Protecting our historic glazing

Images courtesy Winco Windows

by Kurtis Suellentrop, EIT
According to the National Park Service (NPS), “when historic windows exist, they should be repaired when possible. When they are too deteriorated to repair, selection of the replacement windows must be guided by Standard 6 [of the Secretary of the Interior’s Standards for Rehabilitation].”

Repairing historic windows is an admirable goal, but the reality of today’s tough requirements for windows often makes their replacement a more logical and economic choice. Whether the project calls for repair or replacement, there are new technologies and products that can help designers replicate historically accurate sightlines and aesthetics. This can be particularly difficult to achieve on existing structures while maintaining historic accuracy, but several manufacturers now offer good historic replication solutions.

Today’s environmental and manufactured threats to buildings range from seismic forces, high winds, and temperature extremes to security breaches, air infiltration, and acoustic issues.

Originally, windows were designed for weather resistance, ventilation, and daylighting, but current building and construction environments require windows to do much more. Historic structures were not designed to meet current International Building Codes (IBC) and wind load requirements.

Therefore, it can be exceptionally difficult to integrate protection from these hazards in new construction, but can be even more difficult to achieve on existing structures while maintaining historically accurate sightlines and aesthetics. There are many factors contributing to a building’s significance and historical value, but according to NPS, they are generally at least a half-century old and contribute to:

…the historic significance of a district …by location, design, setting, materials, workmanship, feeling and association adds to the district’s sense of time and place and historical development.

This article suggests several aspects to consider when designing a historic window repair or replacement project. Many of these concepts can also be used in new construction when the building is designed to replicate the feel of a historic building.

Understanding the terms
Considering the costs and benefits of the repair/replace decision requires a firm understanding of terms and definitions. It is also beneficial to understand historic construction techniques and methodology.

Depending on the region of the country, this component can also be called mutin, muntin, or bar. They were originally used to receive the monolithic glass in wood and steel windows (Figure 1). The styles are also replicated in more modern aluminum assemblies, with simulated divided lites (Figure 2).

Shadow line
The shadow line refers to the depth of the trim, window, or glass from the referenced vertical or horizontal surface. For example, in Figure 3, there is a shadow cast at the horizontal meeting rail and the head trim in this hung wood window from a 1950s Atlanta courthouse.

Originally, the limitations of few pigments, lead, and linseed oil paints were used for wood or metal windows. Various light stone colors and dark-purple, brown, chocolate, oak, drab, and green were common finishes used. Over the decades, improper maintenance through the layering of heavy paints can damage and alter the appearance, as well as hinder operation. However, it is possible to replicate these finishes using modern, more durable products. Current metal finishes include epoxy, liquid-applied polyvinylidene fluoride (PVDF) resin, polyester triglycidylisocyanurate (TGIC) resin powder coat, and anodized (Figure 4, page 26). Modern powder coat finishes are also able to achieve a wood grain pattern appearance on an aluminum substrate. Restored materials must typically be mechanically or chemically stripped to the raw surface, and then repaired, primed, and painted.

The vertical separations between windows are typically referred to as mullions. On most original wood double-hung windows, they were very wide to accommodate multiple counter-balance weights in the window cavity. Many historic elements in traditional cast iron or wood mullions can be preserved and reused or replaced by more modern materials. They are critical to the overall aesthetic of the building envelope.

Typically, historic windows do not have to match the original ventilation operation, or even open at all based on the mechanical or HVAC requirements. However, the sash (i.e. operable portion of the frame) and perimeter sightline should replicate the existing system. Over the past few decades, fixed windows became more common, but recent feedback and design codes highlight the importance of natural ventilations for building performance and occupant comfort.

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