Preservation by Prevention: Getting to Know Your Historic Double-Hung Windows

Now that fall is in the air, it’s time to switch out the screens for storm windows at my house. This project has forced me to look a bit more closely at the windows than I typically do. My house still has its double-hung, single-pane, true-divided-light, six-over-six wooden windows, which date from the early 1940s when the house was constructed. The original windows add a lot of dimensionality to the appearance of the house, and they are one of the reasons that I fell in love with it. One small casement window in an upstairs closet has been replaced, but the rest of the windows are still operating after almost 65 years of use.

Nevertheless, after decades of service, the windows do need a bit of attention. One window in an upstairs bedroom sticks because of too many layers of paint; one in the kitchen is nearly impossible to open because the sash cords either have broken or were cut, so there is nothing to counterbalance the weight of the lower sash. Historic windows need periodic maintenance to operate properly. With proper care they can last indefinitely because all of their components are repairable or replaceable. Even quality replacement window manufacturers only warranty their windows for 10 to 20 years. When compared with the proven lifespan of my original windows, replacement windows don’t look like such a good deal.

A double-hung window, perhaps the most common type of window in residential applications, has two sashes that move up and down in a vertical plane. A sash is the framework of the window; it can be fixed or moveable. With a double-hung window, the upper and lower sashes move in tracks separated by a parting bead, with the lower sash inside of the upper sash so that it appears slightly recessed from the outside. A single-hung window, commonly installed in modern construction, is one whose bottom sash raises and lowers but whose top sash is fixed. See diagram below.

A sash has a number of component parts, including rails, stiles, muntins, and lights or panes. The rails are the horizontal members at the top and bottom of the sash. The rail in the middle, where the top and bottom sashes meet and lock, is the meeting rail. The stiles are the vertical members that comprise the sides of the sash and join the rails. The muntins are the narrow horizontal and vertical members that hold the individual panes of glass. A pane of glass is often called a light, which is defined as “an aperture through which daylight is admitted to the interior of a building.” Double-hung windows are usually referred to by how many panes are in each sash, e.g., a two-over-one has an upper sash with two lights and a lower sash with one light. With true-divided-light windows, the terms panes and lights are interchangeable because each pane of glass is truly a light. However, many replacement windows are manufactured with a single pane of glass divided into multiple lights with false muntin bars.

Windows are set within a frame that does not move. For most double-hung windows, the weight of the sash is counterbalanced by sash weights that hang from sash cords or sash chains. The sash cords run along channels on the sides of the stiles, after which they continue over pulleys into the weight pockets located in the wall. The weights make opening and closing the windows easier, and they keep them open without stops or other devices.

Like any other wooden component of your house, the most important thing you can do for your historic wood windows is keep them covered with a sound coat of paint, which will help protect them from the effects of precipitation and condensation. However, too many layers of paint can make windows stick and hide their molded details. If you need to remove old paint from a window sash, the safest method is to remove the sash from the frame and place it on a flat surface. The paint can be softened with a heat gun or chemical stripper and then scraped from the wood by hand. Heat guns must be used with care as improper operation can crack the glass, scorch the wood, or even set the sash on fire. The best practice is to heat small areas while constantly moving the gun. If you suspect or know that your windows are coated with lead paint, you must follow all laws related to its removal and disposal.

There are a few areas of the windows that should be left unpainted, or be painted sparingly. The sides of the stiles that abut the tracks should never be painted because the paint can prevent the windows from operating smoothly. For the same reason, you must be careful not to apply too much paint to the tracks in the window frame. Painting the sash cords can cause them to deteriorate and break more quickly than they otherwise would, so they also should be left natural.

Weatherstripping, caulk and glazing putty cut down on air infiltration around and through windows. Weatherstripping fills the joints between components of a window and between sashes and the window frame, e.g., the space between the meeting rails or the space between the bottom rail and the sill. Caulk closes the spaces between the window frame and the window opening. As such, caulk often connects two materials with different rates of thermal expansion. Therefore, it is very important to use a caulk that is compatible with both materials and that is capable of spanning the width of the gap. Glazing putty holds the panes of glass against the muntins and closes the gaps around the glass. Caulk and glazing putty are both materials that degrade over time from exposure to light and air. Replacement of these materials is a standard part of regular window maintenance. Similarly, weatherstripping does wear out and lose its capability to spring back and fill gaps; it should also be replaced periodically.

Storm windows, where I began this discussion, are the traditional way of sealing historic windows tightly against winter weather. Many studies have shown that a historic single-pane window in good repair that is well fitted with a storm window can be as energy efficient as a double-pane replacement window. Because existing windows take no additional energy to manufacture or install, and because they won’t end up in a landfill if you keep them, repairing them and adding a storm window is often the greener solution to saving home heating energy when compared to window replacement. Better yet, retaining your historic windows will protect the historic character and integrity of your home.

Additional Reading

Gibney, David. “Restoring Windows Sashes.” Fine Homebuilding 161 (2004): 84-89. Accessed 19 October 2009.

New York Landmarks Conservancy. Repairing Old and Historic Windows: A Manual for Architects and Homeowners. New York: John Wiley & Sons, Inc., 1992.

Sedovic, Walter and Jill Gotthelf. “What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows.” APT Bulletin 36, no. 4 (2005): 25-29.

Young, Robert A. Historic Preservation Technology. Hoboken, New Jersey: John Wiley & Sons, 2008.