The near-pressurized condition you describe might be true on some airplanes or in a submarine, but this is generally not the case in home construction. It is extremely rare to find doors in typical residential construction that close so tightly air will not pass through at least the door and jamb at the butt, head and hardware sides (not to mention below, even with carpet). Additionally, if heat register exists in a room, it can provide ample air supply for a vent fan (whether the heater is operating or not).
We can understand where the condition you suggest is a possibility, but it is our considered opinion that the possibility is somewhat remote.
The original question was:
Help! the paint on the walls and ceiling above my shower is cracking and peeling. The bathroom has a vent fan. Can you suggest proper ventilation of the wallboard, the type of undercoat and the type of paint (oil, water, enamel)? - Richard
We suggested a procedure to repair the walls and cracked paint, and we cautioned Richard to ensure that the vent fan was operating or that the window was open during (and after) steaming up the room.
Richard complained of existing damage to his walls and requested guidance on a method of repair. He indicated that he had a vent fan, but did not say that he was using it. So we cautioned him to do so. The flaking and cracking paint could indicate that Richard either was not using the fan (very common), or the deteriorating wall finish was originally done improperly, or both! In any case, the walls needed repair, and the procedure to repaint (fan problem or not) seemed necessary.
