Key findings
Proportion of area increase: One of the key findings was that there’s a carbon penalty for increasing the building area beyond the capacity of the given existing building. The larger the extension, the more work we have to do to the structure and substructure, resulting in an embodied carbon number similar to that of an efficient new build structure of the same size.
To provide insight into the whole life carbon of the different options available – if the client requires a significant increase in gross internal area (GIA) – three extension scenarios were compared: Scenario 1, Scenario 2 and Scenario 3. These vertical extensions double the net internal area (NIA). As with the retrofit vs new build comparison, the extended new build scenarios would emit more carbon over a building’s lifespan per floor than the vertical extension scenario. Over the 60-year lifespan, the vertical extension could achieve a saving of CO2e/m2 or 70kgCO2e/m2 in comparison to the baseline and intermediate new build scenarios respectively.
Our key takeaway is that the proportion of increase (in proportion to the existing) is a primary consideration when deciding whether to reimagine an existing building or build new. Where possible, utilise CLT floor decking, rather than composite steel and concrete slabs, to reduce the embodied carbon associated with a steel frame.
Comparing operational and embodied impacts: Operational carbon, which includes emissions from energy use over a building’s lifetime, made up the largest proportion of whole life carbon emissions in all scenarios, varying from 63% to 79%. However, this proportion is expected to decrease in the coming decades with the decarbonisation of the national grid. Embodied carbon on the other hand, which includes emissions from construction materials and processes, accounted for 36% to 78% of total emissions, underscoring the significance of considering embodied carbon in all projects.
For the scenarios modelled, the operational efficiencies were not sufficient to justify the demolition and replacement of the original building. In fact, it would be extremely difficult to achieve sufficient operational savings to justify demolition and rebuild in a building with such a deep floorplate. A new build structure of this nature would need to achieve a whole life carbon of 750kgCO2e/m2 or less to justify the demolition of a similarly dimensioned existing building with an internal courtyard. The deep floorplate prevents the use of mixed-mode ventilation and natural lighting and, as such, limits the operational carbon savings gained from the typical efficiency strategies.
Deep floorplates and carbon efficiency: Buildings with deep floorplates, like the hypothetical office building in this study, limit opportunities for daylight and natural ventilation, thereby restricting potential operational carbon savings. We looked at filling in the courtyard with a core and some additional floor area, but to understand the impact of removing the courtyard, an operational carbon comparison was undertaken between the deep retrofit, which includes a courtyard infill, and the same scenario without the courtyard infill. Operationally, the design where the courtyard is retained is shown to emit 25kgCO2e/m2 less over the building’s 60-year extended lifespan, due to reduced lighting and cooling energy use. The embodied carbon of this design would be slightly lower due to the omission of the courtyard infill structure .
Recommendations
Prioritise retrofitting: Given the findings of this study, especially for buildings with similar characteristics to the hypothetical office building studied. A light refurbishment, which involves improving thermal performance and updating mechanical, electrical and plumbing (MEP) systems, offers significant carbon savings with minimal intervention. That said, the study does also demonstrate significant embodied carbon penalties in scenarios when extensive infills, strengthening and extensions are required. The WLC target of 750kgCO2e/m2 appears to demonstrate a suitable figure to aid decision making between a ‘reimagine’ and new build development, but every project should be judged on its own merits.
Vertical extensions as a sustainable option: For developers seeking to increase usable floor area, vertical extensions present a more carbon-efficient alternative compared to complete demolition and rebuilds, but the impact of this depends on the quantity of area increase. The study estimated that each additional floor in a vertical extension could save between 250 and 575tCO2e over a 60-year lifespan, depending on the fit-out specifications of the new build. Alternative structural options should also be prioritised, for example, utilising CLT floor decking rather than composite steel and concrete slabs to reduce the embodied carbon of the structure.
Embrace circular economy strategies: The high proportion of use-stage carbon emissions, particularly from the replacement of MEP systems, highlights the need for circular economy strategies. Promoting the remanufacturing, re-use, and repair of MEP components, rather than replacement and disposal, can further reduce the whole life carbon footprint of a building. This approach aligns with sustainable practices and extends the life cycle of building materials.
Comprehensive whole life carbon assessments: The study emphasises the importance of comprehensive whole life carbon assessments which consider both operational and embodied carbon emissions. Upfront embodied carbon (A1-A5 RICS Whole life carbon) can often become the primary focus of carbon discussions, which do generally favour retrofit, but a life cycle view can reframe this discussion. Undertaking whole life carbon testing earlier is key in identifying carbon hotspots and developing strategies to mitigate them effectively.
Grid decarbonisation and its impact: As the national grid continues to decarbonise, the relative significance of embodied carbon emissions will increase. This shift necessitates a greater focus on reducing embodied carbon through sustainable construction practices and material choices. Future construction should increasingly favour options that minimise embodied carbon, such as retrofitting existing buildings or opting for vertical extensions instead of new builds.
This study provides compelling evidence that retrofitting existing buildings is generally more sustainable than opting for new builds, particularly when considering whole life carbon emissions. Light refurbishments, which make minimal but impactful changes to existing structures, appear to be the best option for reducing carbon footprints. Furthermore, vertical extensions offer a viable and less carbon-intensive way to extend a building’s life while offering a new reimagined building, but the extension should be carefully assessed to ensure the improvements to the existing structure and quantum of new structure don’t outweigh an efficient new build.
The fourth and final blog of this retrofit series will look to apply these concepts to real-world projects.
