Energy-efficiency Guidelines for Timber and Alu-clad Windows

Wood Window Alliance 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.

How is energy-efficiency measured?

The energy-efficiency of a window 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 and are expressed in W/m2 K (Watts per square metre per degree Kelvin), with lower values indicating greater efficiency.

‘Whole window’ U-values, which measure the heat loss through the whole window, are the most accurate way of expressing this. Some manufacturers may quote ‘centre pane’ U-values, a measurement of the rate of heat loss through the glass alone that will show a lower figure.

Some manufacturers have had their windows tested by the British Fenestration Rating Council (BFRC). In this method, a window’s energy rating (WER) is determined by a formula that takes into account the total solar heat transmittance (heat gain) of the glass (usually referred to as ‘G-value’), the U-value of the window (the window frame and glass combined) and air leakage
through the window seals.

Currently the ratings range from a best of A+ to G.