Understanding Skylight Energy Performance: What is the U-value, the G-value and Energy Balance?

1. What is the G-value?

The measure of solar energy entering a building through the glazing is the total solar energy transmittance, or g-value. It is the ratio between the solar gain transmitted through the glazing and the incident solar gain on the glazing, expressed as a value from 0 to 1. Accessories, such as automatic or user-controlled shading, can work in combination with the glazing to give a dynamic g-value, which can therefore be changed in response to internal or external conditions.

G-value facts:

• Measures how much solar energy enters through your skylight
• Rated from 0 to 1 (higher numbers = more solar energy)
• Can be dynamically controlled with shading accessories

2. What is the U-value?

The U-value of a building component expresses the amount of energy transmitted from the warm side to the cold side. The lower the U-value, the less energy is transmitted. It is often the aim to reduce the U-value of building components in order to reduce the heat loss, and thereby the heating demand, of the building.

The U-value is expressed in W/m²K. In glazing constructions, heat is transferred from the inside through the insulating glass unit to the outside by radiation, convection (warm air rises, cold air falls) and conduction.

For both g-values and U-values, performance can be quoted for the whole glazing unit, or just the centre pane. As the names suggest, whole-unit values take into account both the glazing and the frame, while centre pane values refer to the glass only.

Centre pane values appear lower, because the effect of the frame is not accounted for. It’s important to make sure that like-for-like comparisons are made between different products - and that representative values are used in whole-building assessments. Read more about the U-value.

U-value facts:

• Measures heat transfer from warm to cold areas
• Lower U-values = better insulation
• Measured in W/m²K
• Heat transfers through:
  • Radiation
  • Convection (air movement)
  • Conduction

Sloped vs. Vertical Windows: What You Need to Know

Roof windows are installed on sloped surfaces, which means they have a higher U_w-value (a measure of heat loss) compared to vertical windows. This is because the gas between the glass panes moves more as the window tilts, increasing heat loss. The type of gas and the thickness of the gap between the panes also affect this movement. Generally, if the gap is around 10 mm or less, the slope doesn’t matter much. This higher U_w-value means more heat can escape through roof windows, affecting the building’s energy efficiency. However, roof windows also let in more sunlight and daylight because they are also exposed to a larger part of the sky than facade windows and are normally installed without any constructive shading. This can increase both solar gain and natural light inside the building.

Traditionally, the U-value is the single parameter used for evaluating the energy performance of windows. It is common practice to declare U_w for roof windows at 90°, i.e. as facade windows.

Even though heat transmittance increases with increased slope, passive solar gains increase even more. So, the vertical value leads to fairer indication of the performance than the sloped value. VELUX is striving to have the U-value of windows replaced by energy balance.