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Double-Glazing Existing Steel Windows

Double-glazing historic steel windows can be an easy operation. With small residential steel casement units common in early twentieth-century high-rise apartments, application of a horizontally-sliding aluminum storm unit mounted on the inside reduces noise from the outside while improving energy performance. When finished in a dark color, this type of interior storm tends to have little visual impact from the outside and can also be unobtrusive from the inside. There is some sacrifice in the optimum performance of the casement for natural cooling and ventilation, since as much as 50 percent of the venting capacity of the window is blocked by the slider window. In an age of mechanical cooling, however, this may not be a concern.

On larger steel windows such as those found in medium-sized factories, the steel sections sometimes were designed to accommodate either single or dual glazing. Where these windows are in repairable condition and sized to accept either dual or single glazing, it may be possible to install sealed insulating glass units in place of the single glazing. An evaluation should be made beforehand, with particular attention to the operable portion of the window, to determine whether the steel windows can accept the additional weight of the insulating glass. While this retrofit approach to double glazing has been effectively used, most steel windows installed in the past were not designed to accept dual glazing; the glazing bars (muntins) are too shallow or too narrow in width.

Fisher, Chuck. Rehabilitating Windows in Historic Buildings: An Overview (GSA 08500-03), Historic Preservation Technical Procedures, U.S. General Services Administration

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