Guestrooms and habitable rooms within a dwelling unit shall be provided with natural ventilation by means of openable exterior openings with an area of not less than 4% of the floor area of such rooms with a minimum of four square feet. Exterior openings for natural ventilation shall be provided with tight fitting insect screens. Bathrooms, water closet compartments and similar rooms shall be provided with natural ventilation by means of openable exterior openings with an area not less than 4% of the floor area of such rooms with a minimum of 1-1/2 square feet. In lieu of required exterior openings for natural ventilation in bathrooms containing a bathtub, shower or combination thereof; and similar rooms, a working mechanical ventilation system connected directly to the outside.