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 exterior doors, patio doors and skylights, heat loss is the migration of heat energy from the interior of the building to the exterior through the different components. 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 glazing and generally represent about 66% of total heat loss in a standard glazed unit. 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 occur primarily through the edges of the glazing and through the sash and frame. Advances in glazing unit construction, such as the use of warm edge spacers, and in frame construction such as improved thermal breaks, can dramatically reduce conduction heat loss.


Convection losses occur due to air movement between the glazing layers. 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).


Air Leakage is a significant contributor to heat losses. Operable units will usually have more air leakage than fixed ones and most leakage in operable units occurs between sash and frame. As well, the seam where panels in sliding patio doors meet in the middle of the frame is a potentially significant source of air leakage.

Installation also plays a huge factor in air leakage. Poor or careless installation, inadequate insulation and improper sealant between the outside perimeter of the frame and the rough opening will all have significant sources of air leakage.