Building facades contribute carbon to your design – and by a surprising amount. Teni Ladipo and David Metcalfe offer some things to consider to cut the carbon
A facade can contribute up to 31% of the total embodied carbon of a building, according to a 2021 study by the World Business Council for Sustainable Development. But when calculating facade-embodied carbon, considerable experience and knowledge is required to avoid underestimation and to support architectural design decisions that might aim to mitigate emissions.
Architects may not always be responsible for calculating the embodied carbon of a facade, as sustainability consultants and facade engineers often take on this role during design development. But architects can have a huge influence on the results. This is why the Centre for Windows and Cladding Technology (CWCT) is advocating for a better understanding of environmental impacts of design decisions on facades, so they can be better informed.
Architects have seven major factors to consider when designing and specifying facades to reduce their embodied carbon and keep them sustainable. The CWCT has guidance on what the design team can do at each RIBA stage. Adapting to sustainability requires co-ordination and identifying the most helpful tips for each project.
1 Form factor and building massing
The form factor of a building is the ratio of external facade surface area to internal floor area, which is also typically visualised as the complexity or efficiency of the building shape. A building’s form factor can have a significant impact on the facade’s contribution to the total embodied carbon of a building when measured per square metre of internal floor area. Low form factors (more efficient shapes) will decrease the facades’ contributions to the total embodied carbon of the building. When designing the building’s massing, architects should be aiming for a low form factor. Input from the rest of the design team will influence the design and performance of the facade, structure and MEP systems, and help the architect identify the project’s optimal form factor.
2 Number and materiality of facade system types
Having a high number of different facade systems and material types loses the advantages of repetition and optimisation of components during production and installation. The more facade types and varied materiality a building has, the higher the number of materials and components coming from different sources. This also means more interfaces and trades, which can negatively affect quality, waste and efficiency if not co-ordinated well. More repetition and fewer facade systems, and efficiency in facade materiality, can help reduce embodied carbon in production, transportation and installation.
3 Facade detailing
Differently shaped and sized cladding panels typically require multiple processes, which wastes material. Such processes and results are not always apparent, even for more standard shapes such as stone corner quoins. Architects can work with material suppliers and consultants to better understand the effects of detailing decisions and design out waste as early as possible. Dedicated ‘design out waste’ workshops can be planned to facilitate this, so that facade detailing and geometry can be optimised to reduce supplier waste. Any remaining waste should be planned for repurposing where possible and specified as such.
4 Finish systems and processes
Even the colours and finishes can create different levels of carbon emission. Caution is advised when specifying finish systems to achieve a desired aesthetic – the particular material quantity, production location, batching limits, processing and fuel source can add or reduce embodied carbon. Advice from suppliers will help reveal the impact of finish choices so that less carbon intensive alternatives can be specified.
Visual mock-ups (VMU) are typically full-scale facade assemblies used to review visual quality, while performance mock-ups are employed to evaluate facade performance. The materials are manufactured specifically for this purpose and usually discarded. VMUs and PMUs involve a significant amount of embodied carbon not only to manufacture, transport and test, but also due to the individuals that may be required to travel large distances to inspect them. Combining a VMU with a PMU where possible would lower emissions, as would reducing travel where alternative methods – such as virtual inspections or using local representatives – were available. Further, re-use of mock-up materials on the project building or repurposing them for other needs should be planned for.
Look for ways to reduce visual standards by allowing for greater tolerances
6 Visual quality
VMUs and control samples come with acceptance criteria for their visual quality. Over-restrictive acceptance criteria can produce more rejected materials and waste during manufacturing. Glazing distortions also result in material rejection, as well as overdesigned systems to compensate (eg thicker glass to mitigate pillowing). Additionally, high-clarity glass (low-iron) permits less post-consumer recycled glass to be used. To cut energy and materials waste, architects should look for ways to reduce visual standards by allowing for greater tolerances, but also work with facade consultants to balance this with required performance.
7 Design for disassembly and replacement
The potential for facade system components and materials to be re-purposed or recycled at the end of their design life must be considered early. Facade components that will be replaced during the life of the building must be accessible and easily replaced, with simple parts that do not require complex or bespoke processes or need substantial dismantling of adjacent systems. Thought should be given to the appropriate systems and details to allow for this. This should be co-ordinated with facade consultants during design and specification, including requiring replacement and disassembly method statements, and demonstrations during mock-ups to ensure solutions are viable.
Architectural design decisions have the potential to save facade embodied carbon with carbon assessments to support effective decision-making.
Teni Ladipo is an associate facade consultant at Buro Happold and a part-time sustainability engineer at the CWCT. David Metcalfe is the director of the CWCT. See more at CWCT Sustainability | CWCT
Method for facade carbon calculation
The CWCT has developed a methodology, How to calculate the embodied carbon of facades, for calculating embodied carbon on facades to bring consistency across the industry. It sits among a suite of guides already available for other building elements such as the primary structure IStructE guide: How to calculate embodied carbon, and building services guide, CIBSE TM65. The CWCT methodology was developed with input and collaboration from architects, consultants, contractors and manufacturers in the UK and throughout Europe.