The window company is not delivering the agreed roller shutter boxes

And they certainly won't let themselves be caught cheating.

Installation and sealing according to the guidelines of the Institute for Window Technology Rosenheim or sealing with 3-level sealing tape. But who had already dealt with something like that at that time?

You said it. Therefore, many thanks for your answer. Will be saved in the construction folder...

well. The BlowerDoor test is coming up for us soon (January?).

The window installer will be there together with our architect/energy consultant. BlowerDoor should ALWAYS be done before the drywall on exterior walls as well as the plaster. Otherwise, it only shows the current status which can no longer be changed. That’s pointless and should be obvious - right?
Our architect also recommended the window installer to us. At the latest during the BD test, it becomes clear whether it’s airtight or not. But it’s not just about airtightness. In addition to (very narrow) compribands, foam was also applied inside and outside and then taped immediately afterwards. As a result, the foam bulges are sometimes so thick that the reveal cannot be worked on properly. In addition, the taping, to put it politely, is terribly wrinkled and uneven. I suspect: Not airtight.

Apparently, you can botch it in a thousand different ways :mad:

The Blower Door Test at the right time saves a lot of trouble.

This foam nonsense should be banned. Otherwise, there are regulations and DIN standards for every damn thing, but anyone can seal windows however they please. This is what sealing number 3 looked like for us.

How can one ensure nowadays that the windows are still installed properly?

Exactly the inability to find a convincing satisfactory answer to this was a very significant reason for me to temporarily withdraw from the window industry. Basically, as a builder, you have to reverse the whole thing, i.e., first find a good installer and have them deliver the windows as well. But customers also have to do their part so that this sick market can get back on track – because customers let their price expectations be spoiled too much by the inclusive botched-installation cheap providers. Therefore, every good installer knows horror stories but understandably does not want to come to the experience of "biting the hand that feeds the savior" for warranty reasons. Summarized, what a customer can do: 1. Be aware (also in terms of price) that installation is an essential part of the "overall product quality"; 2. Understand that the installer does not want to work for people who do not grant him the trade margin; 3. For safety, read up a bit on what should be considered during installation (just like for the ETICS, there are various expert videos about examples of horror); 4. (also in other areas, e.g. kitchen installation no less) avoid the type of tradesman who comes like an action hero with a whole ammo belt full of silicone and foam guns – because he usually uses them "one gun fits all" for everything that is not up in a tree "past three"; 5. Plan vacation days to show presence at the construction site – see point 3 for this, because playing the nitpicking inspector who embarrassingly quotes the wrong standard does nothing but destroy his own authority; 6. Only when you can positively check off points 1 to 5 (because otherwise it’s little use) ! also contribute a bit to the working atmosphere on site with sausage roll catering & co.