- At a rough estimate such a building costs 600.000 € - 1.000.000 € nowadays.
- Costs for insulating the exterior walls, the roof and ground floor against the basement is approx. 30.000 - 60.000 €.
- New, insulated windows: approx. 10.000 – 30.000 €.
You'pre good then on energy saving.
And yes, they usually just put plates of foam on the outside. Somehow the mold issue appears to be solved by now as a house-painter told me recently.
I'm not persuaded that the issue of mold simply disappeared. It might be the type of foam they use nowadays, or the way they insulate the houses. For example, I've seen houses insulated from the front and not from the back. At first, I thought it was a cost cutting measure, i.e. "look good, be cheap," but it's actually so common that I don't think that's the answer.
Molds can become toxic in extreme cases, and once they appear, it's amazingly difficult to get rid of them. It's possible that in a town that's highly historic, there are companies that specialize in insulating historical buildings (though I assumed that the vast majority of historical buildings in Germany was destroyed during the wars).
As a side note, I'm not saying it cannot be done. My point was that it was far from "slap some foam around the house". I'd love my building to be insulated.
And if you are confident that the house will still stand in 60 years, then an investment in solar+insulation that takes 20 years to pay for itself is quite attractive (and something your local bank will probably give you a loan for).