minimum, especially when the VB/VR is a foil product whose aging resistance and robustness during on-site processing are not really guaranteed. The function of the VB/VR should better be taken over by more suitable building materials with a corresponding sd-value, classically the OSB board in the roof with taped joints and connection seams to the adjoining components, on the inside/the DWD board on the outside as an underlay board.

The disadvantage of foils is that they can never be processed 100% airtight (faulty connections/damages/missing taping). Warm, moist indoor air penetrates the construction by convection/vapour pressure, condenses at cold spots, and may want to dry back into the room in summer during the phase of reverse diffusion. In doing so, it is hindered by the foils. Foils also prevent capillary re-drying, which is significantly more effective but requires capillary-active building materials. Mineral wool, for example, is not.

Conclusion: As much VB/VR as necessary and as little as possible. An externally ventilated construction is always significantly more advantageous than a vapour-tight one that must be sealed on the inside. The overall structure must be calculated based on the sd-values ("inside tighter than outside"). The selection of materials should not be left to the executing company or must be proven physically consistent by them.

I would definitely also prefer an underlay membrane/underlay sheet to a DWD board on the outside.

Now you didn't really answer his question, but rather presented your opinion on the slides.

It is rather contrary to many other opinions...

Grym, you need to check the U-value calculator - no critical condensation occurs with the low insulation thicknesses, especially since the real rafter insulation is behind it anyway.

I refrained from it because it's a lot of work, little effect, and I already have additional external rafter insulation anyway.

To put it simply: if two vapor barriers are installed, a plastic bag effect is created. If moisture gets in there (and unfortunately this cannot be guaranteed 100% despite all care), a greenhouse literally forms inside because nothing can evaporate "outwards". That is why the rule is "always more permeable to diffusion on the outside". By the way, I found the first post very fitting because it sensibly explains that two vapor barriers are simply too much of a good thing.

But a completely diffusion-open construction would be better? Does this achieve a U-value of 0.14? This is a requirement for Kfw55 according to reference values and also, according to precise calculations, a good value to just achieve KFW55. Insulation materials that make a vapor barrier unnecessary unfortunately usually have worse values, and the roof construction with mineral wool WLG032 is already "massive" with 280-300mm insulation.

Why "completely diffusion-open"? A vapor retarder is sufficient, two defeat their purpose. Only as much vapor retarder as necessary, not as much as possible (e.g., vapor barrier). By the way, there are also partly diffusion-open or one-sided "open" membranes that allow any moisture that has gotten behind the membrane to escape again towards the room side.