Thermal Windows

Windows serve many purposes in commercial buildings, merchandise displays, aesthetics, natural daylighting, ventilation, etc. Unfortunately, windows are poor insulators and heat can easily escape through them, wasting your energy dollars. Thermal or storm windows are an energy- efficient alternative to single-pane windows. Your energy cost can be reduced and comfort improved by installing thermally improved windows.

Many older buildings have single-pane or single glazed glass windows. In an average building, windows cover about 20% of the total wall area. In a well insulated structure, 20 to 50 percent of the total energy loss may occur through and around the windows. Currently the new construction standard is double-pane glass. Triple-pane glass is even more energy efficient. All single-pane glass should be replaced with either double- or triple-pane glass.

A window's insulating level is referred to as its R-value. The higher the R-value the better thewindow is at keeping heat in (or out in summer). Additional panes of glass seal in air, which is a better insulator than glass. The R-values for different types of windows are listed below.

Single Pane
Double Pane    
  .25" air space
  .5" air space
Triple Pane    
  .25" air space
  .5" air space
*These are representative R-values. The size of the window and
type of frame will affect the average R-values for a specific window.

A relatively new innovation in windows is called low emissivity or "low-E" glass. Emissivity is the ability of a substance to emit or radiate heat. Low-E glass is double- pane glass that is coated with a colorless coating that transmits high levels of light but reflects heat. A window is considered to be low-E if its emissivity is 0.35 to 0.05. Standard clear glass has an emissivity of about 0.84. As a window's emissivity decreases, its R-value increases. A double-pane window with a low-E coating will have an R- value equivalent to or better than a triple-pane window. Windows in warm climates, where cooling is the primary concern, should have the low-E coating on surface 2 as shown in the figure below. In cold climates the coating should be on surface 3.

Options for Improvement

There are basically two options for improving your windows. One is to leave the existing single-pane windows and add a pane of glass, or storm window. This is a good option as long as the existing windows are tightly sealed. A second pane can be added on either the interior or the exterior side of the existing pane. Plexiglass can also be used instead of glass and is suitable for areas where added security and durability are desired.

A second option for better windows is to replace existing windows with new ones. This should be done in cases where the existing windows are drafty or need to be replaced for other reasons, such as aesthetics or building remodeling. If the building has an extensive window area, reducing the window area to an acceptable level should be implemented. Total window replacement can be quite expensive and the energy savings alone typically will not justify replacement.

All windows should be properly sealed so that infiltration of air around the frame is minimized. Movable windows are especially prone to causing drafts and should be properly caulked and weather stripped.