Close ventilation opening

Take a photo, long story short
Foam is for gaps, not for canyons. Styrofoam is, as mentioned, quite sturdy and does not absorb.

At the moment, I don’t have a picture of the breakthrough here. The one in the attachment (from the internet) shows the situation quite well though. The section on the construction site is not quite as long.

I don't have a picture of the breakthrough at the moment.

It’s not urgent.

The one in the attachment (from the internet) shows the situation quite well.

Here is also the internet and unfortunately the picture shows the wrong side. I was concerned about where your problem (from below) is: from above it disappears into the finished floor and is closed. What do you fear from below: that something crawls in, moisture for example?

What are you afraid of from below: that something might crawl in, moisture for example?

I just don't know. Insulation (on the breakthrough) would then be too unstable, or I also thought about moisture from below, but what could harm the insulation... That means we could first have the top insulated + the rest of the floor structure and then sometime (quite soon) fill it from below with styrofoam or foam? A board on it, plaster, and done.

My problem area can be seen quite clearly here. The hole goes vertically through the bathroom wall above and ends in the unheated attic...