The U-value (heat transfer coefficient) describes the efficiency of a window in terms of thermal insulation and heat transfer. Since the introduction of the German Energy Efficiency Directive (Energieeinspar-Verordnung, EnEV), this has replaced the k-value. This allows the measured values to be compared across Europe. The U-value indicates how much energy per m2 of window area can escape from the inside to the outside. A lower heat transfer coefficient means that less heat can escape through the window, representing a higher level of energy efficiency. The thermal transmittance of the entire window (UW) is divided into a Uf and a Ug value, where the Uf value reflects the thermal transmittance of the window frame and the Ug value reflects the value of the glazing