53 Pa. Stat. § 25020
No room (other than basement rooms, provision for which is made in section two of this amendment) in any tenement house hereafter to be erected or constructed, or altered into a tenement house, shall be leased or occupied for living purposes, unless it contains at least one window having a superficial area of not less than one-tenth of the floor area of the room, which window opens upon a street or alley, or upon a yard or court, having a sectional area of less than one hundred square feet; and no room shall be so leased or occupied in any now existing tenement house, unless it contains a window of the foregoing dimensions, opening upon a street or alley or upon a yard or court having a sectional area of less than twenty-five square feet; or unless such room adjoins another room in the same apartment, which other room has such a window, opening upon such a street, alley, yard or court, and between which two adjoining rooms there is a sash-window having at least fifteen square feet of glazed surface, the upper half of which is made to open easily.
53 P.S. § 25020