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Exterior Storm Windows

Wood storm.
Wood storm.
Aluminum storm, painted.

The traditional method of double glazing is the use of exterior storm windows, which achieves a U-value for the window comparable to insulting glass. The typical exterior storm window greatly reduces air infiltration, lowers the maintenance cost of the historic window, and extends its useful life. Unfortunately, exterior storms can affect the visual appearance of the historic windows, although less so where single-lite historic sash are involved. While it was common at the turn of the century to match the divided lite pattern in the primary sash with that of the exterior storm, today's single-lite storm panels tend to alter the reflective qualities and shadow lines cast by the primary window, and also obscure features such as muntins.

Several steps can be taken to lessen the visual impact of storm windows. The simplest is to have the storm factory window painted to match the color of the primary window and trim. The second step is to specify a half-screen to be mounted on the inside, since it is the screening material in the typical storm/screen combination that most dramatically affects the appearance of a historic window. By mounting a half-screen on the inside (a typical feature earlier in the century), the sight lines of the storm unit are simplified by the reduction from a typical three-track to a two-track frame. Also commercially available are custom single-track, two-panel units with a simple subframe set within the jamb. Some single- track systems are designed so that the panels can be removed from the inside for cleaning and, for summer use, screen panels inserted for ventilation. This single-track design, can significantly reduce the storm unit's impact on the window's historic appearance. In either case, the storms will also result in considerable sound reduction, which is important to buildings exposed to high street noise.

Fisher, Chuck. Rehabilitating Windows in Historic Buildings: An Overview (GSA 08500-03), Historic Preservation Technical Procedures, U.S. General Services Administration

Exterior-mounted storm windows must have "weep holes" at the bottom of the frame to allow any moisture that collects between the primary window and the storm window to drain out. Even though these drainage holes subtract from energy savings, not having them will eventually cause the primary window frame to rot, and possibly make them impossible to operate.

Storm Windows, Energy Efficiency and Renewable Energy, U.S. Department of Energy

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