Technical advances in window design are being made all the time, and specifiers need to keep abreast of the changes. Barbour Index and Scott Brownrigg outline five things to consider to get the best results

1 - Choice of material

Make sure the window is fit for purpose. Windows are available in aluminium, timber, PVCu, steel (including stainless steel) and hybrids such as aluminium/timber and plastic/steel. Scandinavian and German manufacturers set the performance and quality benchmarks. All windows now have high-quality air seals and good thermal resistance. Thermally broken frames complement the latest high-performance glass units, with their low emissivity and solar rejection coatings, inert-gas filling and low-transmittance edging. High-quality hinges and ironmongery now ensure a long window life. When specifying, consider the following:

  • Hybrid frames, such as timber with aluminium capping, provide high performance and good environmental credentials as well as long life.
  • Durable prefinished timber windows are an alternative to PVCu and aluminium. Look for those with Forest Stewardship Council or Pan European Forest Certification approval, or backing from schemes such as the British Woodworking Federation’s to ensure that the timber is sourced sustainably.
  • Also available are building management system-linked self-contained window operating systems, which automatically provide environmental control, and which can be synchronised with powered louvres.

2 - Compliance with rules and regulations

Make sure air leakage targets of 10 m3/m2 at 50 Pa are met when required.

The right window specification can help ensure compliance with Part L1/L2. Commercially available low-emissivity argon-filled units can achieve 1.1 W/m2°K in hybrid windows with a U-value of 1.5 W/m2°K, which easily exceeds the Building Regulation’s requirements of 2.0 W/m2°K.

Thermally engineered frames give minimal cold bridging, which eliminates the problem of condensation.

Ensure that your windows are of a sufficient acoustic standard to meet Part E when required. Acoustic reduction is often measured using the Rw weighting scale, which allows different products to be compared. The standard is Rw=30–40 dB depending on glass and cavity thickness but a value of Rw=58 dB can be achieved.

3 - Manufacture, installation and delivery

Windows are a factory-made component and high-quality performance can be expected. Principles of performance should be established early on with a preferred supplier. If using toughened glass, heat soaking may be required.

The interfaces between the window and the building must be sealed against air and water penetration. It is important that responsibility for such performance is understood by the supplier and contractor.

Ensure the method of delivery and installation is clear. Careful analysis will ensure minimal damage to the windows’ factory finish and safe installation. Specify fully wrapped windows and keep the wrapping on for as long as possible.

4 - Quality control and warranty

This is a competitive market. Always insist on built examples, preferably ones that have been in use for more than five years, and visit them if possible.

Guarantees vary with the material used. BWF-accredited timber windows have a minimum guaranteed life of 30 years. Warranties of 12 years are available on window systems including the double-glazed units. Seek insurance backing and check the small print.

Agree a quality control sample that matches the specification before going on site.

Ensure security by specifying to the requirements in DD ENV 1627 and Secured by Design.

Request test certificates for window performance. Certificates should cover the whole window including specific glass and hardware.

5 - Health and safety

Consider component sizes and the location – this is the designer’s job under CDM regulations. Internal installation may involve less risk than using external access equipment at high level.

Large opening lights hung on pivots or hinges require friction stays to keep them under control in strong winds.

Ensure safety glass is provided in accordance with Building Regulations Part K. Consider the use of toughened and laminated glass.

Windows forming a means of escape must comply with Building Regulations part B. Childproof locks are available in some window systems or as additional ironmongery for use where there is a risk of falling.

Consider how the windows are to be maintained. Safety access methods are described in BS 8213-1 and Building Regulations Part N. They cover important design considerations, including methods of external cleaning access such as ladder securing, use of safety harnesses, walkways, scaffolds and bosun’s chairs.

BS 8213-1 also addresses the design of windows to be cleaned from within the building. This is especially important in blocks of flats where high-level windows will be cleaned by the occupier and not professionals.

Door closer operating pressures are now covered by Part M and powered operation may be required.

Reference documents

Building Regulations Parts B, K, L, M, N

Secured by Design

British Standards:

  • BS 644 Wood windows
  • BS 4873 Aluminium alloy windows
  • BS 6510 Specification for steel windows, sills, window boards and doors
  • BS 7412 Plastic windows made from unplasticised polyvinyl chloride (PVCu) extruded hollow profiles
  • BS 6375 Performance of windows. Classification for weathertightness
  • BS 6375 Performance of windows. Specification for operation and strength characteristics
  • BS 8213-1 Windows, doors and rooflights. Code of practice for safety in use and during cleaning

Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services.

For further information contact Barbour Index on 01344-899280 or visit

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