Croft Gardens explores the balance of operational and embodied carbon, combining Passivhaus low-energy operation standards combined with a client ambition for a 100-year design life.
As the buildings we design become more energy efficient in use, the emphasis on whole-life carbon shifts to what we build from - with the expectation that it will be standing for 60 years, or most likely longer. Over a 60-year lifetime, it is anticipated the balance of carbon will be split between 70% embodied and 30% in-use within 10 years, with Passivhaus operational energy standards potentially skewing this even more. We need our embodied carbon use to match up to low-energy in-use and to find a low-carbon construction.
Croft Gardens (now called Stephen Taylor Court) is a residential community for students and fellows for King's College Cambridge and a project that seeks to understand this challenge. Passivhaus standards deliver low-energy operation combined with a client ambition for a 100-year design life that reflects their long-term view. What sets a 100-year design life apart from 60 years at first feels indistinct – arguably there is nothing we shouldn’t consider for a typical building life that wouldn’t apply to a longer one. We took a view that it should result in a conscious design approach relating to materials and detail, with a focus on maintaining building performance and minimising future material use. It should also contribute to the creation of an enduring, high-quality architecture.
Material use is inevitably tied to embodied carbon and should seek to balance longevity (not replacing materials unnecessarily) with the likely use of more carbon-intensive materials. The key strategy was to maximise long-life materials, and to use shorter design-life materials where replacement cycles were inevitable. We also had to balance the needs of planning and an appropriate architecture for the location.
The result was the use of brick as the key façade material, with roofs in handmade clay roof tiles supplemented with leadwork for gutters, dormers and eaves details, and copper rainwater goods. We understood that all of these materials would have higher embodied carbon, but that in terms of the whole-life of the building would give an overall more efficient, and lower carbon, approach when considered over 100 year.
Detailing these materials with a CLT structure was a further challenge – it was understood the masonry would expand over time, whereas the timber would contract. The response was to unify this with Passivhaus details; by separating the façade construction from the CLT we could use the brickwork to support lintels and cills, thus eliminating cold bridging connections to the structure, and manage the differential movement at the eaves with more standard leadwork details.
The façade exploration was further challenged to resolve curved masonry facades and was able to minimise carbon in construction through using glass mineral wool insulation and basalt ties. Alternative constructions were explored, but the chosen one was felt to offer a balance of reduced materials, lifespan, carbon intensity and straightforward construction.
The roof construction took a realistic view of the hierarchy of material longevity; the tiles would likely exceed 100 years, but the battens and underlay would probably not. Again using low carbon mineral wool insulations, we needed to include secondary timber as framing, but found this to be preferable to extensive use of steel brackets or fixings.
Maintaining building performance was a complex thought-process, and focussed strongly on window, door and rooflight replacement, driven by seeking to maintain Passivhaus performance throughout the building’s life. If a window required replacement, as was likely over 30 – 50 years, then it must be achievable without undermining the critical airtightness and thermal bridging details.
The design response was a moderating timber window reveal, splayed to maximise daylight into internal spaces, and intentionally removable to allow access to the airtightness seals around the windows if needed as a result of future CLT compression. The material choice reflected the need for longevity and removability, and while the carbon intensity of timber is lower it is balanced with the understanding that over 100 years there may need to be one, two, or even three reveals installed.
We treated the challenge of the detailing as a task guided by research; evaluating options against key criteria and recording outcomes. This was in part to demonstrate our response to the client's ambitions for 100-year design life, but also to allow future re-evaluation with inputs from the construction team post-tender.
As it stands, the approach and details appear to have responded to the test of buildability and construction, with progress and performance on-site exceeding targets. Early modelling suggests embodied carbon has been reduced to match the low operational energy, and we hope it will show how a project can harness the power of an ambition and make it a powerful part of the architecture.
We hope that over the next 100 years the forethought is as successful – we may not be here to find out!