PRESERVATION TECH NOTES     WINDOWS NUMBER 20

	Figure 5. The replacement aluminum muntin accurately replicated original profiles while accommodating insulating glass and thermal stops. Drawing: Bruner/ Cott and Custom Window.

Fabrication
A significant challenge to developing the Sears building replacement window was matching the original narrow muntins. Common industry practice for true divided light aluminum windows was to utilize wider muntins that conceal the spacer bar in the insulating glass unit and protect the edge sealant from light degradation. This approach would significantly encroach into the sightlines of the original windows as a result of the wider muntin and proportion changes to the window. In the case of the Sears building, however, the window manufacturer utilized a narrow spacer bar that permitted an accurate replication of the original 7/8-inch muntin (see figure 5). Substituting a dark bronze anodize finish for the conventional aluminum finish further reduced the visibility of the spacer making it appear as a shadow line when viewed from an angle.

Figure 6. Sections of original and replacement windows. Drawings: Bruner/Cott and Custom Window. Click here to see a larger version of this drawing.

Each replacement window developed for the Sears building was fabricated from over forty new aluminum extrusions. The large variety of custom designed elements allowed for a faithful reproduction of the original profiles. At the request of the developer, the replacement windows were not operable, yet their appearance suggested the functional nature of the original projecting ventilators. Tabs were added to resemble the weathering flange closed flush against the fixed outer frame, while drip caps shielded simulated hinges. The muntin, rail, head, sill and jamb profiles were also accurately reproduced. In order to replicate shadows cast on the original frames, the manufacturer included cosmetic bolt heads on the mullions running between each of the paired and tripartite window arrangements.

The Inside Look
The inside appearance of the windows was important to the developer in marketing the new office space. A similar effort was made, therefore, to duplicate historic interior details and profiles. As was typical of multi-story warehouses, the Sears building’s historic steel windows were glazed on the inside so that the individual panes could easily be replaced when broken.

To simulate the original interior putty profiles, the fixed-light muntins were tapered to the same 1/8? thickness of the original sections while stepped muntins were used in the central vent where steel beads had originally secured the glazing. Jambs in the replacement units were also beveled to recall the shape of the old glazing putty (see figure 6).

Testing and Installation
Being a new window system, the manufacturer had independent firms conduct standard performance tests on both the insulating glass and a mock-up of the complete window. In accordance with ASTM guidelines, tests on the complete window evaluated air infiltration, water resistance, and deflection and structural deformation under uniform load. With the test units meeting required specifications, window fabrication proceeded. Lengths of the new spacer were shipped to a glass fabricator where the insulating glass units were produced with a standard butyl and silicone dual-seal. Completed glass units were then shipped to the manufacturer for final assembly of each window.

While the new windows were being manufactured, a local contractor began the three month long process of stripping, repairing, repainting and reglazing the historic second floor windows and other retained units in stairwells and other non-office locations. As the new windows began to arrive in Boston, the general contractor removed the original units that were slated for replacement and a team of eight to ten workers started installing the new windows.

Evaluation

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