Kampa prefab house from 1978 - but musty smell.

More news.
I was able to measure the indoor air with a Trotec BQ16.

Formaldehyde: 24h without ventilation around 0.15 mg/m^3 everywhere, after 20 minutes of ventilation around 0.06 mg/m^3.
Thus slightly above the limit value of 0.12 mg/m^3.

TVOC: without ventilation around 1 mg/m^3, after ventilation at 0.5 mg/m^3. In the smelly rooms once 2.5 and once 1.8 mg/m^3 before ventilation.

Why?
Formaldehyde too high, okay, the other can be anything or nothing.

Further news:
Indoor air measurement was carried out. The results of the important substances with a request for comments:

1. Formaldehyde: 0.1 mg/m^3, exactly at the limit value. The measuring device value at that time was 0.16 mg/m^3. The expert said that these measuring devices often react very sensitively to other substances (e.g., water vapor, breath) and therefore the measured values are often higher.
Conclusion, not good, but not bad either.
2. PCP and Lindane: PCP is at 10 ng/m^3 and Lindane at 70 ng/m^3. So both values are clearly to at least somewhat below the general target value of 100 ng/m^3.
Conclusion: It could be better, but not alarming.
3. Chloronaphthalenes: These were below the detection limit!
Conclusion: Yay!
4. Chloranisoles. See the table. Detection limit each at 0.1-0.2 ng/m^3.
Trichloranisole Under detection limit
Odor threshold 2 ng/m^3
Tetrachloranisole 1.6 ng/m^3
Odor threshold 100 ng/m^3
Pentachloranisole 9.9 ng/m^3
Odor threshold 200,000 ng/m^3
So this is somewhat surprising. Because there is an odor, but it apparently has another source.

My suggestion here would be windproof hollow wall boxes and sealing of the hole of [der Deckenlampe]. That should further prevent pollutants and thus not significantly worsen. A further analysis in 5-10 years will certainly be necessary.
What would be your conclusion?

Alternative:
How critical would it be from the structural stability of the house to remove the internally placed chipboards, insert new insulation, and then use gypsum fiber boards as interior walls? The whole thing only on the walls; for the ceiling I would only do something with a vapor barrier foil.
Means wall structure old and beneath new.
Klinker-Hinterlüftung-Spanplatte-Dämmwolle-Spanplatte-Gipskarton
Klinker-Hinterlüftung-Spanplatte-Dämmwolle-Gipsfaserplatte

We also have a timber-frame house, built in 1976. The wall structure is similar to yours, only with wooden shingles on the outside instead of bricks. We have already removed the shingles and replaced them with wooden cladding. Inside, it doesn't smell, but we are still gradually removing all the plasterboards; there is ugly textured plaster on them, so wallpapering is impossible. Therefore, for simplicity, the boards are being taken down. Underneath is white-painted particle board. This remains in place, among other reasons because I don't want to deal with the strange and possibly hazardous insulation wool. Also, it's a huge mess. Since we could only add 6 cm of additional insulation outside anyway due to the lack of roof overhang, we are taking the opportunity to do internal insulation. This is absolutely unproblematic with timber framing, as the whole wall already consists of insulation. So the plasterboards come down, everything is smoothed perfectly with filler (especially around the windows, which were enlarged and very sloppily worked on by the window people), then a climate membrane goes on (only glued!), then battens, another 6 cm of insulation, OSB, Rigidur (no more Rigips, I hate that papier-mâché stuff). I find this variant much more pleasant and faster, cleaner, cheaper and more practical than tearing everything apart completely. The climate membrane should solve the odor problem, insofar as it comes from the walls. At least the membrane has a baby icon with the inscription "[zuverlässig aromadicht]". As I said, we have neither odor nor mold problems. Therefore, I notice no difference. Oh yes, about the mice: the protective grid at the ventilation gap is probably missing. That was the case with us as well! We had meter-sized wasp nests IN the facade. We completely sealed all that during the facade work. There are angle grids with different leg lengths, which were then installed at the top and bottom so that air now passes through but nothing larger than 3 mm can get in anymore. Check that and possibly close the gap. Otherwise, you can fight mice however you want, they always come back in.