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Retrofitting Wood Sash Windows

With single-light wooden sash, it may be possible to retrofit insulating glass within the existing sash frame. The sash frame needs to be in relatively good condition and of sufficient size to handle the additional glass weight. Mechanical routing of the glazing rabbet is usually required to accommodate the additional thickness of the insulating unit. In addition, the sash weights probably will need to be augmented if operable windows are desired. It is always important to establish beforehand whether there is sufficient room in the weight pockets to allow for the additional weight balancing.

For the same single-lighte sash, another alternative is to "piggyback" an aluminum-frame storm panel onto the interior portion of the sash. This procedure requires that the inside edge of the rails and stiles on the room side be rabbeted to allow insertion of the storm panel.

As with retrofitting insulating glass, the approach to double glazing has an advantage over separate storms in ease of operation and lower maintenance. In piggy-backing the storm panel onto the sash, care must be taken to get as good a seal as possible where the metal frame abuts the wood sash frame.

A good weatherstripping system should be used on the back side of the metal storm frame, and the metal frame should fit snugly. These measures are necessary to reduce the likelihood of condensation between the two pieces of glass. Weep holes should be provided, preferable on the stiles rather than the rails. Removable clips or set screws to secure the storm panel to the wood sash are recommended; these permit occasional cleaning of the glass and allow access for maintenance work. Local glass shops can provide the necessary materials.

The use of retrofitted insulating glass or piggyback storm panels is generally limited to single-light sash, although it may be possible on some two-light sash to achieve similar results, provided the muntin is wide enough and strong enough.

One additional use of insulating glass with existing wood sash is worth mentioning. For buildings in downtown urban areas or along busy highways, there is an increased desire to reduce noise from the outside. While sound reduction can be achieved through good weather-stripping, the addition of a storm window with insulating glass in combination with the existing window can yield superior sound attenuation over a triple-glazed replacement sash.

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

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