Window offers like day and night

Invest 100 euros for a maximum refund of 50 euros and in the end it’s all for nothing

I wouldn’t do it

Also don’t want to say anything, old industrial building, what about the [Denkmalschutz]?

The windows were just measured again - especially the outer frame. It actually sticks out 2cm (not just 1cm). So the plan is: 4cm insulation on the left and right, making use of the extra 2cm, and then 2cm less width on the left and right. This would result in a total loss of glass area of 4cm if the frame is completely plastered over.

altes Industriegebäude wie sieht es da mit Denkmalschutz aus?

There is no heritage protection on it.

4 cm loss is not much. At most, it is more like loopholes

Then it is good, which is one way to avoid the insulation around it.

Where can you find such a person?

You can search for consultants in your area on the Energie-Effizienz-Experten site. Then check them out. Becoming an "energy consultant" apparently isn't particularly difficult and for many (I particularly feel like chimney sweeps) it simply represents an additional source of income, because builders are pressured by the KFW to involve a consultant. Our first one asked to be released from the contract when I brought up the airtightness concept...

The windows are "bricked up" with clinker on the outside. There is only a 1cm window frame there.

You mean the windows have a so-called "stop," so the window opening is larger inside than outside? In that case, I wouldn’t do anything about it either.
Insulating the window reveal is generally a sensible measure, but it actually has nothing to do with the Energy Saving Ordinance (there is no regulation that says you have to insulate it); it’s really just about the thermal bridge "window reveal," and you can indeed mitigate that quite well from the inside with a calcium silicate board.
The purpose of the measure is to raise the surface temperature of the reveal so that condensation and consequently mold do not occur there.

Yes, we are also just wondering why and whether the window reveal needs to be insulated at all? That's nonsense, right? At least as far as the energy saving ordinance is concerned. For that, only the wall is important. Also, I wonder how directly I can apply the 40% buffer for old buildings? So if a new building has to comply with a U-value of 0.24, would an old building actually only need to comply with 0.24 x 1.40 = 0.336? Then, with our 60 cm brick wall at 0.366, we would almost be within the standard and only need to worry about the other walls.

You have 2 options: Either comply with the values according to the Energy Saving Ordinance for each component handled, or comply overall with the values of the reference building + 40%.

The latter becomes difficult if, for example, you only want to renew the roof, because then you would very likely also have to address heating, walls, etc. to achieve the necessary values relating to the entire house.
Since you are planning a comprehensive renovation anyway, the 140% rule works in your favor because you can compensate for the (too) poor exterior wall with better windows, more insulation thickness in the roof, or indeed a lower thermal bridge surcharge.
This works like in a KFW efficiency house.