Weathertightness of windows

The weathertightness of windows is a key concern for building owners and users alike. Traditionally, windows were installed into openings in masonry walls and guidance on specification of appropriate performance has evolved. This is now given in BS 6375 (Guidance and advice on performance of windows and doors) and is appropriate for applications in low-rise buildings. Windows are now used more extensively, for example, coupled together to form assemblies or within curtain walling and other forms of systemised building envelope, where their performance may be different. These applications often involve high-rise buildings where the exposure is greater than for low-rise buildings. CWCT has been developing guidance on appropriate testing and specification for these uses.

Where windows are installed as an assembly, the components used to join the windows need to be evaluated to ensure that the properties of the entire assembly are satisfactory. The main characteristics of the assembly that may be affected by the joining components are air leakage, rain penetration, structural integrity and thermal performance.

Alternative joining details should not be used unless they have been fully evaluated and tested.

Test procedures for windows are designed for testing single windows and guidance is being developed on appropriate procedures for assessing the joining components of window assemblies. Extensive window assemblies which extend over more than one floor should be considered as a form of curtain walling and tested accordingly.


A typical window assembly

When windows are incorporated into systemised building envelopes such as curtain walling, it is recognised that the interface used to join the window to the curtain wall should be tested as part of the curtain wall assessment. However, it is not always recognised that the performance of windows, when supported in a curtain wall, can differ from that when supported by more rigid forms of construction or a window test box. It is therefore desirable to test windows as part of the evaluation of the weathertightness of the wall system. This can require project-specific tests as the standard test procedures required for CE marking permit testing of windows and curtain walls as separate products.

In medium and high-rise buildings, remedial action in the event of water leakage is likely to be more difficult and hence more costly than for windows in low-rise buildings. Replacement of windows in systemised building envelopes can also be more complex than in traditional forms of construction. Windows in high-rise buildings may also be exposed to higher levels of water for the same wind pressure than low-rise buildings. It is therefore recommended that higher levels of performance are specified. Experience suggests that testing for watertightness under an air pressure equivalent to 25% of the design wind load gives satisfactory performance provided that the installation is carried out proficiently. This level of performance has been required by the CWCT Standard for building envelopes for many years.

A CWCT Technical Note presenting this guidance is expected to be published later in 2015.

For further information please contact Alan Keiller (01225 330945; E-mail:


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