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Most architects know that they like timber windows and that there is a feel-good factor to them. But quantifying the benefits can be tricky. This article, prepared by the British Woodworking Federation (BWF) and the first in a series supported by Wood for Good, adds the facts to the feelings.

Timber windows, whether painted or stained to show the natural grain, can add significantly to the aesthetic appeal of a building. Advances in manufacturing techniques mean that modern timber windows are available in a wide variety of opening designs; including multi-light casements; vertical-sliding sash and fully reversible and tilt-turn sash; and styles from traditional small panes to large contemporary designs. It is now possible to create a style of timber window to match virtually any building, whether for replacement or new build.

Over the last 10 years, the evolution of timber windows in terms of quality, durability and design has been dramatic.

UK manufacturers are now producing timber windows that will take single-, doubleor even triple-glazed insulating glass units to suit particular specifications and capitalise on advances in timber technology.

While the initial cost of timber windows may be higher than that of U-PVC, the 'whole-life' cost, in terms of durability, maintenance and environmental impact, is lower than that of other substrates.

This increases their financial appeal, particularly in social housing where the developer has a vested interest in longterm maintenance.

STANDARDS The main British Standard for timber windows is BS644 - Timber windows - factoryassembled windows of various types - specification. This refers to a number of other standards and codes of practice covering details of quality of timber, glazing requirements and so on. The simplest way for specifiers to ensure that their windows meet all these requirements is to specify those certified and tested independently. Certificationscheme requirements take into account all the relevant building regulations and British and European standards.

Most modern timber windows include basic security provisions. However, for enhanced security for windows where easy access is available from outside the building, windows that have passed BS 7950 should be specified. These products will usually incorporate multipoint locking systems for extra security and should be supplied factory glazed with laminated safety glass. If this BSI test is combined with third-party certification, such as the BSI kitemark, the windows can be used on 'Secured by Design' (SBD) registered developments.

SBD is the UK police flagship initiative, which supports the principles of 'designing out' crime by the use of effective crime prevention and security standards for a range of products and applications.

DURABILITY For timber windows, durability is probably the area where the greatest challenge existed and where most technological progress has been made.

The challenge for timberwindow manufacturers has been to make the barrier to the elements as effective and long lasting as possible and to make renewing that barrier simple and cost effective when it becomes necessary. No windows are maintenance free.

Properly maintained, in line with guarantee guidelines, timber windows are durable, with simple maintenance procedures restoring their original good looks.

The durability of timber windows has been enhanced by manufacturing processes that incorporate full factory finishing. Most timber windows are now manufactured from soft wood, which is classified as non durable. However, if finishing coats are applied and maintained correctly, there will be no deterioration of soft wood. With new spray coating systems, manufacturers apply a coating which is up to six times thicker than can be achieved by brush applying on site, because it is applied under factory-controlled conditions using specialist methods.

High-quality manufacturers, such as those certified within the Timber Window Accreditation (TWA) scheme, run by the BWF, offer guarantees of eight years (some as long as 10 years) on paint finishes, and five years on stains, before recoating is necessary.

When recoating does become necessary, all that should be needed is a rub-down of the surface and the application of a single coat of paint or stain.

Most timber windows from UK manufacturers have also been treated with preservative to improve their durability.

Recent developments have seen the introduction of new environmentally friendly formulations for preservative materials. These avoid the use of organic solvents and tend to use water as a dynamic carrier medium. In addition, application techniques have become more environmentally friendly, using dip-and-flow techniques as opposed to the more energy-consuming vacuum techniques.

Independent research by the BRE on behalf of the TWA scheme has found that the shorter the projection of sills, the longer the coating on timber windows will last. Stone, brick or tile sub-sills that allow the window to be recessed (now returned as a requirement of Approved Document L of the Building Regulations) protect the window and also provide protection to brickwork.

THERMAL EFFICIENCY The thermal efficiency of a window is affected by factors such as the frame material (which should itself be thermally efficient), the glazing system, insulating glass and draught sealing. The glazing unit (whether it is filled with air, argon or krypton) helps determine the amount of heat lost through the window, as well as the amount of heat gained from sunlight.

U values indicate the heat that is lost from the inside of a building to the outside, with a lower number indicating that less is lost. Timber itself is a natural insulator and timber windows are manufactured to produce a variety of thermal efficiency levels. A value less than or equal to 2.0W/m 2K is the recommended level for specification. The new Part L, which came into force this month (April 2006), also refers to window energy ratings; a more accurate calculation of energy efficiency that takes air leakage and solar gain into account. Window energy ratings follow an A-G scale, with E being the equivalent of a 2.0 U-value and C or above qualifying for the Energy Savings Trust's Energy Saving Recommended blue triangle.

SUSTAINABILITY Timber is the only truly sustainable construction material, because it is renewable, and it is now generally accepted as the best environmental choice for windows. Greenpeace, in its 1997 Briefing Number One, Installing New Windows, stated: 'As long as timber is sourced from properly managed forests and care is taken in the choice of preservatives, paints and stains used on them, timber windows are by far the best environmental choice.'

Specifiers should request that timber can be proven to have come from a sustainably managed forest, preferably by means of a certification scheme such as FSC (Forestry Stewardship Council) or PEFC (Pan-European Forest Certification).

The development over the last 10 years of forest certification schemes, which confirm that forests are managed sustainably, has created a means of tracking timber back to its source.

Timber-window manufacturers must be able to demonstrate that the raw material they are using has been sourced from sustainably managed forests. Copies of certificates or other documentation showing the chain of custody all the way from the forest to the supplier should always be available to confirm timber's sustainability credentials.

Ideally, the manufacturer should have their own chainof-custody certification through the manufacturing process to maintain traceability.

COMPOSITE WINDOWS Some of the technological innovations that have developed in north America are now being marketed in the UK.

These include aluminium and even PVC cladding, to give greater durability on the external face coupled with the aesthetics of wood on the interior. The first wood-plastic composite window, which is claimed to combine the water resistance of U-PVC with the aesthetic appeal of timber, was launched two years ago. It is not clear how far these products have penetrated the market.

The TWA scheme represents manufacturers of timber windows and entrance doorsets who have undergone a stringent accreditation process. A series of fact cards published by the TWA provides information on a range of subjects, including specification, sustainability and maintenance.

For further information, call the British Woodworking Federation on 0870 458 6939, email windows@bwf. org. uk or visit www. bwf. org. uk For more information on Wood for Good go to www. woodforgood. com

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