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Madison, Wisconsin. Decorative transoms were often integrated into the overall storefront design as seen here. Source: Photographer Mark Fay.

Your historic commercial building may have two glass features that are significant to the character of your building: windows on the upper façade and transoms above your entrance door and display windows. Your upper-floor windows serve the same functions as most windows: to let in light and ventilation to the upper floors. Transoms are glass panels designed to be both decorative and functional. You should preserve and maintain your upper-floor windows and transoms to retain the historic integrity of your building.

Save Your Historic Windows

Windows are some of the most significant architectural features and visual components of your historic commercial building. Your building’s window design, placement and arrangement (known as “fenestration”) all serve to convey the early character of your building. Your windows provide scale and visual interest to your building. They might have a unique ornamental trim, hoods or surrounds that help to define your building’s style. Your windows also contribute to the unique visual and historic qualities and character of your neighborhood or downtown district.

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A multi-light transom such as the one seen here is a character-defining feature and should be retained. Source: Phil Thomason.

Your upper-floor windows might be either arched or rectangular. Windows from the mid-19^th century are generally of wood sash design with six-over-six, four-over-four or two-over-two glass panels. Advances in glass technology allowed larger and less expensive panes of glass to be manufactured, and by the end of the century, many windows were one-over-one wood sash design. Metal windows such as steel, aluminum and bronze, were introduced and widely used into the mid-20th century. Decorative elements were also added to windows. For example, sheet metal, brick and cast-iron hood moldings were popular in the 19^th century, and cast concrete and terra cotta moldings were popular in the early 20^th century.

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Cleveland's Hall & Blacksmith Shop, 1873

Brooklyn, Wisconsin. Multi-light windows are common on all types of buildings in Wisconsin. Here is a double-hung multi light window on a recently renovated building in Brooklyn, Wisconsin. Source: WHS - State Historic Preservation Office. View the property record: AHI 89478

The character of your windows can be determined by any and all of these features:  

• Frame
• Glazing
• Head
• Jamb
• Molding
• Mullion
• Muntin
• Operation
• Sash
• Sill

The pattern or grouping of your windows in relationship to other windows or building features is also important.

Maintain Your Historic Windows

If you preserve and maintain your historic windows, the old-growth lumber that was used to construct them could last indefinitely. Modern replacement windows, such as vinyl windows, will not last as long as your historic windows. For example, the vinyl elements of modern windows expand more than twice as much as wood and seven times more than glass when exposed to temperature changes. As a result of this expansion, the seals between the frame and glass often fail and cause poor window performance. When modern windows fail, they usually cannot be repaired or recycled and end up in a landfill.

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Riverside Printing Company, 1913

Milwaukee, Wisconsin. Steel casement windows were common in industrial buildings allowing plenty of natural daylight along with some ventilation. Source: WHS - State Historic Preservation Office. View the property record: AHI 41964

Although energy efficiency is frequently cited as a reason to install modern replacement windows, most historic windows are inherently energy efficient and durable. You can make your historic windows more energy efficient by maintaining them properly and installing storm windows and weatherstripping. With these cost-effective treatments, your windows can match or exceed the performance of replacement windows. If your windows are deteriorated, you can save and repair your windows in keeping with their original design.

Save Your Transom Glass

The transoms above your display windows are a significant component of your historic storefront and should not be removed or concealed. Most transoms are made of wood with wood frames, but some may be made of copper or other metals. A structural element known as a transom bar divides the transoms from the display windows below them. Some transoms are fixed in place to allow additional light into the building, while others can be opened and closed for air circulation.

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Probably the most common type of window found on historic buildings is a double-hung. Here is an example of a one over one, wood, double hung window. Source: Phil Thomason.

Some transoms consist of large sheets of glass while others are divided into smaller panes. Most transoms on Wisconsin’s 19^th and early 20^th century commercial buildings have clear glass to allow plenty of light to enter the building. By the end of the 20^th century, other types of glass materials came into use, like stained glass and leaded glass. One notable new glass material was colored prismatic glass, which allowed diffuse light to enter a building. Prismatic glass transoms are contained purple or other colored glass and were divided by small lead frames. The Luxfer Prism Company marketed this material, and it became popular as a decorative element. Luxfer glass is readily identifiable by its small purple-tinted and ridged lights, and glass that is stamped with one of several different designs.

Another type of glass you might have on your storefront is block glass. Block glass was introduced beginning around 1920. Block glass was used at entrances or as fixed transom lights above storefronts. These hollow glass squares allowed light in and added an interesting texture to a storefront.

Preserve and Repair Your Transoms

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Madison, Wisconsin. Prism glass was used above the clear merchandise windows to allow diffused daylight into the store. Source: Photographer Mark Fay.

If your transoms have been damaged, you can repair them as you would other windows in your building. Do not remove transom lights to install air conditioners, fans or other kinds of heating and cooling units. If you have a damaged Luxfer glass transom, check with your local salvage yard or internet sources to find replacement materials, or match the glass as closely as possible using stained glass panels. 

If your storefront was remodeled in the past, your original transoms could be hidden under mid- to late-20^th century remodeling materials. You could rebuild your storefront to its original design, including the transoms, based on photographic or physical evidence. 

Follow Best Practices

When you are making maintenance and repair decisions about the windows and transoms on your historic building, follow these best practices:

• Preserve and maintain your original windows. Your windows, window details and the size and shape of these elements help establish the rhythm, scale and proportion of your building and contribute to its architectural style and character. Retain your historic windows and replace only those elements that are necessary. For example, you can replace missing panes or damaged sashes rather than an entire window. Replace a window only if it is beyond repair.
• Repair deteriorated wood windows as needed. Use epoxy to strengthen deteriorated wood on your historic windows.
• Preserve, maintain and repair your original metal windows. Preserve and repair your original metal windows with materials that match the original as closely as possible. You can improve the energy performance of your metal windows by adding storm windows and applying weatherstripping and security fittings. Other weather stripping options include spring-metal, vinyl strips, compressible foam tapes and sealant beads. You can also replace your window's original single glazed glass with thermal glass panes (3/8 to 5/8 inch thick) as long as the rolled metal sections are at least 1 inch wide and you retain the window’s historic design
• Match the original materials of your windows. If you must replace your original wood windows, wood is the best material to use. Other acceptable alternatives may be aluminum clad wood, fiberglass or composite materials. Most major window manufacturers have appropriately sized wood windows for historic commercial buildings. If you must use metal as a replacement material, the metal should have a baked-on enamel finish. Anodized finishes are not appropriate. The color of your replacement window will depend on the age and architectural style of your building. However, for multi-story buildings where maintenance is a concern, it may be appropriate to install wood windows on the second story and aluminum windows with a baked-on finish on the third floor and above. Replace your historic metal windows with like materials.
• Match the original appearance of your windows. If you must replace your original windows, match the appearance of your historic wood or metal windows with the appropriate dimensions, depth of frame and appearance of true divided lights. True divided lights for windows are preferred, or windows with lights that are bonded to the glass on both the interior and exterior with spacers and grid profiles.
• Preserve and maintain original transom glass and designs. Repair your deteriorated or damaged transoms so they retain their historic appearance as much as possible. Do not cover, conceal or obscure your original transom lights.