Purchase decision small semi-detached house 100 sqm from 1930

I wouldn't have thought that 6cm could make such a huge difference.

The insulation effect is an e^-x function. So if you save, for example, half of the heating energy for the wall with the first 6cm (just as a calculation example), then for 12 cm it's only 3/4 and for 18 cm 7/8, for 24 cm 15/16. Here you would save half with the first 6 cm and roughly the other half with 18 cm (that is, three times as much).

That certainly applies to the same extent, for example, to the basement ceiling insulation? So that a small insulation thickness already has a relatively large effect? Our basement is unfortunately very low and not much will fit under the ceiling ...

The principle applies to every type of insulation. In limited space, it can also make sense to achieve a higher effect with higher-quality insulating materials at the same thickness. This is known from the winter jacket. A thin down jacket is just as warm as a thick jacket filled with the cheapest plastic scraps.

Or real virgin wool sweaters. Worn once and never again, because they are simply too warm. In German average winters, complete overkill

The thread has become quite long by now, so please forgive me for not remembering what windows you currently have (?). If they are really junk (or can they still be saved?), then just replace them and consider the insulation later, or you live with the lower blown-in insulation. It's better than nothing in any case. If you absolutely have to replace the windows, I would definitely have the insulation done. And if your basement is too low and you’re tearing everything up anyway, then consider insulating the ground floor floor. Our basement has a vault and all the pipes on the ceiling, so it’s not suitable for insulation either, which is why we will also insulate the ground floor floor.

Yes, the ground floor floor should be insulated. It will depend on the available room height. I measured 2.52 meters yesterday, we then have to see how much space the suspended ceiling for the ceiling heating takes up. As I said, new windows are indispensable, blown-in insulation seems very sensible to me. I believe that together with a new roof including external insulation we should do quite well for now.