We are actually considering replacing the windows. However, "only" for aesthetic/functional reasons. Due to the uninsulated walls, it probably hardly saves any energy. So it’s also with some misgivings, since they are still in very good condition and the overall ecological balance is not good.
Well, what year were the windows made? Probably 1980, when the extension was added, windows were often replaced then as well.
The U-value of these windows (glass) is about 3.0 W/m²K. New double glazing has 1.0 and triple glazing 0.6 W/m²K. Per m² window area, you save about 160 kWh/m² window per year by switching to double glazing, about 200 kWh/m² with triple glazing.
The ventilation losses of such old windows add to this, with new windows these are significantly lower. We have triple glazing in our old building, built in 1959, walls as existing. With a hygrometer in winter (<5°C outside) keep the indoor air always at 50% humidity and everything is fine. Works very well for us by ventilating in the morning and evening, that’s all it takes. Window rebate vents are just rubbish. If you want to be on the safe side, install decentralized fans with heat recovery in four locations and you will always have good air with few losses at reasonable additional costs.
The floor slab (no basement) is presumably(!) partially insulated. We will only know more precisely when we remove the floor. Whether it makes sense to insulate there and possibly even install underfloor heating on a large scale is certainly an open question we will gladly take with us.
The floor against the ground in this building age was usually done with 2 cm mineral wool (WLG 040) as impact sound insulation and decoupling for the floating screed. Usually, 5 cm cement screed with wire mesh reinforcement on top.
In bathrooms and entrance areas, the floating screed was often omitted and done completely without insulation. Due to the available build-up height and connections (doors and stairs), you cannot build higher there either and consequently cannot achieve more insulation with a classic screed. Tiles are not an option there, it becomes ice cold and parquet in the living area is not really comfortable.
If you want to renovate anyway: Remove the entire screed on the ground floor (!) and instead of screed lay 5 cm PUR floor insulation boards (WLG 028) or resol hard foam (WLG 023) or glue them with PUR insulation board adhesive. On top of that, lay 2-3 XPS boards with glass fiber cement coating (building boards, Wedi boards, Jacko boards) and tile directly or glue parquet on them.
You don’t need impact sound insulation on the ground floor without a basement.
A floor heating system can also be milled directly into the mentioned building boards. More details can be found on the Wedi manufacturer page. For that, you have to use at least 3 cm Wedi boards. You can also only use the XPS boards, but their WLG is worse than PUR and unevenness compensation of the floor slab is easier over two layers.
Exterior walls are plastered, not clinker. So no cavity or the like.
That would not have been usual anymore in 1960 anyway. If clinker, it was placed directly in front of the load-bearing wall since the late 1950s and only set with mortar joint directly in front of the wall.
For a 1960s settlement house, exterior wall insulation can even pay off financially and also improve comfort. These houses were very different regarding the quality of exterior walls and therefore also the U-value. Anything between 1.5 and 0.5 W/m²K was included. The wall thickness already gives some indication. How thick is the wall?