Thermally efficient windows & doors - Information Hub Green Building Store

Thermally efficient windows & doors

What are U values?

PROGRESSION windows at Vowchurch Passivhaus

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

PERFORMANCE triple glazed timber windows and doors at Sheffield retrofit project

Building regulations, BREEAM and Passivhaus standard can all have requirements for the whole window U values of windows and doors. Green Building Store ranges of windows and doors can help you meet these requirements, whatever the U value you need for your project.

Whole window U values of Green Building Store ranges*

1.1 W/m2K – Sliding sash (triple – argon-filled)

0.95 W/m2K – Sliding sash (triple – krypton-filled)

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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