29 June 2020

A natural evolution: the wooden window

TRADA image

Windows are a fundamental component of the fabric of any building. Not only do they allow us to view the environment around the building but, in so doing, they allow sunlight to enter the building, providing warmth and light. Windows insulate the building’s fabric, both thermally and acoustically, and while they provide controlled ventilation, which is essential for maintaining indoor air quality, they resist the uncontrolled ingress of air and water.


Windows also form part of the physical security of buildings and contribute to the safety of people who visit, live or work in the building, providing a potential escape route in the event of a fire and protection from falling and impact injury if the window contains low-level glazing.


This is a big role to fulfil, and modern wood windows successfully provide solutions across a broad spectrum of project types.


Performance through engineering

Over the past two decades there have been significant technological advances in the wood windows industry, from design to the manufacturing processes and materials used in their production. The development of engineered and modified timber components, and improvements in the formulation of coatings and the factory application of these, mean that wood windows provide levels of strength, durability and stability that meet the tough challenges presented by any commercial or domestic project brief.


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 they also provide lower whole-life costs. In its study Whole Life Analysis of Timber, Modified Timber and Aluminium-clad Timber Windows, Heriot Watt University looked at the service life planning (SLP) and whole life cost (WLC) of wood windows compared with other types of materials. The specification used for wood windows was the standard set by the Wood Window Alliance (WWA) and it was found that these modern, factory- finished and glazed wood windows had an expected service life of between 56 to 65 years in average UK conditions.


This, for example, is almost double that of PVC-U window frames, which were found to have an expected service life of 35 years in the same average climatic regimes.


Manufacturing criteria have 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, microporous protective coatings applied under factory-controlled conditions
  • factory controlled drained and vented glazing systems suited to double or triple glazing units.


The above factors 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 superior to finishes manually applied on site with a brush. WWA members typically provide a 10-year guarantee for opaque coatings (paints) and a seven-year guarantee for medium- or high-build 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 if maintenance is carried out before any visible signs of coating degradation are evident.


In the same study, Heriot Watt University took levels of planned maintenance programmes into consideration in the calculation of WLC 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 WLC.


How manufacturing standards influence service life

The measures listed below are part of the Wood Window Alliance (WWA) specification for wood window frames and all work together to contribute to the expected service life of 56 to 65 years.


  • Improvements to the timber in the windows by using:
    • specially selected slow-growth timber grown in cold climates
    • a higher proportion of heartwood
    • engineered components incorporating defect-free, laminated and finger-jointed timber, increasing stability and reducing knots and resin exudation.
  • Improved component design, using a slight slope – especially to the horizontal sections of the windows – to encourage water run-off to prevent standing rainwater, and water ingress into vulnerable areas.
  • Rounded edges (arrises), rather than sharp edges, to prevent the coatings thinning at these points, thus maintaining durability.
  • End-grain sealants prior to assembly, to prevent water ingress into vulnerable areas around and into joints.
  • Timber treatment systems, to extend durability when required.
  • Opaque and translucent coatings applied in a controlled environment in the factory, providing more consistent protection than brush-applied finishes.
  • Improved drained and vented glazing systems, increasing glazing unit service life.


The sustainable choice

To achieve net-zero carbon emissions by 2050, wood as a construction material is increasingly being specified for low- carbon emission homes.


Over the years, the WWA has commissioned several studies on the environmental credentials of window frames made to its specification, to inform specifiers of the overall carbon footprint of members’ products. As part of membership criteria, all WWA manufacturing members must hold either FSC® or PEFC™ chain of custody certification. Timber sourced from sustainable forests not only means that more trees get planted than are felled, but it’s a proven way to reduce carbon dioxide in the atmosphere. Wood products are carbon stores and growing forests are carbon sinks.


In a study undertaken by Davis Langdon to compare the embodied carbon emissions of WWA windows with equivalent PVC-U units, it was found that each wood window made to the WWA specification saved 89kg CO2e when used instead of a comparable PVC-U window – this is a saving of around 0.75 tonnes CO2e per average house (the equivalent of driving around 6,500km in a small family car).


The Heriot Watt University independent study found that a typical wood window made to WWA standards has a negative global warming potential over its estimated 56- to 65-year life service. Planned maintenance prolongs the life of the window and its carbon store effect, reducing the impacts caused by new replacements. Wood is the only window frame material that can be repaired in such a way.


About the author

Kevin Underwood, Technical Director, British Woodworking Federation


This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here


More information on the standards set by the Wood Window Alliance