By Kevin Underwood, Wood Window Alliance

With the Government announcing their plans for the construction sector to help fast track Britain’s recovery by ‘building back better, building back greener and building back faster’, there has never been a better time to look at the use of timber as a construction material. In this article I have chosen to focus on wood window frames as they are often viewed as a ’nice to have’ rather than a ‘must have’. If we are to truly place resource efficiency and sustainability at the heart of the built environment, then we cannot allow ourselves to keep specifying PVCu window frames. Even those that are produced with recycled plastic content, which we all agree is a positive step, have an environmental impact due to the energy consumption during the manufacturing process.

With the drive to adopt a circular economy approach to specifying, it’s crucial that both a product’s embodied carbon and operational carbon are taken into consideration. Independent research by Heriot Watt University shows that wood window frames made to the strict standards set by the Wood Windows Alliance (WWA), can have negative Global Warming Potential over their estimated service life. In the following sections I will explore how the timber used, the design and the manufacture of modern wood windows, enable the delivery of better value and whole life performance.

Enhanced performance of modern wooden windows

Independent research by Heriot Watt University demonstrates that not only do wood windows have a long-service life, often longer than other common window materials, but also provide lower whole life costs. In their study Whole Life Analysis of Timber, Modified Timber and Aluminium-clad Timber Windows, they looked at the Service Life Planning and Whole Life Cost of wood windows compared with other types of materials. The specification they used for wood windows was the standard set by the WWA  and it was found these modern, factory finished and glazed wood windows had an expected service life of between 56 to 65 years in average UK conditions.

Manufacturing criteria has played a strong role in extending the service life of wood window frames, encompassing the following key elements:

• Choice of sustainable, defect free, engineered or modified timber;
• Window design elements such as rounded edges, water shedding angles on horizontal surfaces such as sills and beads, and joint and end grain sealing;
• Flexible, micro-porous protective coatings applied under controlled factory conditions;
• Factory controlled drained and vented glazing systems suited to double or triple glazing units.

Minimal maintenance of modern wooden windows

The design and manufacturing factors described above have also contributed to modern wood window frames requiring less regular maintenance. The coating systems used today, particularly when applied by a factory controlled process enabling consistency in coverage, greater coating thicknesses and with managed drying conditions are vastly superior to finishes manually applied on site with a brush. WWA  members typically provide a 10-year guarantee for opaque coatings (paints) and a 7-year guarantee for translucent coatings (stains) for average UK climate conditions, meaning that a simple refresher coat is often all that is required to be applied to the window within the first decade of it being installed.

In their study Whole Life Analysis of Timber, Modified Timber and Aluminium-clad Timber Windows Heriot Watt University took levels of planned maintenance programmes into consideration in the calculation of Whole Life Cost of window frame materials. With a service life of up to 65 years in average UK climate conditions, wood windows made to the WWA specification were also found to have the lowest whole life cost.

Creating a better understanding of use of wood windows

It’s important that architects and specifiers working on both domestic and commercial buildings keep up to date with the evolution of timber products and avoid common misconceptions about wood. Correctly specified timber products are crucial to helping move the construction industry towards building more sustainable buildings that create the optimum environment for occupants, and capitalise on timber’s inherent properties as a construction material.

To help with continued learning the WWA have invested in an Interactive Wood Window and Door CPDi. The educational programme is broken down into bitesize chunks based on key themes, and each session can be taken alone, or as part of the wider series.

Four modules are currently available to access free of charge, these explore:

• the circular economy;
• the benefits of timber school buildings;
• building nature into architecture; and
• the natural evolution of the wooden window.

Providing practical guidance and support with specification, the modules offer a comprehensive overview of how high performance, quality buildings can be created with the use of timber products, with a specific focus on windows and doors.

Window Specification for the future

In their Roadmap to Recovery, the Construction Leadership Council places a focus upon reducing carbon emissions and improving the sustainability and resource efficiency of the construction and built environment sector. This will mean that during the specification process the value across the whole life of the building will need to be a top priority.

So, in addition to the procurement fundamentals of product testing and certification, a greater emphasis will be placed upon the environmental impact. Specifiers will be responsible for assessing a window’s contribution to the overall whole-life carbon building assessment, taking into account both embodied and operational carbon.

More details can be found in the Interactive Wood Window and Door CPDi. Visit https://windows.bwf.org.uk/ to find out more.