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How to install a window if there was an error and the window is smaller than it should be?

The windows were delivered yesterday. The window installer noticed that one window was 10 cm narrower than needed. He said it was his fault, he could order a new one (it would probably take 14 days+ again) or install this one. On the left edge, he wants to put Styrodur in between, which insulates very well.

Basically, I don't want to wait that long since all the upcoming work (electricity, gas, plumbing) will not be done until after ALL the windows and doors are installed.

Is it safe to use Styrodur, or can it cause cold bridges and mold? After all, it is also used to insulate all sorts of concrete parts.

The masonry is 36.5 Poroton bricks. And this is a new building, the roof truss is just finished.

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