[brnt]

That's clever, but this [1] is already common and I guess not much worse?

[1] https://www.ventilatieshop.com/terugslagklep-in-ventilatieka...

[nice__two]

It is much worse. The Berbel retains the insulation provided by the wall, when closed. This is nothing more than a fancy hole in the wall.

Opening and closing, means you get the best of both worlds.