What is the U-value?

Understanding the U-value

The U-value measures the amount of heat that is transferred through a building component from the warm side to the cold side. It is expressed in watts per square meter per degree Kelvin (W/m²K). Essentially, the lower the U-value, the better the insulation properties of the material, meaning less heat is lost and more energy is conserved.

How U-value works

In the context of glazing constructions, such as rooflights, heat transfer occurs through three main mechanisms:

• Radiation: Heat is transferred through electromagnetic waves.
• Convection: Warm air rises and cold air falls, creating a cycle of air movement.
• Conduction: Heat is directly transferred through materials.

Importance of the U-value in rooflights

For rooflights, the U-value is a critical measure of energy efficiency. Rooflights installed on sloped surfaces typically have a higher U_w-value (a measure of heat loss) compared to vertical windows. This is due to the increased movement of gas between the glass panes as the window tilts, which can lead to greater heat loss. However, rooflights also allow more sunlight and daylight to enter, which can enhance natural lighting and solar gain inside the building.

The U-value is the measure of heat loss. The lower the U-value, the less heat is lost.

Comparing U-values

When comparing U-values, it’s important to distinguish between whole-unit values and centre pane values. Whole-unit values take into account both the glazing and the frame, while centre pane values refer only to the glass. Centre pane values are usually lower because they do not account for the frame’s effect. For accurate comparisons and whole-building assessments, ensure that like-for-like values are used.

At VELUX, we strive to provide rooflights with optimal energy performance. While traditionally the U-value has been the primary parameter for evaluating window energy performance, we are moving towards a more comprehensive approach that includes energy balance. This ensures that our products not only minimize heat loss but also maximize solar gain and natural light, enhancing the overall energy efficiency of buildings.

Source: Daylight, Energy and Indoor Climate Basic Book