Heat Loss

What Is It?

It is a basic law of nature that heat energy will move from warmer areas to colder areas. In the context of windows, heat loss is the migration of heat energy from the interior of the building to the exterior through the different components of the window. It occurs in 4 different ways — radiation, conduction, convection and air leakage — and there is no way to avoid it. We can only find ways to slow these processes down.

Radiation losses occur through the window glass and represent about 66% of the total heat loss in a standard window. Ordinary glass readily emits heat energy to colder surfaces so it is said to have high emissivity. Technologies to counteract radiation heat loss include low-E, or low emissivity coatings on the glass.


Conduction losses in windows occur primarily through the edges of the glazing and through the sash and frames. Advances in glazing unit construction, such as the use of warm edge spacers, and in frame construction such as built-in thermal breaks, can dramatically reduce conduction heat loss.

Convection losses occur due to air movement between the glazing layers of a window. The convection movement, or rising and falling of the air between the layers of glass as it first warms and then cools, passes heat from the warm interior side to the cooler exterior side. Other gases, such as argon or krypton, are used to replace air between the panes because they conduct less heat.  The optimum space width to minimize heat loss is 1/2” (13mm) for air and argon, while for krypton it is 3/8” (10mm). (See Gas Spacing Chart in Gas Fills Section, How It works)

Air Leakage is a significant contributor to heat losses. Operable windows will usually have more air leakage than fixed windows and most leakage in operable windows occurs between the window’s operable sash and frame or the meeting rails of a sliding sash.

Installation also plays a huge factor in air leakage. Poorly or carelessly installed windows with inadequate insulation and sealant between the outside perimeter of the frame and the rough opening will be a significant source of air leakage. Proper installation includes ensuring the space around the window is completely insulated and sealed.