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.
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 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
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