Money Out the Window
A Discussion of Energy Transfer and Money Lost to Infiltration and Conduction
"Windows are thermal holes. Most are 10 times less energy efficient than the wall area they replace. An average home may loose 30% of its heat or air-conditioning energy through its windows."
Windows: Understanding Energy Efficient Performance
Heat loss associated with windows is the result of infiltration and conduction. In other words the amount of "money out the window" you lose each year is determined by the energy transferred over under, and through your windows.
Energy loss can be greatly reduced by decreasing leaks and lowering convection through windowpanes. Let's take a look at the sources of infiltration and the problem of conduction.
The majority of cold air infiltration occurs at each window at its periphery where the movable part of the window (the sash) meets the inner frame. A tight seal must be provided between the sash and the frame while still allowing free movement of the sash vertically within the frame. Old windows did not incorporate extensive weather seals at these edges. Modern windows, however, are superb in this regard. Traditional restoration, "Better Than Ever" Window Preservation by Jay Marco, and quality replacements also address this energy loss adequately. All of these window improvements greatly reduce energy losses between the sash and inner frame that otherwise may account for 50% of energy losses in unimproved double hung windows.
A minor source of infiltration is addressed at the framework of each window where it must be inserted and attached to the wall of the building. Inevitably, cold air infiltration occurs to some extent around the framework where it is secured to the structure. Properly constructed window installations should yield very small energy losses at this area. Water damage and rot may compromise the integrity of the weather seal at this point, but typically little energy is lost to this source.
A much more egregious source of infiltration in double hung windows is the "sash weight cavity." Sash weights attach to double hung window sashes and are made of iron or steel weights equal in weight to the sashes. These weights provide counter balance and are intended to move up and down corresponding to the movement and position of the sash within the frame. The heavy sash weights are suspended by chains or ropes and are contained inside boxes or "cavities"on either side of the window opening. Sash weight cavities are part of the outside wall of a building, and cannot be insulated adequately as long as they are required to accommodate sash weight movement. Consequently, infiltration of cold air through sash weight cavities is responsible for as much as 20% of the energy loss associated with double hung windows. Window replacement, sash replacement, jamb liner installation and "Better Than Ever" Window Preservation obviate the need for sash weights and cavities. Traditional restorations require that sash weights continue to be used, and therefore that sash weight cavities be retained.
A final source of infiltration in windows is the leakage of air around the glass panes and the caulking material used to secure them to the sash. Modern windows are usually weather tight at these seams, but older windows in disrepair may be considerably compromised and loose considerable energy at this point. Re-caulking is a straightforward procedure, and can be accomplished for minimal cost.
The areas of infiltration discussed thus far account for approximately 80% of energy loss associated with double hung windows. The remaining 20% of energy loss in unimproved windows is due to the conduction of energy through glass surfaces. All materials conduct heat and energy to some extent. Unfortunately, glass is a fairly good conductor, allowing heat energy to migrate through windowpanes and be lost outside. Modern window materials such as double and triple pane glass, "double-e" coatings, and window films drastically reduce energy losses through window glass. Window improvement strategies may incorporate any of these options, determined by the needs to maximize efficiency, preserve authenticity, and minimize expenses.Window Repair and Restoration Options