§ 173. Windows in rooms. 1. Except as provided in subdivision four of section one hundred seventy-seven every living room shall have one or more windows opening directly upon a street or upon a yard or court with dimensions conforming to those specified in section one hundred seventy-two, or upon an inner court or shaft three feet nine inches or more in width and eight feet or more in length.

2. Such windows shall be so located as to light properly all portions of the room and in each room shall have an aggregate glazed area of at least one-tenth of the floor area of the room, unless in the opinion of the department such room is already adequately lighted and ventilated. The top of at least one window in each living room shall be seven feet or more above the floor except when such room is in the basement or on the top story, in which case the top of at least one window shall be six feet or more above the floor. Every such window shall be twelve square feet or more in area, and shall be so constructed that one-half or more of its area may be opened. However, such window need not be twelve square feet in area provided that each room shall have an aggregate glazed window area of at least one-eighth of the superficial floor area of the room.

3. Whenever a basement conforming to the provisions of subdivision four of section one hundred seventy-seven is permitted to be occupied for living purposes, every living room, bathroom, water-closet compartment, kitchen and cooking space therein shall have one or more windows opening directly upon a street; or upon a yard which is at every point at least fifteen feet in depth and, above the window sill level of the first story above the basement, at least twenty feet; or upon a court at least two feet in width and extending the entire depth of the lot from the street to the yard. All such yards and courts shall be measured at the ground level from the lot line to the building line opposite on the same lot. The windows in each such room shall have an aggregate glazed area of at least one-eighth of the total floor area of the room, and in no event less than twelve square feet. The top edge of the glazed area of every window shall be nine inches or more below the finished ceiling and one foot or more above the level of the adjoining ground. Every such window shall be so constructed that one-half or more of its area may be opened, and shall be so located as to light and ventilate adequately all portions of the room.

4. Any room on a top story may be lighted and ventilated by a skylight of the dimensions specified for windows and arranged to provide ventilating openings of six square feet or more. Such a skylight shall be accepted in lieu of a window. Nothing in this subdivision shall be construed to prohibit the installation of a skylight without ventilating openings of six square feet or more, provided, however, that such skylight shall be equipped with at least one hundred forty-four square inches of ventilation and provided that the glazed area of such skylight together with the glazed area of any existing window shall be not less than one-eighth the superficial floor area of such room.

5. When required by the department transoms, or louvres, or partition sash to private halls or to adjoining rooms in the same apartment shall be provided to secure thorough ventilation, but no such transom or louvres or partition sash shall be required in rooms having two windows opening to the outer air if each window contains at least twelve square feet of area nor in rooms having a mullioned window with an aggregate area of at least twenty-four square feet.