Preserving and restoring old windows
Preservation experts from the National Park Service recommend the following steps in restoring any historic structure. First, identify the historically significant features. Second, protect the features that are historically significant. Third, repair damage to historically important features. Fourth, replace what cannot be repaired. Fifth, recreate missing features that were historically significant. And lastly, avoid new additions that alter the historical appearance inside, or outside. These steps are based on guidelines developed by the Department of the Interior.
Windows: An introduction
Technology and prevailing architectural styles have shaped the history of windows in the United States starting in the 17th century with wooden casement windows with tiny glass panes seated in lead cames.
From the transitional single-hung sash in the early 1700s to the true double-hung sash later in the same century, these early wooden windows were characterized by the small panes, wide muntins, and the way in which decorative trim was used on both the exterior and interior of the window.
As the sash thickness increased by the turn of the century, muntins took on a thinner appearance as they narrowed in width but increased in thickness according to the size of the window and design practices. Regional traditions continued to have an impact on the prevailing window design, such as with the long-term use of "French windows" in areas of the deep South.
Changes in technology led to the possibility of larger glass panes so that by the mid-19th century, two-over-two lights were common; the manufacturing of plate glass in the United States allowed for dramatic use of large sheets of glass in commercial and office buildings by the late 19th century.
With mass-produced windows, mail order distribution, and changing architectural styles, it was possible to obtain a wide range of window designs and light patterns in sash.
Popular versions of Arts and Crafts houses constructed in the early 20th century frequently used smaller lights in the upper sash, set in groups or pairs; and saw the re-emergence of casement windows.
In the early 20th century, the desire for fireproof building construction in dense urban areas contributed to the growth of a thriving steel window industry along with a market for hollow metal and metal clad wooden windows
As one of the few parts of a building serving as both an interior and exterior feature, windows are nearly always an important part of the historic character of a building. In most buildings, windows also comprise a considerable amount of the historic fabric of the wall plane and thus deserve special consideration in a rehabilitation project.
Step 1: Identify, retain, preserve
Identify, retain, and preserve windows--and their functional and decorative features--that are important in defining the overall historic character of the building.
Such features can include frames, sash, muntins, glazing, sills, heads, hoodmolds, panelled or decorated jambs and moldings, and interior and exterior shutters and blinds.
Conduct an indepth survey of the conditions of existing windows early in rehabilitation planning so that repair and upgrading methods and possible replacement options can be fully explored.
Not recommended:
- Removing or radically changing windows which are important in defining the historic character of the building so that, as a result, the character is diminished.
- Changing the number, location, size or glazing pattern of windows, through cutting new openings, blocking-in windows, and installing replacement sash that do not fit the historic window opening.
- Changing the historic appearance of windows through the use of inappropriate designs, materials, finishes, or colors which noticeably change the sash, depth of reveal, and muntin configuration; the reflectivity and color of the glazing; or the appearance of the frame.
- Obscuring historic window trim with metal or other material.
- Stripping windows of historic material such as wood, cast iron, and bronze.
- Replacing windows solely because of peeling paint, broken glass, stuck sash, and high air infiltration. These conditions, in themselves, are no indication that windows are beyond repair.
Step 2: Protect and maintain historically significant features
Protect and maintain the wood and architectural metal which comprise the window frame, sash, muntins, and surrounds through appropriate surface treatments such as cleaning, rust removal, limited paint removal, and re-application of protective coating systems.
Making windows weathertight by re-caulking and replacing or installing weatherstripping. These actions also improve thermal efficiency.
Evaluating the overall condition of materials to determine whether more than protection and maintenance are required, i.e. if repairs to windows and window features will be required.
Not recommended:
Failing to provide adequate protection of materials on a cyclical basis so that deterioration of the window results.
Retrofitting or replacing windows rather than maintaining the sash, frame, and glazing.
Failing to undertake adequate measures to assure the protection of historic windows.
Step 3: Repair damaged historical features
Repair window frames and sash by patching, splicing, consolidating or otherwise reinforcing.
At right, a worker is preparing steel windows for repairs and refinishing.
Such repair may also include replacement in kind--or with compatible substitute material--of those parts that are either extensively deteriorated or are missing when there are surviving prototypes such as architraves, hoodmolds, sash, sills, and interior or exterior shutters and blinds.
Not recommended:
- Replacing an entire window when repair of materials and limited replacement of deteriorated or missing parts are appropriate.
- Failing to reuse serviceable window hardware such as brass sash lifts and sash locks.
- Using substitute material for the replacement part that does not convey the visual appearance of the surviving parts of the window or that is physically or chemically incompatible.
Step 4: Replace what cannot be repaired
Replacing in kind an entire window that is too deteriorated to repair using the same sash and pane configuration and other design details.
If using the same kind of material is not technically or economically feasible when replacing windows deteriorated beyond repair, then a compatible substitute material may be considered.
The photo shows a deteriorated lower window sash shown prior to its replacement in kind.
For example, on certain types of large buildings, particularly high-rises, aluminum windows may be a suitable replacement for historic wooden sash provided wooden replacement are not practical and the design detail of the historic windows can be matched. The lower window sash in the picture on the left has been replaced, based on physical documentation.
Also recommended are historic color duplication, custom contour panning, incorporation of either an integral muntin or 5/8" deep trapezoidal exterior muntin grids, where applicable, retention of the same glass to frame ratio, matching of the historic reveal, and duplication of the frame width, depth, and such existing decorative details as arched tops should all be components in aluminum replacements for use on historic buildings.
Not recommended:
- Removing a character-defining window that is unrepairable and blocking it in; or replacing it with a new window that does not convey the same visual appearance.
Step 5: Recreate missing features
Design for missing historic features represents the particularly complex technical or design aspects of rehabilitation projects. It should only be considered after the preservation concerns listed above have been addressed.
Design and install new windows when the historic windows (frames, sash and glazing) are completely missing. The replacement windows may be an accurate restoration using historical, pictorial, and physical documentation; or be a new design that is compatible with the window openings and the historic character of the building.
Not recommended:
- Creating a false historical appearance because the replaced window is based on insufficient historical, pictorial, and physical documentation.
- Introducing a new design that is incompatible with the historic character of the building.
Optional Step 6: Alternations/Additions for new use
The following work is highlighted to indicate that it represents the particularly complex technical or design aspects of rehabilitation projects and should only be considered after the preservation concerns listed above have been addressed.
Permissible:
Designing and installing additional windows on rear or other-non character-defining elevations if required by the new use. New window openings may also be cut into exposed party walls. Such design should be compatible with the overall design of the building, but not duplicate the fenestration pattern and detailing of a character-defining elevation.
Providing a setback in the design of dropped ceilings when they are required for the new use to allow for the full height of the window openings.
Not recommended:
- Installing new windows, including frames, sash, and muntin configuration that are incompatible with the building's historic appearance or obscure, damage, or destroy character-defining features.
- At right, this building's historic character suffers from an incompatible replacement window.
- Inserting new floors or furred-down ceilings which cut across the glazed areas of windows so that the exterior form and appearance of the windows are changed.
To see a list of all of the stories in this series, click here. And for many other stories on windows, follow this link.
The Old House Web
