What is a U Value?

A ‘U value’ is the term frequently used to describe the thermal performance of glazing and window installations. This term ‘u-value’ (or U factor) isn’t a very specific term and can be used to describe various elements of thermal performance. All elements of U factors are generally calculated using BS EN 673:1998, the standard test method for determining thermal transmittance in glass and glazing.
The calculated value indicates the heat loss through a material or installation and is measured in units of W/m2K. When the ‘U value’ decreases it represents a better thermal performance, so the lower the U value the better the thermal performance. There are various elements of ‘U values’ that are relevant when thinking about the thermal performance of glazing and glass installations.

  • Ug Value = indicates the thermal performance of the glass panel.
  • Uf Value = indicates the thermal performance of the framework.
  • PSIG = indicates the thermal performance of the glazing spacer.

These thermal performances are combined together with the sizes of the opening/framing to give you an overall U value for the installation referred to as the Uw value.

internal glazing

It is the Uw value that is important when attempting to achieve good thermal performance on glass installations, for instance it is the Uw value that matters when looking to achieve levels of Code for Sustainable homes, and in 2013 building regulations will change to ensure that all new build glazing elements have a maximum Uw value of 1.5 and refurbishments 1.6 W/m2K.
There are various ways of improving the Uw value of an installation. Obviously improving the individual U factors in the calculation will improve the overall thermal performance. Using things like ‘Super Spaces’ will improve the Psi value and using a higher performing glass panel.
It is also important to note that all standard Uw, Ug or Uf factors given for glass and installations are generally standardisations, the size and finishes do affect the thermal performance of the installation. This can be calculated for you using a standardised Uw calculator.

Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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