Heat loss

Heat loss is influenced by three mechanisms: radiation, conduction and convection.
The electromagnetic long wave heat radiation that every entity emits due to its temperature transfers thermal energy without transmitting the entity or medium itself.
Heat conductivity is the heat flow within a medium caused by temperature differences. In this case, the energy always flows in the direction of the lower temperature.
Convection is a flow of gas particles in the interspace due to the difference in temperature between the inner and outer panes of insulating glass. The particles drop onto the colder surface and rise again on the warmer side. Consequently, the gas circulates, creating a heat flow from warm to cold.
Insulating glass consisting of just two uncoated panes of float glass and with air filling the interspace loses around two-thirds of the heat that the room would otherwise have due to the radiation loss between the two panes, and a third due to heat conductivity and heat convection to the outside air.
 

In the case of older insulating glass units without coatings, this results in an extreme difference in temperature between the inner pane and the warmer room air in the cooler seasons of the year, resulting in a significant loss of heat due to the heat transfer from the inner pane to the outer pane. Most of today’s insulating glass consists of a minimum of one float glass pane equipped with a low-E coating.

These coatings can have emissivity values of less than 0.02 (2 %) and reflect more than 98 % of incident long wave heat radiation, so that radiation loss is almost completely eliminated. This represents an improvement of approx. 66 % compared to insulating glass units without coatings. Heat conductivity and convection are not affected by coatings. However, the heat conductivity can be reduced by using an inert gas such as argon. Inert gases have significantly lower heat conductivity than air, thereby reducing the heat flowing through the insulating glass system. The convection is minimised at a certain gas space, depending on the filling gas. For example, this is approx. 16 mm in the case of air, 15 - 18 mm for argon and 10 - 12 mm for krypton.