Wall construction - Timber frame construction - Installation level - Inquiries

Hello,

we are currently planning the renovation of a typical prefabricated house from 1978 by Nordhaus.

The current wall composition of the exterior wall is as follows (from inside to outside):

Gypsum plasterboard -- chipboard (V100 - 13mm) -- 100mm mineral fiber board -- chipboard (V100 - 13mm) -- 16mm air -- 8mm chipboard as a curtain with plaster

Since we now want to renovate the electrical and building services accordingly, I would like to install an installation level from the inside. My idea was to additionally insulate the level later with wood fiber boards (60mm).
According to dew point calculation, this would not be a problem, my question is whether it makes sense to apply a vapor barrier in front of the "old" wall from the inside (so in front of the 1st chipboard) or whether this would be rather counterproductive. The OSB board already fulfills part of the vapor barrier. Or would the wood fiber boards get fully saturated over time?

In another forum, the answer so far has been that condensation would occur (with my new planning below). But there have been no further answers there. That’s why I’m trying my luck here.

The rule that the interior should be more airtight than the exterior should apply here, or can someone clarify what would be wrong?
I have googled several times and the wall build-up always came out roughly as I imagined:

The planned composition is as follows (from inside to outside):

Gypsum plasterboard -- OSB3 18mm -- installation level (filled with 60mm wood fiber boards) -- chipboard (V100 - 13mm) -- 100mm mineral fiber board -- chipboard (V100 - 13mm) -- 16mm air -- 8mm chipboard as a curtain with plaster

Thanks in advance!

OSB boards are vapor retarding, but not at the joints, and a considerable amount passes through the numerous openings for light switches and sockets. A vapor barrier with a low sd-value between the old wall structure and the installation layer is certainly not harmful with intact insulation. With the division of 60/100 mm inside/outside, there should be no dew point undershooting. For a 40-year-old house, however, I would check the wall structure at an appropriate location for damage-free condition.

Otherwise, no one will surely give you a guarantee that nothing will go wrong with a retrofit without a vapor barrier on the inside, and replacing and doubling up the existing insulation from the outside is, of course, free of thermal bridges. In general, however, this is not economical with the existing wall structure.