Wood is a natural insulator. The energy efficiency of wooden windows when manufactured to the high standards set by the Wood Window Alliance (WWA) exceed those required in the Building Regulations. Our members design and manufacture their windows to ensure they surpass the performance requirements defined in BS 6375 Parts 1 and 2 relating to weather tightness, operation and strength.

The type of glazing used in any window will impact the energy efficiency. Members of the WWA manufacture windows with double or triple-glazed units (with the exception of specialist period windows). Due to the strength of wooden window frames, they are particularly suited to triple-glazing. Depending on the window design, triple-glazed units are generally fitted in wooden window frames without the need for additional thick sections, helping to preserve the aesthetic integrity.

How to measure the energy efficiency of wooden windows

The energy-efficiency of windows is measured in two ways:

1. U-values – are a measurement of the rate of heat loss through a material. Most manufacturers quote ‘whole window’ U-values, which measure the heat loss through the entire window. A small proportion of manufacturers will quote the ‘centre pane’ which is a measurement of the rate of heat loss through the glass alone.
2. Window Energy Ratings (WERs) – were originally launched by the British Fenestration Rating Council (BFRC) in 2004. The rating system is based on a scale of G to A+, with A+ windows being the most energy-efficient. Under this system, a window’s rating is determined by a formula that takes into account the total solar heat transmittance of the glass (the ‘g value’), the U-value of the window (the window frame and glass combined) and air infiltration through the window seals.

Our members offer windows with an A+ to C Window Energy Rating (the glazing unit specified and window type will influence the rating) with whole window U-values down to under 0.8, Passivhaus standards.

Energy efficiency and solar gain

Increased solar gain is seen as ‘free energy’ and so high solar gain can be beneficial. However, to prevent overheating in the summer, a lower solar gain may be preferable. Windows with ultra-low U-values, such as Passivhaus windows, have relatively low solar gain thanks to triple glazing and the design of the window frame.

Considerations for the energy efficiency of wooden windows

At the WWA we recommend that the U-value, WERs and solar gain be taken into consideration when choosing the best fit for your project. To help you with this, we have produced an Advice Note – click here to download the Energy-efficiency guidelines for Timber and Alu-clad windows.