During a time when interest in the restoration of historic buildings is at an all-time high, strict preservation guidelines make these projects more challenging for building teams and owners. The main challenge is preserving historic integrity while at the same time modernizing the building for energy efficiency, client use and to meet fire- and life-safety codes.
"Any restoration project that has to meet historic preservation standards — such as the Secretary of the Interior's Standards for the Treatment of Historic Properties — can be difficult, because they're only concerned with keeping the original historic fabric of the building," says Karl W. Stumpf, head of Baltimore-based A/E RTKL Associates' Historic Preservation studio.
Windows are certainly at the center of this debate, as building teams struggle with whether to rehabilitate or replace the windows.
"Many of our clients often get frustrated because they figure by replicating the windows they'll achieve huge savings in energy and maintenance costs," says Stumpf. "But that argument alone does not work with the preservationists. The only way to sway them is to show a massive difference in the cost between replacement and rehabilitation, or prove that the windows are in such poor condition that they can't be saved."
Many historic restoration projects involve a mix of rehabilitation and replacement. For instance, Arlington, Mass.-based engineering firm Simpson Gumpertz & Heger Inc. (SGH) restored 600 and replaced 600 bronze-framed, leaded-glass windows on Yale University's 75-year-old Sterling Memorial building, the school's main research library. In order to preserve approximately $1 billion in books, artifacts, coins and paintings housed in the building, Yale installed a humidifying system to control the climate in part of the building, which required windows with better thermal properties. In addition, numerous window units were found to have water leakage.
"In the areas that were going to be humidified, we designed a replacement window system that could handle humidification without developing water condensation," says Niklas W. Vigener, project manager with SGH. "In the other portions, we rehabilitated the windows."
Where the windows were rehabilitated, each unit was dismantled and masonry repair work was completed to prevent water penetration. Original plans were to replace the lead came — lead rods that hold together glass panes — on each window unit and then reinstall the windows. But further assessment by SGH showed that the windows needed to be strengthened.
"When the leaded glass was first installed in the mid-1920s, it almost immediately started bowing and sagging," adds Vigener. "So we knew we couldn't just rebuild those windows exactly as we found them and expect them to perform, especially since they're operable. We had to somehow strengthen the glazing without changing the look."
SGH, with the help of Mount Vernon, N.Y.-based leaded-glass consultant Rohlf's Stained & Leaded Glass Studio Inc., inserted stainless-steel reinforcing rods within the lead came to strengthen the windows.
Vigener says that the labor involved with documenting the leaded glass and the logistics of keeping the original pieces organized made this rehabilitation project considerably more expensive compared with purchasing new glass. "However, the subtle hues and texture, such as waves and air bubbles, of many historic glass pieces is very difficult to match," he adds. "So a complete replacement of all glass will typically result in a less-than-perfect match of the existing leaded-glass panel."
On most projects, says Vigener, it's imperative that designers follow preservation guidelines, such as those of the Secretary of the Interior, to assess the significance of windows when considering restoration or replacement.
"It may be acceptable to replace a utilitarian piece of plate glass, but mandatory to preserve rather than replace a hand-painted leaded glass panel," he says. "The window frames, rather than the glazing, are often a significant character-defining feature of historic buildings. For example, the thin sight lines of steel frames — an important design feature of early 20th century institutional and industrial buildings — generally cannot be matched with wide, off-the-shelf extruded aluminum frames. Special aluminum frames, however, can be designed to replicate steel-frame sight lines on the exterior."
In the case of the Sterling building, however, historical research showed that some of the original bronze-framed windows could be replicated using aluminum-framed units, which provided a cost saving and better thermal performance. "We found that the windows in the book stacks were built in a utilitarian way, because they are in nonpublic areas," says Vigener. "So we took the liberty of reinterpreting them using modern materials: a bronze-colored aluminum, double-glazed window detailed to replicate the original. We used colored glass and applied lead to match the lead came in the original glass."
Existing windows are often in good aesthetic condition, but do not meet modern design standards for thermal performance, air-infiltration and water-penetration resistance.
One common solution for upgrading historic windows is to install an interior or exterior secondary glazing, or storm window, to create an insulating layer of air. Moreover, storm windows can be fitted with laminated glass to filter out ultraviolet light or to meet code requirements for fire and life safety.
RTKL specified interior, operable storm windows for the Interstate Commerce Commission building in Washington, D.C. Completed last year, the project also included the rehabilitation of 1,500 steel, double-hung windows.
"We looked at both restoration and replacement options on a life-cycle cost basis," says Stumpf. Because the existing windows were in good condition, it was more cost-effective to rehabilitate them and install storm windows.
"There was very little rust or corrosion, but many layers of paint," says Stumpf. "So we stripped the paint off and then primed and repainted them."
Operable storm windows were then installed to meet the client's requirement for keeping all windows fully operable. "If we didn't have that requirement, we would have most likely just caulked all of the windows shut, because testing proved that would produce the best insulating performance," Stumpf adds.
There are potential drawbacks to installing storm windows, says Vigener. For instance, heat buildup and condensation can occur between the storm glazing and primary glazing if the airspace is not properly ventilated. The incorporation of weep holes in the sill of the storm glazing will help prevent such a problem, he says.