Understanding skylight energy performance: What is the U-value, the G-value and energy balance?

A building’s carbon dioxide emissions and energy use are heavily influenced by the balance of heat loss against solar gains. Therefore, when designing a building, it is helpful to understand the relation between thermal transmittance, glazing and the different levels of thermal performance.

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

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 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.

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), and conduction

Sloped vs. vertical windows: What you need to know

Roof windows are installed on sloped surfaces, which means they have a higher U-value (a measure of heat loss) compared to vertical installation. 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-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 obligatory by the Construction Product Regulation (CPR) to declare the CE-marked Uw for roof windows at 90°, i.e. same as façade 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.

Energy balance: The complete picture

The term energy balance is used to describe the energy characteristics of a window. The intention is to communicate the balance between solar gain and heat loss. Energy balance is calculated as the sum of usable solar gain through the window during the heating season minus any heat loss. Energy balance is a more accurate way of describing the energy characteristics of a window than the U-value alone, as energy balance includes both Uw-value and g-value to provide a more complete picture.

What to consider:

• Building type and design
• Window orientation
• Geographic location
• Balance between solar heat gain, natural light benefits and heat loss concerns

Professional insights: Making the most of your skylights

In conclusion, skylights play a crucial role in a building’s energy dynamics by influencing both heat loss and solar gains. Understanding the interplay between g-values and U-values is essential for optimizing energy performance. While skylights can increase heat loss due to higher Uw-values, they also enhance solar gain and daylight, contributing to a more energy-efficient and comfortable indoor environment. By focusing on the energy balance, which considers both solar gain and heat loss, designers can make informed decisions that maximize the benefits of natural light while minimizing energy consumption. Ultimately, the strategic use of skylights can lead to significant improvements in a building’s overall energy efficiency and sustainability.

1. Consider both U-value AND g-value when selecting skylights
2. Factor in installation angle for accurate performance estimates
3. Look at overall energy balance rather than individual metrics
4. Account for your specific climate and building needs

Source: Daylight, Energy and Indoor Climate Basic Book