PERFORMANCE | FAADES WINDOW OF OPPORTUNITY More stringent regulations coupled with factors such as embodied carbon, depth of faade, daylight, and overheating risk make envelope design a challenge. Arups Rob Buck looks at how it might be overcome T he latest version of Breeam (2018) makes attaining Excellent and Outstanding ratings harder to achieve. This is in part because of the greater emphasis now put on the thermal performance of the faade to improve the energy efficiency of buildings. The level of performance may need to improve further with the introduction of the draft London Plan, which requires a 35% reduction in carbon emissions over Part L, and a 10-15% improvement on energy efficiency. This, and the recent release of the amendment to the Building Regulations Part B (Fire) and the approved guidance banning combustible high-performance insulation for buildings above 18m high has made it harder to meet thermal performance requirements. When important factors such as embodied carbon, depth of faade, user interface, daylight, buildability, risk of overheating and cost are put into the mix, getting the right balance requires more work. For example, if large areas of glass are introduced to allow for high levels of daylight, this might make it difficult to achieve the required overall U-value, unless designers introduce triple glazing or increase the thickness of the insulation for opaque areas. A holistic approach at the start of projects is very beneficial in defining the faade concept with the right amount of glazing in relation to other design elements. High-rise residential buildings with anoverall faade U-value requirement of 0.64W.m-2.K-1, plus a windowto-wall area of around 30%, are common. But with previous building energy targets, it was possible to design faades withoverall U-values in the region of 0.80-1.0W.m-2.K-1. The high-rise nature of these developments means it is wise to consider a pre-assembled approach to the manufacture of the faade, reducing time spent working at height and improving the quality of workmanship. A unitised curtain walling system, however, has more aluminium framework which compromises the thermal performance than a built-on-site approach using a rainscreen system. Detailed design is required much earlier in the development phase of the project to clarify whether an aluminium, thermally broken, unitised approach would meet the thermal requirements. This may require the architect to compromise on their design by simplifying the architecture for example, by removing features such as balconies that could penetrate the faade, employing triple glazing, and/or increasing the percentage of opaque elements. Finding a balance Enhanced, thermally broken, aluminium systems are now available and can help with achieving higher thermal performance. However, these materials are made from a combustible plastic with a foamed core. In the Building (Amendment) Regulations 2018, combustible thermal breaks are permitted BENEFIT OF SHUTTERS In the April 2019 issue of CIBSE Journal, Bill Bordass described how the integrated design of the 2+1 window system a double-glazed inner pane, with venetian blinds on the outside protected by an outer pane of glass overcame the usually conflicting requirements of daylight without the heat gain. The window can also open to provide natural ventilation. My favourite is the sash window, coupled with an internal shutter system, commonly used by the Victorians. The window, with openings at low and high levels, offered single-sided ventilation, while the shutter system gave solar shading and security. A modern update is to integrate movable louvres into the shutter, offering more flexibility with daylight and ventilation control. Lessons are also to be learned from continental Europe, where external shutters or roller blinds, coupled with an inward opening window, can offer similar benefits. The system at Gasholders, Kings Cross, is a modern example of using external shutters to control heat gain, with internal opening windows to give natural and purge ventilation. The shutters are motorised and require a service agreement. An alternative approach could be to lease the products, with ownership remaining with the manufacturer, who would be responsible for repairing and maintaining the system. Currently, a lack of information is being passed back to the manufacturer to improve product design as, generally, their products are installed and maintained by others. This information would also allow the manufacturer to recycle or reuse the product at the end of its life. 36 May 2019 www.cibsejournal.com CIBSE May19 pp36-37 Facade.indd 36 26/04/2019 17:09