1 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 performance and quality benchmarks. All windows now have high-quality air seals and good thermal resistance. Thermally broken frames complement the new high performance glass units using 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 a viable alternative to PVCu and aluminium. Look for those with FSC or PEFC certification to ensure that the timber is sourced sustainably and specify windows backed by schemes such as the British Woodworking Federation's.
- Also available are building management system (BMS)-linked self-contained window operating systems, which automatically provide environmental control including synchronising 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 L. Commercially available low-emissivity argon-filled units can achieve 1.1 W/m2K in hybrid windows with a U-value of 1.5 W/m2K, which easily exceeds Building Regulation requirements of 2.0 W/m2K.
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 that allows different products to be compared. The standard is Rw=30–40dB depending on glass and cavity thickness but a value of Rw=58dB can be achieved.
Consider aluminium and PVCu windows when an architecturally sensitive profile is required.
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. Be clear regarding the glass performance, if toughening heat soaking is 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 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 enable control in strong winds.
Ensure safety glass is provided in accordance with Building Regulations part K. Consider use of toughened and laminated glass where glass has to be retained in place after breakage.
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 boatswains 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.
Building Regulations Parts K,L,M,N
Secured by Design www.securedbydesign.com
BS 644 Wood windows
BS 4873 Aluminium alloy windows
BS 6510 Specification for steel windows, sills, window boards and doors
BS 7412 Plastics 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