Whole Window U-Values Explained: Why They Matter When Choosing Windows

When comparing windows, you will often see a U-value quoted for the glass. But is that the number you should use to compare one window with another?

Not necessarily. For an accurate picture of a window's thermal performance, you need to consider the U-value of the complete window (Uw) - not just the glass. The whole window U-value takes into account the combined performance of the glass, frame and edge of the glazing, giving you a much more realistic indication of how much heat the complete window allows to pass through.

For homeowners building or renovating with energy efficiency in mind, understanding the difference can help you make a more informed decision.

Key Takeaways

• The lower the U-value, the better the thermal insulation.

• The glass U-value (Ug) describes the performance of the glazing only.

• The frame U-value (Uf) describes the performance of the frame - important when comparing!

• The whole window U-value (Uw) combines these elements to provide a more realistic measure of window performance.

• Windows with the same glass specification can have very different whole-window U-values.

• Always compare Uw values when assessing the thermal performance of complete windows.

A U-value measures how readily heat passes through a building element. It is expressed in W/(m²K). Put simply: The lower the U-value, the better the insulation.

For windows, however, there are several different U-values, and understanding what each one measures is important.

There isn't one single U-value that is suitable for every project. The right specification depends on the building, climate, orientation, glazing area, energy-efficiency targets and project requirements.

For a high-performance or Passivhaus project, very low Uw values may be particularly important. However, thermal performance should always be considered alongside other factors such as solar gain, daylight, ventilation, acoustic performance and overheating risk.

can achieve Uw values as low as 0.62 W/(m²K), depending on the product and configuration. This demonstrates why it is worth looking at the performance of the complete window rather than simply choosing the glass with the lowest Ug value.

Not necessarily. Thermal insulation is important, but a window is a complete system. A good specification should also consider:

• Solar heat gain

• Natural daylight

• Acoustic performance

• Security

• Ventilation

• Frame material

• Installation

• Durability

For example, a window with excellent thermal insulation may not be the right choice for every orientation or climate if solar gain and overheating aren't considered. The best-performing window is therefore not necessarily the one with the lowest U-value, but the one whose overall performance is appropriate for the building.

When comparing windows, ask for the Uw value of the complete window, and make sure you are comparing equivalent sizes and configurations where possible.

Don't rely solely on Ug = glass performance. Instead, look for: Uw = complete window performance

This simple distinction can make it much easier to compare window systems accurately and understand what you are actually buying. However, the Uw-value as an overall measure of the window might hide a poor performing frame. So always get the Uf-value from manufacturers. That way, you can make the best informed decision when buying , French doors, and particularly large glazing like

• What is the difference between Ug and Uw?

  Ug measures the thermal performance of the glass, while Uw measures the thermal performance of the complete window, including the glass, frame and glazing edge.

• Is a lower Uw value better?

  Generally, yes. A lower Uw value means less heat passes through the window, indicating better thermal insulation.

• Why can two windows with the same glass have different U-values?

  Because the frame and glazing edge also affect the thermal performance of the complete window. Different frame materials and constructions can therefore produce different Uw values even when the same glass unit is used.

• What is a good Uw value for a Passivhaus window?

  Passivhaus projects typically require very high levels of thermal performance, but the appropriate Uw value depends on the specific project, including the window size, orientation and overall PHPP model. The complete window should therefore be assessed as part of the building design rather than in isolation.

If you require further information, please contact the experts at , who will be able to explain the difference between windows to ensure that you make the right choice for your project.