The thermal and optical behavior of a fenestration system can be described with the following parameters:

• Thermal transmittance coefficient or heat transfer coefficient (U-value),

• Solar transmittance coefficient or solar heat gain coefficient (g value) - Shading coefficient (SC)

• Visual transmittance ( v ).

These parameters apply both for glazing and windows.

However, the energy performance of a window cannot be isolated from the performance of the building. Especially in cooling dominated climates, control of excessive solar heat gains is desirable, leading to fenestration with low g-values; for conventional glazings, visual transmittance would range in low levels, as well, and consequently, the use of artificial lighting instead of daylighting would increase the internal heat gains and finally the cooling loads. It is evident that a holistic approach is essential and therefore, descriptors, which characterize the impact of the fenestration systems on the energy performance of the building, are needed with regards to their thermal behavior and daylight potential.

The parameters that describe the energy behavior of the fenestration products (energy performance indexes for heating and cooling season), as well as their impact on the building performance are presented in this document.