PRESERVATION TECH NOTES     WINDOWS NUMBER 20

Figure 7. Installed replacement window. Photo: Bruner/Cott & Assoc., Inc.

Evaluation
Decisions involving the treatment of the original Sears building windows were reached after understanding the significance of the windows’ historic appearance and by evaluating their condition and the requirements imposed by the rehabilitated building’s new function. Various proposals were explored to determine how well they reconciled these factors. The process suggested that the most appropriate solution was to retain approximately 18% of the original windows while replacing the remainder with aluminum true divided light units that carefully matched the originals in both detail and general appearance (see figure 7).

The replacement window system used on floors three through eight had two important advantages over earlier design proposals. First, it did not rely upon applied muntin grids that read less as individual glass “panes.” Second, by developing new custom extrusions, the replacement window successfully matched the dimensions and sightlines of the original muntins.

The design of the Sears building windows shows the degree to which aluminum windows with insulating glass units can accurately replicate historic windows. Attention to seemingly small details such as drip caps and the interior appearance of the window proved crucial to the success of the replication effort (see figures 8 and 9). This understanding led to a new engineered window that met the challenge of combining narrow muntins with insulating glass units and true divided lights.

Although there were many advantages to the window scheme developed for the Sears building several drawbacks should also be acknowledged. One of the most significant disadvantages was the loss of historic material and integrity that accompanies any window replacement. In this case, the loss included steel frames and glazing that were removed as well as the functional nature of the once-operable projecting window.

While the replacement window frames are virtually indistinguishable from the original frames, the uniform, factory-produced nature of the units is in contrast to the look of historic steel windows that have aged over time. Also, the true divided lights, though superior in appearance to large insulating glass units with applied muntins, still have the reflective quality of modern insulating glass.

A final concern, relevant to all dual-glazed replacement windows, involved the integrity of the insulating glass unit seals. Although the dual-seal used in the Sears building replacement windows is currently state-of-the-art, the lifespan of insulating glass units in general has varied widely and is undoubtedly shorter than traditional monolithic glazing. The combined effects of the true divided light design and the narrow spacer bar suggest that the Sears building windows be periodically inspected. Small divided lights significantly increase the perimeter area that is sealed—and thus vulnerable to degradation—while the narrow spacer reduces the amount of sealant that can be accommodated along that perimeter. The glass fabricator is currently offering a ten-year limited warranty for the window system.

Figure 8. A close up view of the replacement window showing the replicated ventilator drip cap and narrow muntins. Photo: Bruner/Cott & Assoc., Inc.		Figure 9. The interior profiles of the original windows were accurately reproduced in the aluminum replacement units. Photo: Bruner/Cott & Assoc., Inc.

Conclusion and Project Data

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