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In this example, notice the replacement windows do not fit the masonry opening of the original window. Instead the new window was reduced in size dramatically changing the historic appearance of this commercial building. Source: Bob Yapp

If an original window in your historic house or building has deteriorated beyond repair (or is simply missing), you will need to replace it with a window that closely matches the original windows. You should consult a contractor, such as a carpenter or painter who is experienced with historic houses, to determine if your window is deteriorated beyond repair.

When you are working with a window manufacturer to design a replacement window for your historic house or building, you should make certain your replacement window matches these three characteristics of your original windows:

• Design
• Dimensions
• Material (when possible)

Replacement Window Checklist

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A replacement window should match the original in design, dimension and, where possible, materials. Here is an excellent example of a replacement window. Source: Bob Yapp

The checklist below describes various design considerations for a replacement window. You might find it useful to bring this list with you when you meet with a window manufacturer.

• Window unit placement. The location of the window within the wall plane affects the overall appearance of your historic house. If you still have an original window in place, look at the window unit placement in relation to the wall plane. 
• Window frame size and shape. Your replacement window should be the same size and shape as the original window. Even a slight modification to either the size or shape of the window can greatly impact the overall appearance of your historic house or building. For a wood window, you should also match the original casing, which includes the brick mold, blind stop and sill. Steel windows were nearly always installed while the building’s walls were being constructed, so they have very little of their outer frame exposed. Your replacement steel window will need some added dimension, but this added dimension must be minimal.
• Glass size and divisions. Historic windows were constructed with single panes of glass held together with wood pieces and glazing compound. These wood pieces formed a grid called a muntin. Modern window units have two panes of glass. The appearance of your original window (several small pieces of glass held together with muntins) can be reproduced as simulated divided lights. The reproduction should have an exterior grid, spacer bars between the glass and an interior grid. If the dimensions and profile of the exterior grid are the same as the historic muntin, and the interior and exterior grids are permanently affixed tight to the glass, your replacement window will approximate the original appearance of your historic house or building.
• Sash elements.  The sash elements of a wood window include the rails, stiles, and muntins. The sash elements of a steel window include the operator frame and muntins. Make certain to use the same sash depth, or thickness, as your original window. The depth of the sash in a double-hung window affects the depth of the offset (the separation between the two panes of glass) at the meeting rail of a hung window, and this depth is perceived through the shadow that it creates. Because a muntin is small, even a slight change in the dimensions of a muntin will also have a noticeable effect on the overall character of a window. The visual effect of a muntin is also affected by its shape.
• Materials and finish. It is theoretically possible to match all the significant characteristics of a historic window with a substitute material. However, the finish, profiles, dimensions and details of a window will all be affected by the switch to a substitute material. For example, vinyl-clad or enameled aluminum-clad windows may have joints in the cladding that can make them look very different from a painted wood window. Secondary window elements that do not match the finish or color of the window can also diminish the strength of the match. Some examples of secondary window elements include white vinyl tracks on dark-painted wood windows or wide, black, glazing gaskets on white aluminum windows.
• Glass characteristics. A replacement window can have insulated glass as long as it does not compromise other important aspects of the window match. The glass must be clear and nonreflective with a visual light transmittance of 72 or higher.

Replacing a Window Where No Historic Window Remains

A replacement window for a missing or non-historic window must be compatible with the historic appearance and character of your house or building. If all of your original windows are missing, you should start the process of designing a replacement window by looking for historic photographs. Even one historic photo of your house or building could offer many clues about your original windows.  

If you are unable to find a helpful historic photograph, you should follow a few general guidelines to design your replacement window:

• The appearance of the replacement window must be consistent with the general characteristics of a historic window of the type and period of the house or building.
• The replacement window must always fill the original window opening.
• The appropriate type of window, such as industrial steel or wood double-hung, can usually be determined from the period and historic function of the building, and from the proportions of the window openings.