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Thermally Efficient Windows & Doors

What are U values?

U-Values measure how well heat is transferred by the entire window/ door – the frame, sash and glass – either into or out of the building.  The lower the U-Value number, the better the window or door will keep heat inside a building on a cold day.

Two different U-Value figures are used in the glazing industry:

  • Ug = thermal transmittance of the glazing itself (or the ‘centre-pane’ U-Value)
  • Uw value = thermal transmittance of the whole window – including glazing and frame.

Whole window U values

Building regulations, BREEAM and Passivhaus standard can all have requirements for the whole window U values of windows and doors. 

Building Regulations

L1A (new buildings): The backstop is 2.0 W/m2K, but 1.4 W/m2K is recommended for the notional building Target Emission Rate (TER). 

L1B (retrofit) 1.6 W/m2K or Window Energy Rating C.

Passivhaus requirements

Passivhaus design in Germany generally requires whole window U values of less than 0.8 W/m2K*

*It is not necessarily required to use a Passivhaus Institut certified window when designing or certifying a Passivhaus.  The actual requirement for Passivhaus Institut certification is either that the Uw value should be < 0.8 W/m2K when modelled with glazing Ug of 0.7 W/m2K, or that the Uw installed should be < 0.85 W/m2K with the same glass, and including the linear thermal bridging coefficient of the installation.

Whole window U values of Green Building Store ranges*

Green Building Store ranges of windows and doors can help you meet these requirements, whatever the U value you need for your project.

0.85 W/m2K – PERFORMANCE (triple – outward/ inward window)

0.75 W/m2K – ULTRA (solid frame – outward/ inward window)

0.68 W/m2K – ULTRA (insulated frame, Passivhaus certified – outward/ inward window)

0.68 W/m2K – PROGRESSION Passivhaus certified  A-rated component

*Overall U values to EN12567-1/ EN10077-2

More information on Passivhaus windows & doors

Positioning of windows & doors

In low energy and Passivhaus projects detailing around window installation is critical. Care must be taken to:

  • Minimise thermal bridging
  • Ensure continuity of insulation by wrapping insulation around the frame
  • Pay attention to junctions with the wall to achieve high levels of airtightness

More information

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