Energy efficiency | Double or triple glazing

Our members manufacture some of the most advanced, energy-efficient windows and doors available in the UK, whether timber or aluminium-clad. All are double or triple-glazed (with the exception of specialist period windows) and can be specified with low-emissivity glass, gas-filled glazing units and warm-edge spacer bars.

They exceed the energy-efficiency requirements in the Building Regulations and the performance requirements defined in BS 6375 Parts 1 and 2, relating to weather tightness, operation and strength characteristics.

Our members offer windows with A+ to C Window Energy Ratings and with whole window U-values down to under 0.8, Passivhaus standard.

Which is better, wood or PVC-U? Both wood and PVC-U are good insulators and make equally energy-efficient frames. Higher levels of energy-efficiency are achieved through changing the specification of the glazing unit, rather than the frame material. Wood works better for triple-glazing, because of its strength.

The energy-efficiency of doors and windows is measured in two ways: U-values and Window Energy Ratings (WERs). U-values are a simple measurement of the rate of heat loss through a material. Most manufacturers quote 'whole window' U-values, which measure the heat loss through the whole window, although some quote 'centre pane' U-values, which are a measurement of the rate of heat loss through the glass alone. Some manufacturers may also have had their windows tested by the British Fenestration Rating Council (BFRC). In this case, a window's rating is determined by a formula that takes into account the total solar heat transmittance of the glass (usually referred to as 'g value'), the U-value of the window (the window frame and glass combined) and air infiltration through the window seals.

Solar gain can be a good thing (free heat from the sun) or a bad thing (overheating in the summer). Windows with ultra low U-values, such as Passivhaus windows, have relatively low solar gain thanks to triple glazing and the necessarily more robust frame sections required to support the heavy glazing units.

The WWA recommends that specifiers should evaluate the three individual elements of a window’s energy rating against specific architectural requirements in order to get the right product for individual buildings and projects.

This means getting the right balance between the thermal transmittance of the frame and glass (U-value), solar gain through the glass (G-value) and air leakage (L-Value).

Increased solar gain is seen as ‘free energy’, and so high solar gain is beneficial. However, there may be times when lower solar gain may be preferable - to prevent overheating in summer, combined with a low U-value to keep the heat in during the winter.

So you need to look below the top line rating to see whether it’s being achieved in a way that will suit your building.

For more information, download our Advice Note, Energy-efficiency Guidelines for TImber and Alu-clad Windows (2015)