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Personal: Residential

Fenestration Products –
Heat Loss Through Windows and Doors

Heat energy tends to move from warmer areas to colder areas. This influences the rate of heat loss through window components. There is no way to get around this fundamental principle; all we can do is slow down the heat-transfer process.

There are four principal heat-transfer processes in windows.

Radiation – Radiation (the transmission of heat energy through space) causes about two-thirds of the total heat loss in a standard window. Because ordinary glass readily emits heat to colder surfaces, radiation losses can be reduced by lowering the emissivity of the glass.

Conduction – Frames and sashes are the primary elements of a window that conduct heat. Advances in materials and designs that more effectively use insulating materials have dramatically reduced losses from these sources.

Convection – Air movement in the spaces between panes of glass results in heat loss from convection. If the space is too small, conduction through the air is significant. If the air space is too large, the still air rises, as it is heated on the warm interior side and falls as it is cooled on the cold exterior side of the window. This convection movement of the air passes heat to the exterior. The best spacing to minimize convection losses is 12 to 16 mm (1/2 in. to 2/3 in.) between the glazings. Gases (argon and krypton) are often used to reduce convection heat loss. Optimum spacing for these gases can be different.

Air leakage – Air leakage is a significant contributor to energy costs during the heating and the cooling seasons. Most of the air leakage of operable windows (windows that can be opened) occurs between the window's sash and frame. Bigger windows tend to leak less air per unit area. Air leakage can also occur in poorly constructed fixed windows between the insulated glass unit and the frame. Windows that have the lowest leakage rates, regardless of type, tend to be windows that are fixed (i.e., they do not open).

Air leakage can also be a big problem if the windows are poorly or carelessly installed in the rough opening. If the space between the outside perimeter of the window frame and the opening isn't sealed properly, air will leak through it. This space should be insulated and sealed before the window trim is attached.