Today’s standards require that a bathroom have a source of ventilation, usually this is a fan but an operable window also qualifies. I always recommend installing a fan in any case. In winter a window will rarely be opened and the added moisture in the room will be absorbed by the home. Before long there will be black spots on the ceiling and condensation on the inside of the windows of your home. Adding a vent is the best way to control moisture and odors for that matter. I will write more about proper venting practices in another issue but I will say that you should never vent into an attic.