Thermal efficiency
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, Code for Sustainable Homes 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*
1.4 W/m2K - Ecocontract (double)
1.1 W/m2K - Ecocontract (triple - door)
0.92 W/m2K - Ecocontract (triple - inward wndow)
0.92 W/m2K - Ecoplus3 (triple, oak)
0.9 W/m2K - Ecocontract (triple - outward wndow)
0.86 W/m2K - Ecoplus3 (triple, pine)
0.77 W/m2K - Alu2wood (Passivhaus certified)
0.75 W/m2K - Ecopassiv (Passivhaus compatible)
0.72 W/m2K - Frostkorken (Passivhaus certified doors)
0.68 W/m2K - Alphawin (Passivhaus certified)
0.65 W/m2K - ENERsign (Passivhaus certified)
*Overall U values to EN12567-1/ EN10077-2
BFRC energy rating
The British Fenestration Rating Council
(BFRC) scheme is the UK’s national system for rating the energy
efficiency of windows and is recognised within the Building Regulations
as a method to show compliance. BFRC window energy ratings use a
traffic-light style A-E ratings guide similar to that used on ‘white’
goods (such as fridges, freezers, washing machines etc). As well as
taking U values into consideration, the BFRC energy ratings also take
into account the nominal solar heat gain provided by windows.
Every window has in practice an 'energy balance' which balances the heat loss through thermal transmittance and air leakage with the heat gained from the sun. The BFRC scheme rates an 'average' window and gives a nominal energy balance, the Energy Index to this. Where this is positive, nominally the window gains more energy than it loses. Of course in practice this will depend on the aspect of the windows, balance of north facing to south facing etc. If the nominal balance is above zero (ie positive) the the window gets an A rating. Many argue that the rating is set too low and that there is no differentiation between an A rating with an Energy Ratio of 1 and an Energy Ratio of 30 (which might be achieved with a good Passivhaus standard window).
Ecoplus3 triple glazed windows achieve an A rating under the scheme.
Passivhaus requirements
Passivhaus design in Germany generally requires whole window U values of significantly less than 0.8 W/m2K*
Green Building Store offers the following options:
0.77 W/m2K - Alu2wood (Passivhaus compatible)
0.75 W/m2K - Ecopassiv (Passivhaus compatible)
0.72 W/m2K - Frostkorken (Passivhaus certified doors)
0.68 W/m2K - Alphawin (Passivhaus certified)
0.65 W/m2K - ENERsign (Passivhaus certified)
*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.
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
