Window delivered too small

Good morning,

the windows were delivered yesterday. The window installer noticed that one window is 10 cm too narrow. He said it was his mistake; he can order a new one (probably takes another 14 days+) or install this one. On the left edge, he wants to put Styrodur in between, which insulates very well.

In principle, I do not want to wait that long again because all upcoming trades (electricity, gas, plumbing) will only start once ALL windows and doors are installed.

Is it safe to use Styrodur for this, or could cold bridges and thus mold develop? After all, it is also used to insulate all kinds of concrete parts.

The masonry is 36.5 Poroton stone. And it is a new building; the roof truss has just been completed.

Thank you

I have now found the following after a longer Google search:

it is described: "...using Styrodur on the inside is tricky if it is not fully glued over the entire surface and without cavities. It is virtually vapor-tight and not capillary-active. Moisture condensed between the wall and Styrodur can lead to mold growth.

Better would be a thin calcium silicate board with a suitable system adhesive. Apart from that, reveal insulation is recommended."

Accordingly, the idea with Styrodur would be dangerous in my opinion, as moisture accumulates between Styrodur and the masonry, which can lead to mold.

So the question arises whether one should not build up the 10cm with Poroton bricks. Then you would have "one" wall and (from my point of view) moisture could not form since there would no longer be a warm/cold gradient?

And how about ordering the window to size and meanwhile installing the wrong window temporarily?

We received new windows in 1991 or so, and one was too small. A board was used as a temporary solution until the delivery and installation of the [passendes Fensters].

Two windows were damaged and therefore not installed. This took several weeks. For that long, OSB boards were simply installed there. Then the other trades are still protected.

And looking at the entire living period, two weeks is really nothing. Do you really want to make compromises because of that?

There are aerated concrete panels exactly 5 cm thick, each side one edge and done.