Insulation of a 1970s workshop with steel trusses.

Addendum:
I had forgotten that a conversion from workshop to apartment is to take place.
Then the requirements for residential buildings apply accordingly, instead of the limits for NWG - but in principle, this does not change anything.
The crucial point is whether the workshop was heated before or not.

What matters is whether the workshop was heated beforehand or not.

First of all, thanks for the good tips.

Yes, it is a metal workshop heated with an oil stove, with an office room.
It is even insulated, unfortunately with this unhealthy slag wool, which I will tear out.

In the meantime, I have come to terms with a "proper" complete renovation.
I will put 16 cm of wood fiber on the roof and 12-16 cm on the wall, each ventilated behind, with a sheet metal roof and a wooden facade on top.
That should get me to a U-value ≤0.24 for the existing building.

I have contacted a structural engineer to prove (or not) the load-bearing capacity of the roof trusses (*shiver*).

I have drawn below the detail of the roof construction for the existing building and renovation:

Best regards,
Wolfgang

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The expert will also tell you what you ARE ALLOWED to do before you decide what you want.

How is the expert supposed to know what he has to talk me out of if I haven't come up with it beforehand?

How is the expert supposed to know what to talk me out of if I haven't thought it through beforehand?

He, or someone else, will then tell you that converting commercial premises into residential spaces generally requires approval, and accordingly, U-values must be complied with during any renovation. So it's best to first ask the building authority what they need, and then someone has to provide the thermal insulation certificate.