Bathroom ventilation in one form or another is not only desirable, it is also required by building codes. Although its main purposes are to provide a healthful exchange of air and remove odors, ventilation also helps in controlling excessive humidity. Thus, in a residential bathroom with no window that opens to the outside, you need a ducted electric fan to meet code requirements. The problem here is that a fan will only remove stale air and moisture when it is turned on, and a window will only do the job when it is opened. Both are at the mercy of the occupants. Of course, moisture removal from a bathroom can be accomplished simply by leaving the bathroom door