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Understanding U Values

There are many factors to take into consideration when specifying a glazing system, including the frame, the type of glazing itself, air permeability, as well as general aesthetics.

There are many factors to take into consideration when specifying a glazing system, including the frame, the type of glazing itself, air permeability, as well as general aesthetics.

However, one of the most important factors to consider at specification stage is the thermal performance of the glazing system, otherwise known as the overall as Ud value*. Call to action at bottom of article: *For more information on Ud values you may wish to read the NARM Technical Document NTD2 (2010)

The overall Ud value performance of a glazing system is one of the most frequent misunderstandings at specification stage. This is because a glazing system, such as a ridgelight or a mono-pitched rooflight, has two different U values - the centre pane and the overall system.

Many specifiers are simply unaware that the centre pane U value does not reflect the overall thermal efficiency of the system. This can be detrimental in some cases, especially when a system needs to comply with the Building Regulations for thermal efficiency.

U values are representative of the thermal performance of the glass or system - the lower the U value, the more efficient the unit is. Typically, double glazed units (DGUs) can boast a U value of anything from 1.4 to 0.9, and no other factors affect the centre pane U value of the glazing other than the type of material used to fill the gap in between the glazing sheets, which is usually dry-air or gas, such as argon or krypton.

However, the overall Ud value of a system does depend on many more factors than centre pane U values. Large ridgelights on vertical legs that encompass many individual rooflights may be ‘stitched’ together using a frame, which is usually made from timber, PVCu or aluminium material.

The type of frame used can vary the overall Ud value performance of the system. For example, an aluminium frame can raise the Ud value to anything upwards of 2.0, because the material used is a conductor and not an insulator. This means that the material loses heat as opposed to retaining it, thus is not considered efficient.

Taking the time to fully understand U-values will ultimately save time and confusion. Our team of experts are happy to answer any questions you may have about U values. Simply call us on 01670 354157 for more information.