Empty conduit from the supply room to the attic 50mm - sufficient

Construction foam is not airtight - if you use HT pipes, you can simply seal them.

Expanding foam is not airtight - if you use HT pipes, you can simply seal them.

At our place, the empty conduits are installed as corrugated pipes. How can I seal them airtight? Another alternative might be duct tape, but that probably isn't a solution that lasts long either.
Maybe spray with acrylic/silicone? But with the 40mm pipe, that probably won't work so well either, right?

Yes, I would use the tape - plenty of it, but no expanding foam, it's a mess anyway...