Wis. Admin. Code Department of Safety and Professional Services SPS 379.10
No room shall be used as a sleeping or living room unless it has a window or windows opening upon a street, alley, or yard, or upon a court connecting therewith, or above the roof of an adjoining building, or upon an inner court not less than 25 square feet in area; or unless it has a vertically-sliding pulley-hung sash window, not less than 15 square feet in area, opening into another room in the same apartment, which other room opens on a street, alley, or yard, either directly or through similar windows. Every such sash window shall be placed as nearly as possible in line with outside windows, and both halves shall be made to open readily, and the glass in the lower half shall be translucent.
Wis. Admin. Code Department of Safety and Professional Services SPS 379.10