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Everything you need to know before buying Windows, Doors or a Conservatory

Window & Glass U-Values

What is a U-Value?

A U-Value is a measurement of insulation in general the lower the value the more efficient the window as less fabric heat is lost through the glass or frame.  U-values are measured in  w/m2K which is the units of watts per square metre per degree Kelvin or in other words the amount of heat loss through a 1 square metre of material for every degree difference in temperature between the inside and outside.  As a measurement to compare to a single 4mm pane of glass center value is 5.4w/m2K.

How are U-Values Calculated?

Q = U x A x dt


U =      Q      


Q = Heat transfer (Watts)
U = U-value (W/m2degC)
A = Cross sectional area of glass(m2) 
dt = Temperature difference between inside & outside (degC)

Center Pane VS Whole Window U-Value

One of the most misunderstood concepts in double glazing is u-values and how they are measured. below shows the difference in areas that are tested.

           Center Pane           Whole Window

Center Pane or whole window Calculations, everything you need to know about U-Values

The center pane only takes in the center of the glass, where as a the whole window calculation takes in the glass, the spacer bar, the window outerframe and sash PVC as well as the reinforcing. The difference between a center pane and whole window can be anywhere between 0.2 and 0.4 so just make sure the when you compare value they are using the same measure. 


  • 1000mm x 1000mm Fixed pane Window
  • 3 Chamber 65mm uPVC profile
  • K Glass
  • Standard Reinforcing

Center Pane U-Value = 1.72 w/m2K

Whole Window U-Value = 2.0 w/m2K

The table below show the typical Centre pane u-values given by Pilkington

Inner Pane

Spacer Bar

Outer Pane


G Value

U - Value

4mm Clear

6mm Aluminium

4mm Clear




4mm K Glass

6mm Aluminium

4mm Clear




4mm K Glass

6mm Aluminium

4mm Clear








Pilkington SpectrumDownload

Free Pilkington Spectrum U-Value Software Click here to Download


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