One of the most important criteria to keep in mind when you are designing your bathroom is the ventilation. Moist air tends to create conducive environment for moss and fern. A house infested with moss and fern is not very good for the health of the inmates. That is why it is very important that you take proper steps to vent out the moist air. In fact there are certain indoor air quality standards that have been set up by the State Department that you should ensure that your bathroom is not flouting.
Of course you should ensure that the windows and doors provide for cross ventilation. But because of privacy requirement you would actually have to keep the windows closed when the bathroom is really in use. That is why an exhaust fan is very important. Just ensure that this moist air that is being vented out is not drained out into another room. You need to insulate the pipe that is responsible for venting the moist air out as well.