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U-values for windows explained

Posted by Bath Bespoke on 29 November 2017

Our guide to U-values for windows

U-values are becoming increasingly important for architects, builders and homeowners, as the need for energy efficient and sustainable housing becomes more significant.

U-values are a measure of the effectiveness of a material as an insulator. The lower the U-value number, the better the window is at keeping heat inside a building.

The value is measured in watts per square metre per degree Kelvin (W/m²K). For example, if we took a 12 mm double glazed window with a U-value of 2.8, for every temperature degree difference between the inside and outside of the window, 2.8 watts will be transmitted every square meter. Basically, the larger the U-value the more energy that is transmitted/lost from the inside to the outside.

U-values for windows

When making a window, it’s necessary to distinguish between U-values for materials. That being:

  • Uw (the overall U value of the window)
  • Ug (the U-value of the glazing)
  • Uf  (the U-value of the frame)

Uw relates to the entire window and incorporates the U-values for the glazing and the frame. It’s influenced by several factors, including the size of the window. Larger windows normally feature better values as U-values achieved in glazing are better than in the frame material. Therefore a larger glass area is able to produce a better thermal insulation value.

If you’d like more information about U values, please feel free to get in touch.