Product review (3 stars): good on flat surfaces or slightly tilted back for full closure
hello,
these two deflecto louvered vents fit perfectly onto an exterior vent opening.  they easily open the flaps when any pressure is applied from the inside.  when the pressure is finished, if mounted correctly (level and even) then they will flap down.  if it is off center or not installed flat, the louver flaps will stay open.  if there is a screen inside of some sort, then the opening flap not shutting close, is not a problem.  if there is no screen inside, then the open flaps may remain open inviting all small creatures to a covered hideaway and into the dryer tube.  make sure the flap get closed all the way.

but, they must have foreseen the future as these are made of plastic and will weather like plastic.  that makes them brittle over time and weathering, so be forewarned.