Thermal efficiency
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.
U values measure how well heat is transferred by the entire window - the frame, sash and glass - either into or out of the building. The lower the U-factor number, the better the window will keep heat inside a building on a cold day.
1.4 W/m2K - Ecocontract (double)
1.0 W/m2K - Ecoplus (triple, argon)
0.9 W/m2K - Ecocontract (triple)
0.8 W/m2K - Ecoplus (triple, krypton)
0.75 W/m2K - Ecopassiv (Passivhaus standard)
0.65 W/m2K - ENERsign (Passivhaus certified)
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).
Currently our double glazed Ecoplus windows are rated under the scheme and achieve either an A or a B rating, depending on the glazing specification. We intend to rate our triple glazed (now standard window) shortly, and anticipate A rating for all glass specifications and a higher positive Energy Index than the double glazed version. Until we do so, should you require a rated window then double glazing will need to be specified, and we will offer this as a special order.
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:
0.75 W/m2K - Ecopassiv
0.65 W/m2K - ENERsign (Passivhaus Institut 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.