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What's the U-value of windows?

Posted by Sharon Russell-Verma on 3 July 2015 at 4:15 pm

If you are considering buying new windows for your home, you may have come across the term the ‘U-value of windows’, but what exactly is a U-value and how do you use it? 

In this case the U-value is a measurement of a window’s ability to retain heat and thus keep the building at a stable temperature. Now for the more technical definition; the U-value, sometimes referred to a thermal transmittance, is the rate at which heat is lost through the window and is measured in W/m²K (watts per square metre, per degree Kelvin). It reflects the rate at which heat transfers through 1 square metre of a structure, where the temperature difference between the inner and outer face is 1 degree Celsius. U-values are important measures for understanding the amount of energy that is needed to keep a comfortable inside temperature.

Furthermore, U-values are also used to calculate heat loss in other components of a building such as doors, walls, roofs and floors. The U-value is also referred to as an ‘overall heat transfer co-efficient’ and measures how quickly parts of a building transfer heat. Thus, a higher U-value signifies a less energy efficient building, whilst conversely a lower U-value indicates a high level of insulation. The same is true of U-values for windows; hence a lower U-value means a more energy efficient window.

The glazing/window industry utilise two types of U-values:

  • Ug - is the U-value of the glass pane (also known as centre pane), 
  • Uw - is the U-value of the entire window, in other words both the glass and window-frame together.

The Uw value is most commonly used as Building Regulations have requirements for the whole window U-values of windows and doors. Since 1 October 2010, all window manufacturers are required to comply with the energy efficiency requirements from Part L of the Buildings Regulations which specify that U-values must be 1.8 W/m²K or lower (whole window). Each manufacturer/supplier should be able to provide the fixed U-value for their window products and it is worth double checking that the U–value quoted refers to the entire window rather than just the glass pane itself. Thus in summary, U-values are useful tools that allow consumers to compare energy ratings of windows based on their energy performance. 

To find out more about energy efficient windows see our recent blog 'What to look for in an energy efficient windows’.

Photo: Superhomes

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