1. Introduction
Rapid urban development and population growth have gradually increased energy demand [
1] and carbon footprint [
2], with the United Kingdom (UK) emerging as one of the top fifteen carbon emitters per capita [
3]. In 2019, the UK became the first major economy in the world to enact laws to put an end to its contribution to global carbon emissions by 2050 by setting a target to bring all greenhouse gas emissions to net zero by 2050 (Department of Business, Energy & Industrial Strategy and Skidmore, 2019). Various studies indicate construction and the built environment have the highest potential for reducing energy [
4,
5]. With buildings contributing up to 40% of the global energy consumption annually [
6], it is considered one of the critical areas for improvement in meeting the UK Government’s 2050 greenhouse gas reduction target [
7]. Furthermore, the residential sector produces almost half of the built environment’s greenhouse gases [
8]. Therefore, there is a critical role to be played by the residential sector in meeting the carbon reduction targets.
The energy usage of a building can primarily be categorised into operational and embodied energy [
9]. The Building Research Establishment [
10] defines embodied energy as “total primary energy that has to be sequestered from stock within the earth to produce a specific good or service”. Under normal circumstances, operational energy outweighs embodied energy in buildings. However, depending on the composition of a building, embodied energy can range up to 60% of the total energy spent, especially when the operational energy requirement is low [
11]. Understanding the relationship between operational and embodied energy is imperative to reducing the overall carbon [
9] due to its Pareto optimal nature [
12]. Reports from the United Nations Environment Programme advise that construction, including manufacturing, accounts for between 10–20% of a building’s total energy consumption [
13]. Similar studies by [
14] postulate 10–30%; however, [
15] reduces the range to between 25–30%. In addition, the embodied energy range is highly dependent on the operational requirements and can vary between 9% and 38% when conventional systems are used [
16]. Despite varying percentages based on different case studies, the consensus of the literature would suggest that Leoto and Lizarralde’s [
11] statement is accurate. When considering construction, refurbishment, and demolition over 50-years, the total energy consumption figures alter to 45% and 55% from embodied energy and operational energy, respectively [
17,
18].
Although a plethora of research [
19], legislation and design considerations [
16] have been conducted, there is a stronger focus on operational energy over embodied energy [
19]. However, an emerging argument within literature is that embodied energy is more important over the life cycle of a building as operational energy can be reduced post-construction, unlike embodied energy [
20]. However, embodied energy has received significantly less consideration than operational energy, both in practice [
21] and within academia [
22]. Consequently, some appraisal systems consider only operational energy and exclude embodied energy from calculations [
16]. However, as operational energy is reduced through increased government intervention and improved design, the significance of embodied energy is expected to grow [
23,
24]. This is demonstrated in a study by Azari and Abbasabadi [
25], which confirms embodied energy has gained focus in recent years due to the current trend of reduction in operational energy. BRE [
20] claim that embodied energy should be prioritised as, unlike operational, it cannot be reversed once installed. Thus, embodied energy in buildings requires further attention, especially in the residential sector.
Multiple studies express the criticality of improving affordable housing to meet 2050 energy targets, primarily due to the social and affordable residential sector producing over half the carbon within the residential sector [
26,
27,
28], suggesting that affordable housing can present additional pressures with most housing associations working with Small, Medium Enterprises (SME), subjecting poor knowledge, workmanship, and project fragmentation. Furthermore, they state that the complexity of ageing tenants often causes rebounding issues with operational energy, increasing the importance of embodied energy. A key issue with affordable housing is that costs need to be kept to a minimum to keep the prices ‘affordable’. This presents a significant opportunity and a challenge to innovate and develop low-cost and low-carbon solutions. While there is no universal definition of affordable housing, it is identified as housing of any tenure that is judged to be affordable to a particular household or group by analysis of housing costs, income levels, and other factors in a House of Commons briefing paper [
29]. Affordability here is a subjective term, as it depends on the income, savings, disposable income, etc of a household. The definition of the House of Commons identifies affordable housing as ‘subsidised housing that meets the needs of those who cannot afford to secure decent housing on the open market either to rent or buy’ [
30] provides a more applicable explanation of affordable housing. According to research commissioned by the National Housing Federation [
31], it was projected that England will need about 340,000 new homes to be built per year over the 15 years between 2018–2033, of which 145,000 were expected to be affordable homes (including 90,000 for social rent, 25,000 for shared ownership and 30,000 for intermediate rent). This represents over 40% of the new-built homes. To fulfill the 2050 carbon reduction targets, it is recommended to evaluate carbon emissions from the residential sector by considering its contribution to over half of the carbon emission of the built environment [
8]. Therefore, the category of affordable homes too will need to achieve carbon reduction targets if the wider carbon reduction targets are to be met.
This research seeks to assess the importance placed by both the client and design team on embodied energy in the low-cost affordable residential sector to determine areas for improvement in reducing embodied energy. To help achieve this goal, we calculate the embodied energy produced by three separate single-family residential designs in a selected affordable housing development. This will enable highlighting the most embodied-carbon-significant building elements in social housing. With the help of primary research with project stakeholders, we seek to provide a set of improvements to create efficiency between embodied and operational energy in social housing. The study is significant for understanding the existing level of embodied energy and exploring strategies to lower the total carbon impact of social housing.
2. Materials and Methods
A report by the UK Green Building Council [
9] revealed that embodied energy levels were not governed by legislation. Embodied energy has not been included within Part L of the building regulations [
32,
33], despite the government showing commitment to reducing environmental impact [
16]. The Committee on Climate Change [
34] has reported to Parliament that new policies are required; however, previous studies have highlighted that housebuilders believe that policies do not enforce the same degree of rigour as legislation [
35].
Although the Home Quality Mark (HQM) is not enforced legally, some councils are creating a policy whereby any developments, where feasible, should use the HQM [
36]. The HMQ is a new national standard for housing in the UK created by BRE [
20]. Five percent of the total credits available are for the environmental impact of materials, enforcing life cycle assessments, and environmental product declarations to be supplied [
37]. However, there is limited literature available determining the uptake of the HQM standard, with HQM’s website stating that there are only around twenty-six sites providing feedback [
38]. The Building Research Establishment (BRE) has incorporated life cycle assessments into their material category, making it harder for companies to reach good, excellent, and outstanding certifications on buildings without reviewing embodied energy [
20]. There is no confirmation on the number of homes being tested under the guidelines following the last 2012 BRE digest. The European Union (EU) has included embodied energy as a core indicator within the EU framework for building assessments [
9]. The companies Act 2006 (Strategic and Directors’ Reports) Regulations 2013 require companies whose shares are traded on a stock exchange to report their GHG annually [
39].
Several industry organisations are working with the Greater London Authority to enforce new guidelines for embodied energy, with a requirement for specific schemes to report and meet targets of both embodied and whole life carbon emissions [
40]. Furthermore, as part of the RIBA’s 2030 Climate Change Challenge, they declared that embodied energy should be reduced by around 50–70% before offsetting with a benchmark of 300 kg CO
2/m
2 in domestic buildings [
41]. However, there is no indication of the repercussions of not following this procedure and how it will be policed. LETI-2020 [
24] has a different set of domestic guidelines aiming to reduce embodied energy to 500 kg CO
2/m
2 from 2020 and 300 kg CO
2/m
2 in 2030 from the current baseline of 800 kg CO
2/m
2. Also, there is no confirmation whether LETI’s guidelines are nationwide or applicable to only domestic properties within the London area.
The client or end-user has a significant influence on a buildings’ carbon footprint through purchase decisions, which can similarly extend through to the supply chain [
42]. Farmer [
43] confirms this, recognising the need for clients to enforce change if governmental intervention is absent. When designing buildings, both economic and environmental criteria should be discussed [
44]. A cost-saving of 30–50% has been revealed from Anglian Water due to tracking embodied energy [
9], which is in line with WRAP’s [
45] proclamation that a financial benefit can be made with embodied energy reductions. Conversely, the [
46] estimates the cost to reduce GHG emissions to net-zero at around 200 £/t CO
2e. With the UK emission levels at 460 MtCO
2e in 2017 [
47], this implies a cost of 96 billion £ to the UK to achieve net-zero.
Hendrickson et al. claim calculating embodied enables parties to address cost-effective strategies through a profound understanding [
42]. The parameters for calculating the environmental performance of buildings are outlined within BS EN 15978:2011 [
48]. However, the guidelines are not comprehensive, which led to the BRE guides [
37]. Other guidelines include the RICS Whole Life Carbon Assessment [
49], BS EN 15804:2012, the Life Cycle Assessment (LCA) framework, and ISO14040:2006 [
50].
Due to the broad system boundary definitions available to estimate embodied energy, there are high levels of uncertainty when compared to estimating operational energy [
25]. Additionally, D’Agostino et al. argue that throughout research on embodied energy, methodologies are deficient in both description and consistency, causing “scarce research repeatability and data quality” [
51]. Furthermore, uncertainty can be amplified with Hendrickson et al. [
42] claiming that companies often focus less on the supply chain in their emission calculations, which could be due to the limited availability of data [
25]. Bhardwaj [
52] revealed that transportation could be the most uncertain element within embodied energy as significant variations come from ‘manufacturing location, carriage weight, distance travelled, and transportation type used’ [
53]. Therefore, transportation cannot be included unless named suppliers are specified for the study [
54]. However, it is also argued that transportation should never be discounted from studies as it accounts for between 5% and 20% of total embodied impacts [
54,
55].
4. Research Method
An exploratory, single case study design assessed embodied carbon in affordable housing. An exploratory study can be identified as the initial investigative stage of a more rigorous study to follow. The case study approach allows for a detailed investigation of the issues of concern [
87]. The case study project selected was a typical low-cost, affordable housing project. Multiple housing designs within the project were analysed for their embodied carbon. As Yin [
88] has explained, studying a single case is appropriate if the single case represents a common case, as in the project chosen here. Rather than selecting an exemplary project with targeted action to reduce embodied energy, a typical housing project has been selected as this would provide a better account of the general situation on new-build affordable housing. The houses in the development are intended either to be socially rented or sold on a shared-ownership basis. The study was subjected to methodological triangulation to increase accuracy. Initially, embodied carbon was calculated to identify embodied carbon significant building elements and critical areas for improvement. A total of three common house types used in the case study development were used for this embodied carbon calculation. Secondly, semi-structured interviews were conducted with the selected members of the project team.
An exploratory approach was adopted in undertaking semi-structured interviews with the key members of the project design team. Face-to-face interviews were carried out in the interviewees’ board room. Reasons for conducting face-to-face interviews were to have a great rapport between the interviewer and interviewees, to monitor body language, which helps with the flow of the interviews, and to identify any limitations related to the responses. Semi-structured interviews queried the respondents, which allowed key questions to be answered and probed the respondents on any new themes emerging from the discussion. Accordingly, five well-experienced members of the project design team were interviewed.
Table 1 presents the profile of the interviewees.
The interviews were recorded and transcribed via true verbatim. This allowed a natural flow of conversation without breaks for writing or forgetting vital information. Moreover, interview transcripts were reviewed as a validation technique. Interviewees had the opportunity to read and clarify any statements made during the interviews, and this enhanced the accuracy of the data.
After that, themes on drivers, barriers, and reduction techniques were developed. Non-probability sampling was selected since there is no probability attached to the unit of measurement [
89]. The purposive sampling technique, which comes under non-probability sampling methods, was selected as the most suitable sampling technique to select interviewees since there is no probability attached to the population unit, and selection relies on the researcher’s judgment [
89]. Lastly, project documents, including contract documents, were examined to extract further information.
The main building elements such as substructure, superstructure, roof, internal partitions, and finishes were considered to calculate the embodied energy. The external works were not included within the calculation as this would be divided equally amongst the houses in the development and could not be effectively measured with the resources available. The take-off was carried out as per the Standard Method of Measurement 7 (SMM7) format and transferred to a bill of quantities where the carbon rates were applied. SMM7 was selected as the measurement guide due to it being the standard used in the case-study project and also because of its compatibility with The UK Building Black book [
90], which is one of the limited resources produced in a bill of quantity format [
91]. To select the most appropriate carbon database for this study, an exercise was completed whereby the databases were accessed, and a set of identical searches were completed. Other alternatives such as Ecoinvent, Inventory of Carbon and Energy, WRAP, Greenbook Live, and Eco Platform were considered, but the UK Building Black book was selected as the most suitable tool to obtain carbon data due to the compatibility with the method of measurement used in the project [
92]. This allowed taking account of both materials and operations, a benefit not offered by most of the databases considered. No allowance for wastage will be added as it is already accounted for within the carbon rate [
90]. The UK Building Black book presents embodied carbon of building work adopting a cradle-to-end of construction approach, excluding transport. Therefore, the calculations are based on cradle-to-end of construction, and embodied carbon of transport is not included. While transport is not included, [
93] found that embodied carbon calculation arrived at using the Black book was considerably higher than cradle-to-grave calculations arrived at using BRE Green Guide. As commented by [
93], while there can be disparities between the results produced by different construction carbon counting tools, the Black book is commonly used to calculate embedded carbon in construction work.
Figure 1 presents an example of how embodied carbon for each work item was calculated to provide a better understanding of the calculation involved. The stepwise calculation followed is identified below for ease of reference.
Each work item involved in the project was identified, and quantities of each work item were measured/quantified as per the SMM7 measurement standards, according to the project’s drawings. (This step has already been completed by the project team).
Then the embodied carbon rate for each work item was obtained from the UK Building Black Book.
Embodied carbon quantity of each work item was calculated by multiplying the quantity of work by the unit rate of embodied carbon.
Quantities from all work items were then enumerated to arrive at the total embodied carbon for each house type.
6. Conclusions
This study assessed the importance placed by both the client and the design team on embodied energy in the residential affordable housing sector to determine areas for improvement in reducing embodied energy. The study calculated the embodied energy consumption of three affordable housing units. The results revealed that all three housing units are substantially below the baseline target of 800 kg CO2/m2. Although the current target is satisfied, only two out of three properties fall within the LETI 2020 target of 500 kg CO2/m2. On the other hand, all three housing units would fail to fulfill the RIBA or LETI target of 300 kg CO2/m2. On one hand, caution should be exercised when interpreting the embedded carbon calculations for these 3 properties due to limitations in the embodied carbon calculation method adopted here, including the UK Building Blackbook and the Bill of Quantities from the project. The data, however, shed light on the general level of embodied carbon in typically affordable housing units in the UK. The key observation here is that much still needs to be done to achieve ‘best practice’ standards and to help achieve future national and industry targets concerning embodied carbon.
Therefore, the study explored drivers and barriers for reduced embodied carbon implementation and embodied carbon reduction techniques to bring them in line with the targets above. Accordingly, 11 drivers and 9 barriers were recognised, and 9 reduction techniques were proposed and checked for compatibility. The sub-categories of ‘financial’ and ‘regulation’ were recognised as the top two drivers, and ‘client’ and ‘local authorities’ were highlighted as sub-categories next in line. On the other hand, ‘operational energy prioritisation’ was the most significant barrier to reducing embodied carbon. The study findings on potential reduction strategies and the potential of key enablers and barriers can be exploited to reduce embodied carbon in new-build affordable housing units. Thus the research facilitates UK building stakeholders to mitigate embodied carbon in the residential housing sector, especially affordable housing. The findings reveal that further work is required to future-proof the affordable housing sector in line with the targets set for the decade ahead. Any improvements will positively contribute to the net zero-carbon agenda as the work required to offset embodied energy will be reduced. Further research is required to better understand the financial and operational impacts of the strategies suggested.
This research answers the demand for methodological pluralism in research on embodied energy. Research on embodied carbon is mostly calculative and seeks to quantify embodied energy. This research, however, offers an alternative viewpoint, complementing the embodied energy calculations with qualitative research. The approach adopted not only provided a quantitative account of embodied carbon in the case study project but also shed light on drivers and barriers to reducing embodied carbon and potential strategies for reducing embodied carbon in typical affordable residential development.
As with every research, this research also possesses some limitations. The embodied carbon calculations were based on the Bill of quantities already prepared for the project, and although spot checks have been performed throughout the process, there can be some uncertainty on the accuracy of dimensions. If there were errors in the bills, these would have caused an incorrect reading when calculating the embodied energy. Furthermore, the rate book used for the study (UK Building Black book) has limitations that will have impacted the accuracy of embodied carbon calculations. While the embodied carbon calculations may carry limitations, they present a good account of current rates of embodied carbon in residential developments. Apart from that, although continually stressed the anonymity of the study, there were signs that one or few of the participants were expressing signs of Halo Effect Bias. The researchers believe this may have occurred due to a pre-existing professional relationship.
Based on the results of this paper, the study proposes several research directions in the field of embodied energy. The present study was undertaken as an exploratory study to obtain a general account of the state of affairs in terms of embodied carbon in residential developments, particularly affordable housing, and as a precursor to a more detailed study to follow. Further research can now be undertaken reflecting on the limitations of this exploratory research and to explain the observations further. Future research can be undertaken to evaluate the effect on the buildings from cradle to end of construction. This would allow an analysis of whether a higher initial embodied energy assists in the reduction of the whole life energy when looking at replacement and maintenance of the building. A further recommendation is to include multiple case studies to arrive at more generalisable findings. This would enable the understanding of the drivers and barriers in the housing sector and whether different sectors produce and maintain buildings with a different approach. This may also highlight whether the form of the contract assists in its ability to monitor and reduce embodied energy. Measuring the building works from scratch ahead of carbon calculations rather than relying on an already developed bill of quantities may also improve the accuracy of findings.