Evaluation of offer (5 windows and a balcony door including removal and installation)

But you don't know anything about the U-value of the exterior walls. In that respect, it can only be regarded as a slogan. ... everything has to fit together in a house! And if Joe had built in 1995, he certainly would not have had triple glazing as standard in his new build.

1.1 is as of 1995. And the common saying is that there are problems if the walls have a worse U-value. Mold, provided there are no construction defects, is exclusively related to heating and ventilation behavior.

Double glazing is not outdated, but simply a different approach (or conversely, triple glazing is not necessarily energetically superior). One does not have to live on the moon or be an extreme cheapskate to occasionally choose a well-made double-glazed window even today.

In 2014, we accidentally installed only double-glazed Veka windows. We had everything recalculated for the Kfw 70 energy certificate to present it at the acceptance by the KfW bank. It was close, but it was enough. If it hadn't worked out, we would have had a huge problem with a long construction delay and the window manufacturer would have suffered a huge loss. However, there is one disadvantage: in the summer, the glass gets very hot due to the sunlight. The heat gain is enormous. I think the ventilation system also plays a role in the fact that we don't have any mold. For the roof, we would need to choose the best and most expensive windows, but then you can sit underneath in the summer without getting sunstroke, even with the roller shutter open.

Update and topic closure: I received two more offers at the end and negotiated the first provider down to €5000 including VAT and am now going with the provider due to numerous recommendations.