My old copy of the 1989 CABO One & Two Family Dwelling Code (birth-mother to our current IRC) states under # 203.2: Bathrooms, water closet compartments and other similar rooms shall be provided with aggregate glazing area in windows of not less than 3 square feet, one-half of which must be operable.
Exception: The glazed areas shall not be required where artificial light and an approved mechanical ventilation system capable of producing a change of air every 12 minutes are provided. Bathroom fans shall be vented directly to the outside.

__________________
Jerry McCarthy
Construction Consultant
www.expertbuildingconsultants.com