Ann Cousins, Preservation Trust of Vermont
November 5, 2010
Historic windows are an important, character-defining feature that contribute to an historic building’s significance. The size of the pane, muntin profile, method of joinery, and wood species are clues that help to date windows.
Most historic windows are made of dense wood and, with proper maintenance, can last a very long time—much longer than most newer windows dating from 1940 when peg joinery was replaced with metal staples. Window conservation and weatherization combined with good storms and energy-saving shades can realize a better, more efficient unit than thermopane replacements.
Plus, there are economic advantages! The cost is typically about the same to conserve as to replace, yet a conserved window will likely last 100 years in comparison to most replacement thermo-pane windows with an expected life of 20-25 years. Looking at life cycle costs window conservation makes the best sense.
- It is the green solution. Historic windows can usually be repaired so that they operate easily with light maintenance, lasting another 100 years. Insulated glass loses effectiveness when seals fail resulting in cloudiness and condensation. Most thermopane windows need full replacement rather than just replacing the panes. An exception is Woodstone windows, made in Vermont. Some of the better manufacturers typically guarantee seals for 20-25 years, but when they fail the guarantee is prorated.
- Refitting and weather stripping with good quality spring bronze weather strip (avoid less expensive plastic that loses its spring in approximately 1 year and has a 5-year life) reduces air infiltration, increasing comfort and saving energy.
- The wood in historic sash is dense old-growth compared to plantation grown wood today. Clad windows, in particular, use soft wood that swells with water infiltration, typically at the miters. Many manufacturers use finger-jointed pieces of wood that are glued together rather than full length sticks.
- Mortise and tenon joinery, typically dating from before the 1940s, uses wood pegs so that the deteriorated parts of a window can be easily replaced.
- Steam tables remove paint, putty, and their lead contaminates. Historic windows should never be dip-stripped as this pushes not only stripping chemicals, but also lead, deeper into the grains
- Adding a good quality storm window with efficiency coatings (there are advantages to exterior, but some prefer interior for aesthetic reasons) brings the R-value of windows closer to insulated glass. Adding a Vermont-made Gordon’s EcoSmart insulating shade, can result in a 5.5 to 6 R-rating. Because the shades are operational, there is the advantage of passive solar heating during the day.