(b) All required windows shall be so constructed that at least one-half of their required area may be opened, except that a mullioned casement window, if otherwise large enough to supply the window requirements of the room, need be readily openable to the outer air only to the extent of five and one-half square feet of its area.

(c) Where fresh air is furnished in any room through a mechanical ventilating unit or system which is an integral part of the dwelling structure and capable of introducing not less than forty cubic feet of air per minute, the required window area in such room need be openable only to the extent of twenty-five per cent of such window area but in no event less than five and one-half square feet. 9. Transoms or partition sash, or louvres having a minimum area of 144 square inches and arranged to be opened or closed, shall be provided to private halls or to adjoining rooms to secure through-ventilation whenever required by the department, but no such transom or partition sash or louvre shall be required in a room having two windows opening to the outer air if each window is at least nine square feet in area, or in a room having a mullioned or single window with an aggregate area of at least eighteen square feet. 10. All windows and their assemblies in walls situated on a lot line, except those facing on a street, shall be fireproof, with assemblies having a fire-resistive rating of at least three-quarters of an hour and glazed with wire glass at least one-quarter of an inch thick. Every opening in a wall situated on a lot line which is less than fifty feet in a vertical direction above a non-fireproof roof of another structure within a distance of thirty feet of the wall in which the opening is located shall be an automatic fireproof window.