Construction costs are currently skyrocketing

As if they heard me whining yesterday, the plastering crew came today and after a week’s break, work continues. I hope they finish soon and don’t really wait 2 weeks between the two coats :D We need to talk to the BU about the window, but I also think it can’t stay like this forever. I’ll take another look at it in October, but from November onwards I’d like it closed, because I’m heating out the window. But I can’t just remove it myself. The plug is too thick for the tilted window, and on the other side (inside) there is no plug, an electrician has to disconnect it.

Why no three-phase socket outside?

The drying time for exterior plaster is 1 day per millimeter of plaster thickness. So if 20 mm of plaster is applied, it should be allowed to dry for 20 days. In damp and cold weather, it can sometimes take a little longer.

I would definitely not have my finishing coat applied after just one week but wait at least 20 days.

You can also tape the tilted window with foil sometimes, so that all the heat from the house doesn’t escape.
I can certainly understand all the frustration with craftsmen at times. If you have customers like that, you don’t need enemies.

We have no influence on how long they wait or where they get their high voltage from, that is all managed by the BU.

Which film can I use to tape the window that keeps the heat inside? It would actually be interesting as a workaround until they are finished.

Drying times should definitely be observed. Messing around for a few days now can backfire in the end. If it really can't be helped with the cable, why don't you get some mineral wool or styrofoam and stuff it into the open window? That costs a few euros and half an hour of work.

The problem isn't even that you're heating out the window. Your basement wall or the entire room cools down, the moisture present in the house condenses, and it molds.

That's why you shouldn't overdo it when ventilating in winter. Better 5 times a day for 10 minutes than once for an hour. The room must not cool down too much while airing, otherwise you (in places, preferably on the outer wall) fall below the dew point...

Which film can I use to tape off the window that keeps the heat inside? It would actually be interesting as a workaround until they are finished

Heat transfer is not the problem with regard to the small area. Air exchange should be prevented. A piece of roofing underlay / vapor barrier with fabric adhesive tape should work well. Wrap it accordingly at the penetration. Be prepared that after removal the adhesive residues will need to be cleaned, so maybe don’t use the good adhesive tape that is normally used to secure the vapor barrier – that sticks too well.

I would stuff a foam mat in there.