Historic Windows & Energy Efficiency
By Sarah Donahue Wolff
Across the country, owners of old houses are being encouraged to remove their original windows and replace them with new energy-efficient models. Manufacturers of new window technologies advertise that old windows are inefficient, causing houses to leak energy and contributing to excessive heating and cooling bills, and to deterioration of the planet’s environment. Replacing historic windows is touted as the “green” choice.
In reality, historic windows can be the “greener” choice. Environmentally conscious historic homeowners can keep their historic windows and improve energy efficiency at the same time.
• When the entire life of the window is considered and embodied energy calculated, retaining and repairing historic windows is environmentally preferable to replacing them.
• Homeowners can make relatively simple repairs to increase old windows’ energy efficiency, and storm-window systems can further improve energy efficiency while protecting and retaining the historic character of the home.
Beyond Energy Efficiency: Sustainability
Energy loss occurs in homes through many conduits: attics, basements, doors, even cracks in the walls. Windows account for only 10% for energy loss in a typical home, perhaps even less in North Carolina’s mild climate, less than attics, floors and doors. Improvements in window efficiency, unless there are gaping holes, will only incrementally improve a home’s overall energy efficiency.
Green principles include more than just energy efficiency. When evaluating historic windows, we should consider the environmental and financial consequences of the window’s entire life cycle.
One important consideration is “embodied energy,” the energy that has already been expended in creating and installing the existing window. Keith Haberern, an engineer in New Jersey, estimates that new windows consume approximately 2.3 million BTUs just in production. To recover this environmental cost alone would require four years of energy payback. Haberern’s estimate does not include the energy required for the extraction of the raw materials used in the windows or for their installation, shipping, etc. These costs further extend the energy payback. Furthermore, many new windows are made from vinyl, a nonrenewable, petroleum-based product.
And what happens to a window after it has been replaced? Most replaced windows end up in landfills, certainly not a “green solution.”
Replacement windows themselves have a short useful life. Countless historic windows have been in use for more than a hundred years. New windows are expected to have useful lives ranging from two (as listed in some manufacturers’ warranties) to twenty years, depending on their quality. Either the seal between the panes of glass breaks, reducing their insulation capacity and creating an ideal environment for mildew and other forms of mold, or the vinyl casing cracks, fades and discolors.
In either case you’ll have to replace your new replacement windows before they have even half-paid for themselves in energy savings. Haberern estimates the payback time needed to recover the financial investment in a new window is 41.5 years. Generously assuming a useful life of 20 years, new windows are not in use nearly long enough for homeowners to recover their cost.
New Life for Old Windows: Improving Efficiency
Many repairs can be made to historic windows to improve their energy efficiency. Window restoration expert David Hoggard of Double Hung Historic Window Restoration in Greensboro argues that no historic window is beyond repair. Simple changes include
- Making sure windows are properly sealed, caulked and installed.
- Adding storm window systems, either inside or outside the existing window.
These changes improve a home’s energy efficiency while maintaining its historic character.
Many of the energy leaks found in old windows are a result of poor maintenance. Most were designed to be double hung. A system of weights and ropes allows the top and bottom sections of the window to move independently. Over time, the weights and ropes may sag, resulting in gaps between the sash and the casing. These gaps cause energy loss. Repairing the double-hung mechanism allows the windows to close tight. Proper caulking and sealing around the window casing can also reduce energy loss.
Another solution is to affix the upper sash, remove the weight system and insulate the cavity where the weights had hung. This relatively simple repair retains the character of the window and increases its efficiency.
Attaching storm windows is another historically accurate way to increase the efficiency of original windows. Historic windows used in combination with storm windows have been shown to have even better insulating properties than double-pane windows, an R value of 1.79 compared to 1.72 for new windows. Exterior storm windows can also provide protection for the historic window and can be designed to minimize their aesthetic impact.
Homeowners have many options when choosing storm windows. The cost of installation depends on many house-specific factors including the region and location of the house, the size and number of windows, their accessibility (first floor? second floor? attic?), the labor required, and the degree to which the windows must be customized to fit a particular space
Interior Storm Windows
Interior storm-window systems differ in material (vinyl, wood) and operation. Some are designed to open and close; others remain attached. Although interior storm windows increase the energy efficiency of the historic window system, they don’t protect it from the elements. Moisture can also be trapped between the windows. However, interior storm windows do not obscure the look of the window from the outside, and they are removable.
Aluminum Triple-Track Exterior Storm Windows
Triple-track systems attach to the outside window frame and have two window sash and a screen which move along separate tracks, providing protection while allowing ventilation. They are readily available in a variety of sizes and colors. On the basis of balancing cost, ease and effectiveness, this system is recommended by the National Park Service. Exterior storms can reduce the visual beauty of the historic windows, though some new profiles minimize their impact.
Replacement Traditional Storm/Screen System
New storm/screen systems hang from small hooks on the exterior frame of the window. The system consists of two parts: 1) a storm window with glass in a wood frame and 2) a similar frame with screen instead of glass. The storm windows can be designed to remain closed or to open and close. The wood sash can be painted to blend with the exterior design of the house. This traditional system requires more maintenance than modern storm windows. The storms and screens must be changed manually with the seasons, and homeowners cannot easily switch between storms and screens.
Owners of historic homes have many ways to improve energy efficiency and reduce their impact on the environment. Despite advertising claims, window replacement is not the best way to improve the efficiency of historic windows. Repairing, insulating and adding storm windows are economically viable options for conserving both historic and environmental resources.
Sarah Donahue Wolff received her Masters of Regional Planning degree at the University of North Carolina Chapel Hill in 2007. She now works for Self-Help, a community development financial institution in Durham and spends her free time revitalizing her own historic home. This article was adapted from “Historic Windows and Energy Efficiency,” which appeared in North Carolina Preservation magazine, fall 2007.