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Horizontal fins (five groups of three fins), Velocity Building, Weybridge (right) and horizontal fins (two groups of six fins), University of Warwick, Mechanochemical Cell Biology Building (far right)


‘Upon close inspection, it is relatively easy to assess the effectiveness of a ‘thermal break’ as a feature of a curtain walling bracket’


much as 10 per cent. It is therefore critical to assess a number of curtain walling


brackets from a variety of manufacturers and demand evidence of the thermal transmittance rating or ‘χ’ for each bracket. As a guide, a curtain walling bracket with a good thermal


break should deliver a thermal transmittance rating of around 0.14 W/K, while a bracket without a thermal break would achieve around 0.42 W/K. It is also advisable to ask if testing was carried out by an independent body, such as the Centre for Window and Cladding Technology (CWCT). Upon close inspection, it is relatively easy to assess the


effectiveness of a ‘thermal break’ as a feature of a curtain walling bracket. A thermal break can be defined as “an element of low thermal


conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials”. In the case of curtain walling, the most conductive element


is the grid of aluminium mullions and transoms that are used to secure glazing. The more isolated a conductive material is, such as aluminium, the more effective the thermal break and the lower the U-value achieved by the curtain walling. Many curtain walling brackets purport to feature a thermal break, but when examined in detail, it is clear that while an


insulating material is present in some areas, this is not compre- hensive enough to prevent the curtain walling mullion from being compromised and cold bridging from occurring. This increases the risk of interstitial condensation developing inside the curtain walling structure. Over time, this can weaken the curtain walling structure and lead to mould growth forming. The optimal design for a curtain walling bracket is for it to


fully isolate the mullion at the point of penetration. A low conductivity, insulating element should be used throughout the bracket, perhaps in the form of insulated bolt sleeves and screw caps to ensure the thermal break is comprehensive. When this is achieved, the thermal performance of a curtain walling system can be maximised. A U-value of less than 0.9 W/m2


K is then a


real possibility, which is around 50 per cent lower than an average commercial building facade. As an additional benefit, the inclusion of an insulating


element can help to prevent noise and vibrations being trans- mitted into a building. When it comes to specifying solar control for a building, it


is therefore not sufficient to accept any curtain walling bracket, but demand one that is fully isolated, with a comprehensive thermal break. This will ensure that the thermal performance of a building’s facade is not compromised.


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