Shadow boxes have been used in curtain wall construction for a number of decades. Their use is becoming more widespread, not just in commercial buildings as before, but also in residential construction. This has prompted insurers such as the NHBC to take a keener interest, and this Technical Note has been produced in response to that increased interest.
A shadow box generally consists of clear glazing, a cavity behind the glazing and an insulated panel or tray.
Shadow boxes are used for the particular appearance they give. The cavity behind the glazing adds depth to the appearance, creating greater visual interest than can be achieved with a more typical glazed spandrel. In addition, they allow the use of the same glass as that used in the vision areas, thus giving visual continuity between the different zones of the façade. The appearance will depend on the glass type and coating, the depth of the cavity and the colour and texture of the back panel. The appearance is normally established via a visual mock-up.
The principal issue with shadow box construction is how to deal with the ventilation in the cavity behind the glazing, and a number of conflicts must be fully considered. Various strategies have been used to ventilate the cavity of a shadow box, depending on the climate and the ventilation strategy of the building.
Openings are required for a number of reasons. Firstly, they will help control condensation in the cavity by removing water vapour, thus lowering the vapour pressure and therefore reducing the risk of condensation. The ventilation will also encourage evaporation of any condensate which has formed within the cavity.
The second reason for requiring ventilation openings is to accommodate pressure changes in the cavity due to temperature fluctuations. A variation between the minimum and maximum temperature of as much as 100°C may be possible, which will result in significant changes in cavity air pressure and/or volume. If the cavity was not ventilated this pressure variation would cause either the glass or the panel at the back of the cavity to bow, which may result in unacceptable changes in appearance and/or damage to materials and/or seals.
However, there are disadvantages to ventilating the cavity. A large area of openings will result in higher levels of air exchange between the cavity and the external environment. Whilst this will be best in terms of controlling condensation, high levels of ventilation may allow significant amounts of dust into the cavity.
The area of openings will also affect the thermal performance of the façade. High levels of ventilation may cause significant thermal bridging around the perimeter of the shadow box due to the reduced temperatures within the cavity. This would also negate the thermal benefit of using a glazing unit over single glazing.
In UK-type climates, it is recommended that shadow box cavities have limited openings to the outside. This will provide some ventilation to reduce the risk of condensation and moderate the pressure fluctuations, whilst minimising the amount of dust entering the cavity and maintaining the thermal performance.
For further information please contact Brenda Apted, Centre for Window & Cladding Technology at the University of Bath (01225 330945; E-mail: firstname.lastname@example.org) or visit www.cwct.co.uk
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