Introduction

Windows are a fundamental component of any building. Not only do they allow us to view the environment around us but they also allow sunlight to enter, providing warmth and light. They 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.

This CPDi module explores the solutions that modern wood windows provide across a broad spectrum of project types and identifies key factors to take into consideration when specifying wood as a window material.

2

Why wood windows?

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 Wood Window Alliance has commissioned a number of studies on the environmental credentials of wood window frames made to their specification.
Timber sourced from sustainable forests not only means that more trees get planted than chopped down, but it’s a proven way to reduce carbon dioxide in the atmosphere. Wood products are carbon stores and growing forests are carbon sinks.
As part of our membership criteria, all WWA manufacturing members must hold either FSC® or PEFC™ chain of custody.

3

Why wood windows?

Lower carbon footprint

In a study undertaken by Davis Langdon to compare the embodied carbon emissions of WWA windows with equivalent PVC-U units (1), it was found that each wood window frame made to the WWA specification saved 89 kg CO2e when used instead of a comparable PVC-U window frame – this is a saving of around three-quarters of a tonne CO2e per average house (the equivalent of driving around 6,500 kms in a small family car).
Through a Heriot Watt University independent study, it was found that a typical wood window frame 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 frame 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.

Reference

(1) Comparison of Environmental Impact (CO2e) of Timber and PVC-U Windows, 2010

4

Wood windows - design through engineering

Recent years have seen significant developments in the design of wood windows, the manufacturing processes and materials used to produce them.

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

5

Wood window checklist

When choosing a wood window manufacturer consideration needs to be given to:

The type of timber species used to make the window frame
The type of coatings applied
The environment in which the window is made and the coatings applied
Test evidence of performance – covering thermal and weather performance and security
Responsibly sourced timber

6

Consideration of timber species

Choosing the wrong timber species can compromise a window’s functionality, safety, strength, and durability.

The Wood Window Alliance would recommend:

Using specially selected slow-growth timber grown in cold climates
Using a higher proportion of heartwood
Using engineered components incorporating defect-free, laminated and finger-jointed timber, increasing stability and reducing knots and resin exudation
Using modified timber, such as Accoya®

BS EN 942:2007 defines the standard for timber in Joinery and provides details of quality appearance, timber quality grades, permissible defects, knot sizes and moisture contents which help in the selection of the right species.

7

Consideration of timber coating application

The Wood Window Alliance would always recommend factory-finished wood windows to ensure durability and longevity of the coating performance.
Advances in factory-applied paint and stain finishing techniques provide finishing conditions that cannot be replicated by on-site painting.
Coatings are applied in factory controlled conditions indoors ensuring wet weather and high outdoor humidity do not lead to high moisture contents which hinder the absorption and adhesion of coatings.
Water-based coatings are more commonly used by window manufacturers and have lower environmental impacts than solvent-based paints. All WWA members use water-based coatings.

8

Test evidence – weather performance

Weather performance

In assessing the thermal performance of windows, the weather performance, or the weather tightness, of the window should always be considered in addition to the U-value of the glass units.

Whatever the frame material, there are performance standards for the basic weather and mechanical performance of all types of windows. In the UK the standard for this performance is BS 6375 which is divided into Part 1 for basic weather resistance and Part 2 for the window’s operation and strength characteristics.

When specifying wood windows, the WWA advocates that specifiers request to see performance test evidence, ideally verified by a third-party accreditation body, such as UKAS.

All WWA manufacturing members can provide test evidence for the products
All WWA manufacturing members have third party certification on at least product design to illustrate they meet the standards for BS644 set out in BS 6375, parts 1 and 2

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Test evidence – security performance

The WWA advocates that specifiers check that the window frames meet UK security standards. They can do this in two ways:

Ask for verified test evidence to PAS 24. PAS 24 is the standard for fenestration and ensures enhanced security performance requirements are met for doorsets and windows in the UK. PAS 24 sets out two alternatives for compliance:

1. Meeting the requirements of EN1627 RC2 for window
  OR
2. Meeting the requirements of the details contained in the annex of PAS24

Check to see if the window has Secured By Design certification

We hope you’ve found this module useful. If you would like to keep learning, click on one of the suggested modules or visit the Wood Windows Alliance website to see how one of our members can help you on your next project.

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