Renovation of old buildings - insulation of exterior walls from the inside

Hello,

I live with my sister in a two-family house that is attached to another house. For this reason, I cannot realize insulation from the outside. So I have to address the problem from the inside. Since my sister and I want to renovate/modernize anyway, that should not be a problem. Now, to illustrate, a brief description of the situation:

Heating: electric storage heaters
Electricity: status 1990
Sanitary: status 1990
Windows: status 1995 and 2003
Ventilation: none
Insulation: partly paneling with tongue and groove boards, partly glass wool behind them

The heating system will be renewed and executed as a gas heating system with wall-mounted radiators. Electrical installations will be updated to the current standard (network, TV, intercom, etc.). Sanitary installations will be renewed. The windows will be partly replaced. I will install decentralized ventilation. I also want to redo the insulation, but I still have some problems with that. I have spent days reading on the internet, but somehow I cannot find a clear solution. Use diffusion-open or vapor barrier? Styrofoam or natural materials? Use silicate boards because of mold prevention? Every manufacturer only highlights advantages for their system but no real disadvantages.
At first, I thought of using drywall panels with attached Styrofoam boards. Then I read that the risk of mold formation behind the boards is enormous and that in case of fire they release a lot of harmful substances. Then I came across silicate boards (Multipor). These are not so good for insulation and cannot be heavily loaded, and at €25 per m² are not cheap. But they absorb moisture from the air, prevent mold growth, and are fire resistant. Now I am considering whether I should simply build a wooden stud wall in front of the brick wall, fill the spaces with rock wool/wood fiber boards, and then put drywall panels on top. Drywall should also be diffusion-open and able to absorb moisture from the air. How did you do it? Is there an insulator among you who could help me further?

Hello,

you have to understand each wall as a system. In the case of an exterior wall, ideally the dew point is within the masonry. This is caused by the material and the wall structure. If you insulate it with, for example, Styrodur or a stud frame and drywall, it will certainly start to mold. The reason is that the heat from your side is missing. The wall cools down and condensation moisture forms, which can cause significant building damage. So if you insulate from your side, then use silicate boards.
You must always remember that you are intervening in the building physics through a subsequent modification.