meaning of U-value

The U-value is a factor that expresses the glazing system’s thermal conductivity. It is also known as the overall heat transfer coefficient or thermal insulation value. It indicates the volume of the thermal flow through the glazing material in relation to time and thus also expresses its insulating capability. It is the most important co-efficient in relation to heat insulation. Defined precisely, it indicates the amount of energy (in joules = watt seconds) that flow in one second through a surface of 1m2 if the air temperatures on the two sides differ in a stationary state by 1K (= degree Kelvin). It is a specific co-efficient of the material composition of a component. The lower the value, the better.

The U-value is determined in the glazing’s vertical position. In the case of air or gas-filled double-glazing panes and unfilled polycarbonate panels the values go down by c. 30% whenever they are laid horizontally. As Lumira® aerogel is a solid material, the U-values when glazing is laid vertically or horizontally are identical. This is a key advantage of roof glazing using Lumira® aerogel.