NET ZERO CARBON | AHMM CASE STUDY the whole life model to chart progress of the design against carbon reduction targets. Seventh, and finally, the capability of the design team to support the client aspiration was essential for delivering carbon reductions on this project. This may require wider upskilling of the industry to deliver zero carbon buildings as a matter of course. The AHMM/UCL IEDE guide is available to download at bit.ly/CJFeb23AHMM2 and is intended as a live guidance document. It will be subject to ongoing critical review, and the next release will feature a calculation spreadsheet to help others in industry deliver net zero carbon buildings. CJ DR CRAIG ROBERTSON is head of sustainability and building performance and DR SIMON HATHERLEY is senior building performance architect at AHMM Parametric solar gain analysis at Canada Water The solar radiation analysis shows the peak internal gains for the base case faade of the commercial development (without the second tower). The areas with the highest gains require additional shading strategies to reach comfortable levels. These could be solar shading, window recesses, and the use of glass with lower G-values, which would reduce heat gain. All areas in green appear to meet the target solar gain. CANADA WATER NET ZERO CARBON CASE STUDY Canada Water Plot F includes a significant office building of 38,925m2 and 410 residential units in two towers. The towers are positioned at 45 degrees to one another, to respond to the street pattern. The clients ambition to be net zero in use was developed into primary numerical targets based on industry benchmarks for achieving the aspirations of the scheme, and included specific embodied energy and operational targets. In this case, 90kWhm-2 per year operational energy (aligned with the UK Green Building Councils targets, and recognising the likely landlord-tenant split of 55kWhm-2 per year base build and 35kWhm-2 per year tenant use) and 500kgCO2/m2 embodied carbon for the office; secondary targets supporting the delivery of the primary targets were then informed by team experience and other industry benchmarks, notably LETI proportional breakdowns of total embodied carbon numbers. For example, 16% of the overall carbon number allocated to faades gave the architectural team a componentspecific embodied carbon target to inform design development. Faade development The faade of the buildings represented a key intersection between architecture, structure and MEP engineering. The design of the skin of the building, therefore, illustrates the iterative and collaborative nature of delivering zero carbon buildings. 28 February 2023 www.cibsejournal.com To achieve very low operational energy levels, the MEP engineer set very low upper limits for perimeter solar gains entering the office spaces. This set a challenge for the architects to balance daylight, gains, architectural expression and embodied carbon. So, the first piece of analysis was a simple headline exploration of the gains on the office faades (see figure above). This developed into more detailed and iterative explorations of the faade geometry: parametric analysis of the faades (that also allowed embodied carbon to be assessed simultaneously with operational energy) and CIBSE TM59 assessments of the residential faade proposals. The parametric analysis (above) enabled conversations between architects, engineers, the client and planners, while using carbon and energy performance as the driving metric to assess design development. The whole life carbon assessments for the project revealed that upfront embodied carbon emissions would be as important as operational energy emissions over the life of the building. Simultaneous to the parametric development of the faade geometry, the design team was working on optimising embodied carbon. The structural engineer examined various structural options, providing data to compare each option against the secondary carbon targets while the architect visualised the formal and aesthetic implications. As the architecture evolved, the team developed a hybrid ventilation approach to the perimeter zones of the office building. This reinforced the need for low solar gains and introduced opening vents. The team examined the faade from an upfront embodied carbon point of view, as well as whole life emissions. This dual analysis provided an elemental breakdown of the carbon intensity of a faade bay and informed the design for repair, maintenance and replacement throughout the life of the building. The results highlighted that, for example, the commercial buildings faade had embodied carbon of 88.5kgCO2e/m2 based on RICS A1-A3. This value was almost double the LETI benchmarks for the RIBA 2030 target, which is estimated to be 46.8kgCO2e/m2. Putting this value into a cumulative whole building value allowed the team to use the low structural value to compensate for a higher faade value. The case study takes the reader from first principles to examining more nuanced decisionmaking, identifying some of the trade-offs required to drive down whole life carbon on a live commercial project. The lessons point to a way of working that is much more collaborative and a process that is more iterative in all disciplines particularly engineering, where consultants are perhaps not as accustomed to the trial and error of a design development process that is not always linear.