Smart vents
One thing that I never saw before in houses before moving in Holland are aeration vents in almost all rooms (see picture). One thing that worries me is that even when they are closed, during the winter, they can let cold pass, impacting the temperature inside the room. One thing that I was wondering is any one ever replaced them with some sort of sealing smart vents - something similar to the one in the toilets, but that can "properly" close and that open and make air circulate when needed (ideally when the level of CO2 or humidity are too high for example).
Curious to know solutions that other found!