of SOM, could move vertically along steel rails embedded in the mullions. Two window washers were hired to clean the facade every six days. Each of the building's 1,404 windows could be cleaned within ninety seconds; because the window panes were fixed shut, they could be cleaned in less than one-third of the time it took to clean a sash window. According to Curbed, the scaffold was used for a publicity stunt that "used Lever-brand Surf soap to scrub the windows clean". The fixed-position window panes required that the building be air-conditioned, so steel grilles are also installed on the facade for ventilation intake. The curtain wall cost $28,000 more compared to normal sash windows, while the double glazing cost $135,000 and the window-washing equipment cost $50,000. However, the air conditioning system resulted in $90,000 of upfront savings, $3,600 per year in energy savings, and $1,000 per year in savings from reduced air "leakage". The fixed window panes also saved $2,000 a year on window-washing costs compared to sash windows. Features Structural features The internal superstructure is made of a cellular-steel skeleton with floor plates made of reinforced concrete. Small sections of the floor slabs outside the restrooms, elevator lobbies, and service core are supported by concrete arches. Generally, the ceilings are about high, but the floor slabs are thick. The west end of the slab is cantilevered from the furthest column while the east end is cantilevered . Lever House's wind bracing system consists of transverse bents placed at intervals of , with one set of columns through the middle of the slab. The building's utilities run through the service core on the west side of the slab. Six elevators are provided in the service core: five serving the office stories, as well as a service elevator between the first and third floors. A seventh elevator shaft was provided in the building to serve the upper stories if an additional elevator cab was deemed necessary. The core was placed on the west end of the slab so that, if Lever Brothers had ever built a westward addition to the tower, the elevators could serve the addition. Interior According to the New York City Department of City Planning, Lever House contains of gross floor area. All of the space was intended for Lever Brothers; in exchange for a more prominent structure, the company had been willing to forgo additional space