Future Homes and the Carbon Budget

The Future Homes Standard [1] is a consultation document about changes to the building regulations for new dwellings in the UK. It refers to “our commitment to deliver 300,000 homes a year by the mid-2020s”. In view of the present concern with meeting carbon budget targets, this post seeks to examine the carbon implications of this building program.



A 2010 newspaper article was headed “What's the carbon footprint of ... building a house” [2]. The answer proposed for a “newbuild two-bed cottage” was 80 tonnes CO2e. Details of the house were not provided, and it is clear that the writer was seeking only to give an indication of the carbon footprint. Is this figure still a useful guide today? We might expect that even if the total energy needed is the same, the CO2e figure might be lower, due to decarbonisation of at least some of the energy used.



A 2013 article proposes that “the average house contains about 1,000GJ of energy embodied in the materials used in its construction.” [3].The authors point out that the embodied energy contained in a structure is difficult to assess, and depends on whether, for example, the energy used to transport the materials and workers to the building site has been included, whether the materials list includes fittings, driveways and outdoor paving, how much of the upstream energy input in making the materials is included, and whether the embodied energy of urban infrastructure is accounted for.



1,000GJ corresponds to about 278 MWh, and the 80 tonnes CO2e mentioned above, would, if derived from this amount of energy, correspond to an emission factor of about 288 gCO2e per kWh. This can be considered in the context of a spectrum of emission factors for different energy sources. At the high end, the emission factors in gCO2e per kWh for coal and oil are in the region of 1000, for natural gas closer to 700, for photovoltaics about 100, for nuclear power around 60, and for hydro and wind power in the range 15-20 [4]. We can say that the energy and CO2e figures for the house are compatible in the sense that a mixture of energy sources could plausibly result in an emission factor of 288 gCO2 per kWh. However the great spread in emission factors across energy sources indicates the possibility of a wide range of embodied CO2e figures for similar buildings made in different ways. To make the point by means of two extreme examples, the 1,000GJ referred to above would imply about 278 tonnes of CO2e if derived entirely from coal, but as little as 4.2 tonnes if produced wholly from wind power. Without entering into the detailed analysis which would be needed to arrive at confident estimates, we can say that the method used to generate the energy, particularly the electrical component, is likely to have a large effect on the final figure, and also that this figure is likely to vary significantly over time as electricity supplies are decarbonised.



McAlinden [5] “explores the current ways of measuring embodied energy and carbon, and makes recommendations on how to reduce these on construction projects.” He cites studies of low energy buildings and gives examples (in 2002 and 2010) showing that initial energy embodied in such a structure may account for 40 - 45% of its whole life-cycle carbon. Huang et al. [6] claim that embodied energy typically accounts for 10-20% of total life-cycle energy in conventional buildings, but can be 60% or more in low energy buildings. The absolute values of energy in these cases are not provided.



The web page “Architecture and sustainable buildings” [7] points out that “Energy efficient low carbon buildings can be achieved only by reducing both embodied carbon and operational energy.” If we look at the problem of minimising the lifetime carbon footprint of a new building (or perhaps its carbon footprint up to some significant date, such as 2050) then we have to engage on an optimisation exercise which recognises that the embodied carbon of a given building design is likely to depend on its construction date, and that its operational emission factor will almost certainly change significantly with time as energy sources, particularly electricity, are progressively decarbonised.



Malmqvist et al. [8] present case studies illustrating approaches to reducing embodied impacts from building. The key recommendations of their 2018 paper include replacing structural components with timber; the use of recycled or recovered components and materials, and components such as wood-concrete composite members; minimising site waste and energy use both in construction and demolition, and ensuring materials are locally sourced where possible; separating and recycling or processing waste efficiently; and designing buildings for long lives, and making them adaptable to different usage over their lives. The paper provides numerous references, but concludes “that there are still only limited studies which calculate the reduction of EEG [embodied energy and greenhouse gases] from most of these strategies, and far more are needed”.



Pomponi and Moncaster [9] write that while life cycle assessment (LCA) is becoming increasingly mainstream as a design-decision tool for buildings, “there are considerable variations in how the method is currently used, leading to limitations in comparing the results and the conclusions that can be drawn.” The variety of approaches leads to a wide range of outcomes, and

“very few case studies are detailed enough to allow for an in-depth comparison”, while results “may differ by two orders of magnitude”. They argue that “greater transparency and greater conformity must be embraced by the LCA community and enforced by policymakers and professional bodies.” They note “that data scarcity is a problem only in some life cycle stages, primarily those related to the use stage of a building and its end of life impacts...” As in the preceding paper a wide range of literature references is given.



The paper by Huang et al., cited above [6] differs from those preceding in that it is concerned with the life cycle assessment of a precinct (or housing estate) of more than two thousand buildings, and takes into its energy assessment the commuting habits of its residents, who travel mainly by private vehicle, and contribute more than half of the total energy use of the development over its lifetime.



References



[1] The Future Homes Standard

2019 Consultation

Ministry of Housing, Communities and Local Government

https://www.gov.uk/government/consultations/the-future-homes-standard-changes-to-part-l-and-part-f-of-the-building-regulations-for-new-dwellings



[2] Mike Berners-Lee, The Guardian, 14 October 2010.

https://www.theguardian.com/environment/green-living-blog/2010/oct/14/carbon-footprint-house



[3] Geoff Milne

Embodied energy

Your Home, 2013.

http://www.yourhome.gov.au/materials/embodied-energy



[4] The carbon footprints of energy

Wikipedia

https://en.wikipedia.org/wiki/Carbon_footprint#The_carbon_footprints_of_energy



[5] McAlinden, B.

Embodied Energy and Carbon

Institution of Civil Engineers,

May 2015

https://www.ice.org.uk/knowledge-and-resources/briefing-sheet/embodied-energy-and-carbon/



[6] Huang, B. et al.

Life- cycle energy modelling for urban precinct systems

Journal of Cleaner Production (2016),

http://dx.doi.org/10.1016/j.jclepro.2016.10.144

Also available at https://www.researchgate.net/



[7] Architecture and sustainable buildings

School of Civil Engineering, University of Leeds

https://eps.leeds.ac.uk/civil-engineering-energy-sustainable-buildings/doc/architecture-sustainable-buildings



[8] Malmqvist et al.

Design and construction strategies for reducing embodied impacts from buildings – Case study analysis

Energy and Buildings

Volume 166, 1 May 2018, Pages 35-47

https://oro.open.ac.uk/52956/3/52956.pdf



[9] Pomponi and Moncaster

Scrutinising embodied carbon in buildings: The next performance gap made manifest

Renewable and Sustainable Energy Reviews

Volume 81, Part 2, January 2018, Pages 2431-2442

http://oro.open.ac.uk/49680/1/49680.pdf