It is nearly airtight IMO and its night and day difference with/without flap. It is easy to inspect how it moves when I remove filters from filter box. Im not excluding anything here tbh
It is insane and hard to believe, but its easy to guess what meal my neighbour is preparing while it looks like there is no flow at all. After long cooking it becomes annoying especially with onnion or fish...
The other appartment installed 2 flaps with no difference... In our building I think all 3rd floor kitchens are connected to 1st floor, also 2nd to 4th floor are connected. 10 appartments on each floor means up to 40 appartments with the same issue. They have installed flaps to anyone who complained and no one is happy with results afaik.
I replaced mine with (probably) better build quality one, but there might exist even better flaps. Do you know any source in Europe for the best quality flaps. We wouldnt mind spending much more money if that can help