To reduce losses, you can insulate or improve the management of your windows. The basis for determining the effectiveness of insulation is its R-value or resistance to heat flow. In other words, nearly 20 times more heat escapes through each square foot of window than through an insulated wall. With this R, the product has about 75 percent efficiency, which means it stops 75 percent of the heat loss through the window. Table 1 below lists the R factors of various window types.
Heat losses occur by infiltration when there are cracks around the frame or glass.
Heat is conducted through glass rapidly when it warms the glass and is transmitted to the colder outside environment. Radiation is the process of losing heat through space. For example, a person seated near an uncovered window will begin to chill as their body heat radiates to cold glass surfaces. Insulation value of windows and window treatments. The Inside Story , Gwen Cukierski.