How Heat is Lost Through Windows

Having a basic understanding of the three heat transfer mechanisms, it is possible to more carefully examine windows and their methods of heat retention and loss.

Essentially speaking there are 4 methods of heat loss for a window: (1. Northern Comfort)

	Transmission of Heat Through the Glass	Transmission of heat through the Glass is accomplished primarily on the visible light spectrum, however some infrared transmission does occur.
	Conduction Through the Frame	Conduction through the frame is a very large consideration because the frame acts as a "thermal bridge" (heat is conducted rapidly through a region that is otherwise well insulated on all sides creating a cold point.) (This is shown below in Diagram #3)
	Air Movement Through and Around the Window	Convective currents (Primarily natural convection) takes place on both sides of the window, and if the window is multi pane, within the window itself, thus causing more rapid heat transfer. See Diagram 2 below.
	Radiation	Radiation is a unique method of heat transfer, in that it does not require a medium between the radiant objects, thus it is not affected by insulation thickness so much as special coatings discussed in the Options Available section. See Diagram 4 below

Most window manufacturers base their specifications on U factors, the lower the U factor the more thermal Resistance, (keep in mind that the U factor is the inverse of the summation of the R values).

It is clear from Diagram #2 that the amount of heat loss is drastically increased by the thermal currents created by the variation in air temperature.

As noted above, losses through the frame via conduction are quite frustrating for numerous reasons, first is the fact that many window frames are constructed of Aluminum alloys, which are excellent conductors of both electricity, and more importantly, heat. Second, the temperature drops, as mentioned, often below the dew-point for the given humidity within the structure/ building envelope, causing if nothing else, rot, corrosion, and or oxidation of the interior of the window sills.

This is perhaps the most difficult to understand diagram on this page. Understand that the Heat Source is essentially a person, heater, heated air, etc. and that it is radiating in the infrared spectrum constantly, in accordance to the Stefan Boltzman Equation discussed on the Methods of Heat Transfer Page. Meanwhile, sunlight is actively heating the heat source further, but only in the infrared spectrum, meaning that at night, the only notable heat transfer that is occurring is that of the Heat Source to the outside night air. Here it is useful to note that objects radiate at different rates according to their Emissivity, for which some common values are available at the bottom of the Methods of Heat Transfer page.