Using the link you have provided it has a picture of a french full lite door with a vent that has removed a significant part of the door s base.
Gap under bathroom door for ventilation.
7 mistakes not to make in your bathroom remodel do let your.
And for proper bathroom ventilation your exhaust fan needs to vent outside not into the attic or another area of the house.
However the size of that gap is very unlikely to be greater than 1 2 for any door.
I like the suggestions for wall ceiling jump ducts etc.
Check your bathroom door.
A 10mm gap under the bathroom door is very usefull it allows the fan to work easier as it can draw in air and because its from within the house it stops the bathroom.
This is less than desirable in bathrooms and bedrooms.
My contractor just hung our fancy dancy un painted solid wood door slabs in my 100 year old house with solid hardwood floors throughout.
The flow dropped from an unrestricted 150 160 cfm down to under 100 cfm in each room.
My mom has a house with a central return and as you suggest her doors have about a 1 5 gap underneath to allow air to flow.
Client always resist 1 gap over the floor covering.
The jumper makes sense in some cases.
If your door has very little or no clearance at the bottom you.
If more gap is required it is easier to trim a door than to lengthen it.
There should be plenty of room under the bottom of the bathroom door to allow air to get into the room so the fan has a source from which to pull the air it needs.
If the bathroom vent can t get any air of course it isn t going to work the right way.
I have a problem with the door vents you are passively promoting.