The thermal transmittance, or U-value, is the key parameter for evaluating the thermal insulation performance of windows, doors and facade elements. It is expressed in W/(m²·K). For windows, a distinction is made between the Uw-value of the entire window, the Ug-value of the glazing, and the Uf-value of the frame profile. Modern triple glazing achieves Ug-values below 0.6 W/(m²·K). Energy saving regulations specify limit values that must be observed during new construction and renovation. A low U-value reduces transmission heat losses and improves living comfort by keeping interior surfaces warmer, which prevents condensation forming on glass surfaces.