BWF members such as yourselves frequently use our Member and Technical Helplines for assistance on a wide range of Employment, Health and Safety, Contractual and Technical issues. We’re putting up a frequently asked question every Tuesday in order to help members and illustrate the type of advice we are able to provide. If you have any questions that you would like to see featured, we would be happy to assist. This question and many others like it can be found in our Question Centre. If you want further advice on any of the issues, don’t hesitate to ring the helpline or browse our extensive publications library, which includes our information on Installation, Building Regulations and Maintenance. This week: Does the Glazing Unit affect a window’s performance? The performance of a window is dependent on the quality of the glazing. Here are some factors to take into consideration. One of the essential components of a window is the IGU (Insulating Glass Unit) and the contribution the IGU can make to the performance of the window is greatly affected by the method used to fix it into the glazing aperture. The glazing method secures the IGU in place, contributes to all elements of the window’s weather performance, affects the air permeability of a building (which is important if a new building is to be pressure tested may be called upon to form part of a system which provides protection from falling and can affect the level of security provided by the window. The choice of IGU is important. It needs to be robust where it will be used in a ‘critical location’ described in Approved Document N, primarily at low-levels and in, or near doors, and provide ‘containment’, as described in Approved Document K, where it is used, at low-level, to protect people from falling. The IGU is also the major contributor to the thermal performance of the window, as required in Approved Document L, affecting both insulation and solar heat gain. The choice of IGU will also play a part in the window’s acoustic performance and fire performance. If a window is fitted in a building located in an area which has a high wind speed the ‘windload’ on the IGU can be considerable. The map of basic wind speed for the UK is centred on Oxford, which has the lowest wind speed and then radiates outwards with increased wind speeds in Cornwall, Norfolk and Scotland. The basic wind speed is increased with altitude and the proximity to the top of hills, but is generally reduced in towns. With the accelerating effects, the windload on an IGU could be over 3000 Pa; however, most windows will only experience windloads of around 1200 Pa. But even at this reduced level, the load would be the equivalent of a person standing on the IGU. Whatever the load, the glazing method will need to hold the IGU in place and allow the window to perform to the levels it would have achieved when it was first made. If, under load, and the resulting distortion of the window, the IGU is dislodged, gaps can be created in the glazing system which will let in air and water. So the glazing system needs to be strong enough to hold the IGU in place at the expected loads and this strength will be provided by the combined affects of the sealant or tape used between the IGU and the rebate upstand and bead, the sealant, if applied, in the cavity between the edge of the IGU and the rebate platform, and the mechanical fixing, sealant or adhesive used to secure the beads. Throughout its life, an IGU will be subtly moving. Changes in air pressure and temperature will cause the two panes of glass to move together or move apart and changes in windload, which can be a positive or negative pressure depending on the wind direction, will regularly push and pull the IGU. This constant movement needs to be accommodated by the glazing system and this is best achieved by sealants, tapes or gaskets, which are wide enough and flexible enough to allow for the movement without becoming detached from the frame or the IGU. Under conditions where there is wind and rain together, the wind pressure will force the rain water into any gaps in the glazing system and this could possibly come through into the property. In drained and ventilated systems the effects could be exacerbated as the cavity between the edge of the IGU and the rebate platform is open to the wind pressure and so only the internal seal is effective against air leakage and this can additionally affect internally beaded systems. In addition to being robust enough to withstand the elements, the glazing system will also need to provide security. Under the timber window standard, BS 644, windows are required to offer a basic level of security, partly assessed by a deglazing test which the glazing system will need to be robust enough to withstand. If the IGU is intended to form part of a window which is providing an enhanced level of security, for Secured By Design for example, the standard glazing process may need to be augmented with glazing clips or other devices to mechanically hold the IGU in place.