In Europe apartments are pretty much air tight. Any source of air in there are window and door "leaks". If you don't have enough of them then pressure of the ve

In Europe apartments are pretty much air tight. Any source of air in there are window and door "leaks". If you don't have enough of them then pressure of the ventilation can be high enough to suck the air through the p-traps in your plumbing. So PVC windows have built-in "leaks" in form of slightly opening them so that seals on them are no longer sealed. Or even separate holes with filters and regulated (non-airtight) barriers that allow adjustable (but never down to zero) air intake. Because of that I don't consider any window that can't be made air-tight to be a failed window because otherwise I'd have to consider some of them to never be working by design because they have separate holes in them exactly to prevent air-tightness. All of this is necessary because in general PVC windows can be way more air-tight than wood windows and stay air-tight way longer. Which is harmful to homes with ventilation that doesn't have dedicated air intake.