A single pane has a heat transfer coefficient of 5.7–6 watts per square meter per degree of kelvin; a double pane separated by 6 millimeters (air is a poor conductor of heat) has a coefficient of 3.3. Applying coatings to minimize the passage of ultraviolet and infrared radiation lowers it to between 1.8 and 2.2, and filling the space between the panes with argon (in order to slow down heat transfer) chops it to 1.1. Do that with triple-glazed windows and you drop to between 0.6 and 0.7. Substitute krypton for argon and you can get it down to 0.5.
