If you haven't experienced modern high performance windows before, you'll be amazed at the difference they make to the warmth of your home - and to reducing condensation.

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 windows are more energy-efficient - timber 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.

How is energy-efficiency 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. Some manufacturers may quote 'centre pane' U-values, which are a measurement of the rate of heat loss through the glass alone; 'whole window' U-values, which measure the heat loss through the whole window, are a more accurate way of expressing this.

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.

Just a word about solar gain. 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 due to triple glazing and the necessarily more robust frame sections required to support the heavy glazing units.

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