PROUD SPONSOR OF GLASS & SEALED UNITS EXTRA
Glass & Sealed Units Extra
The Uw value : The measure of a window ’ s energy loss The Uw value determines the loss of heat that passes through a building ’ s structural elements from inside to outside . It describes the flow of heat that is transferred through a 1 m ² component from inside to outside with a temperature difference of 1 Kelvin ( K ). The Uw value is indicated in W / m ² K . The smaller the value , the more efficiently the component is protected against heat loss , allowing considerable costs to be saved in the long term .
It is therefore important to know the precise Ug value of the double or triple glazing and the Uf value of the aluminium , wood , PVC or wood / aluminium window frame . Ultimately , this is the only way to determine the precise Uw value of a window , which is composed of the area-weighted values of glazing and frame as well as the length-related contribution of the linear heat transfer coefficient Psi .
A sample calculation with a double sash aluminium window measuring 1.23 x 1.48 m shows the change in the Psi value as a function of Uf and Ug – once with spacer bars made from aluminium and once with SWISSPACER Ultimate :
Meaningful documents
On the basis of the values determined by ift Rosenheim , Bundesverband Flachglas produces objective data sheets for each of eight representative Psi values per warm edge spacer bar system . These can be found at https :// www . bundesverband-flachglas . de / en / downloads / bf-datenblaetter-fenster .
The explanations on the data sheets refer to the principles of the values shown : The equivalent thermal conductivity was determined in accordance with ift guideline WA-17 / 1 “ Thermally improved spacer bars – Determining the equivalent thermal conductivity through measurement ”. This confirms that the indicated values – in comparison to the merely “ declared ” values – are based on measurements and calculations . The basis for this is ift guideline WA-08 / 3 “ Thermally improved spacer bars – Part 1 : Determination of representative Psi values for profile sections of windows ”.
The only recommendation , therefore , is to opt for spacer bars from manufacturers that have gone through this independent testing procedure and are able to present a data sheet from Bundesverband Flachglas .
In order to determine the U value of a window , this is broken down in idealised terms into
a ) the glazed area ( U g ) b ) the frame ( U f
) and d ) the transitional area between the two , the glazing edge ψ ( Psi ).
From these components , U w is calculated as required by the standard EN 10077-1 in accordance with the following formula :
U w
= ( A g ⋅ U g
+ A f ⋅ U f
+ l g ⋅ ψ g
) / A w
U w
U value of the window [ m ²] ( w = window )
Example : Double sash aluminium window ( 1.23 x 1.48 m ) with a frame width of 0.11 m , calculated with CALUWIN Version 0.134.59 .
© SWISSPACER
A w
Area of the entire window incl . frame
A g
Visible glazing area [ m ²] ( g = glass )
U g
Undisrupted glass U value [ W / m ² K ]
A f
Area of the frame [ m ²] ( f = frame )
U f
U value of the frame [ W / m ² K ] l g
Size of the glazing unit ( length of the glazing edge ) ψ g
( pronounced Psi )
Thermal bridge loss coefficient of the glazing edge in W / mK
Another example was calculated with a double sash PVC window measuring 1.23 x 1.48 m :
Example “ Data sheet of Psi value for windows ” of Arbeitskreis Warme Kante (“ warm edge ” working group ) for SWISSPACER Ultimate .
© Bundesverband Flachglas / Arbeitskreis Warme Kante
Example : Double sash PVC window ( 1.23 x 1.48 m ) with a frame width of 0.11 m , calculated with CALUWIN Version 0.134.59 .
© SWISSPACER
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