The design of fenestration (windows, skylights, etc) requires special attention because of the huge variety of available building components and the several important roles that windows play.
Perhaps thermally most important, they admit solar radiation. This is often advantageous in the winter and disadvantageous in the summer. Also, despite dramatic improvements, glazing still usually has the lowest R value (highest U-factor) of all components of an envelope. Windows and skylights also admit daylight to buildings and often provide desired ventilation.
The physical properties of the materials, coatings, and constructions that make up windows and skylights. Good glazing properties are important because they control the amount of daylight, quality of light, and amount of solar heat gain let into the building.
Window configurations that use low-E coatings, selective transmission films, inert gas fills, and thermal breaks can lead to higher energy performance. The net effect of these measures is to reduce the U-factor, andthe right choice of these features depends on the application.