There shouldnt be such a thing...
Permenant ventilation is a necessary building feature and regulation and should not be closed off completely....
The one with the moveable grill are designed to reduce draughs but not cut out the ventilation completely... to do so is highly dangerous... permenant ventilation is necessary to remove stale air and combustion gases, and to allow ingress of fresh oxygenated air....
if you want to be truely energy efficient the do not incorporate passive vents at all.... you should look to install a Mechanical Heat Exchange Ventilation system.... this system mechanically draws warm stale air out of rooms and replaces this with freash air thats warmed by the stale air going out.......