Environmental Retrofit Buildings are responsible for approximately 40% of energy consumption and 36% of carbon emissions in the EU (European Commission, 2021). Environmental retrofit, green retrofit or low carbon retrofits of existing homes ais to upgrade housing infrastructure, increase energy efficiency, reduce carbon emissions, tackle fuel poverty, and improve comfort, convenience and aesthetics (Karvonen, 2013). It is widely acknowledged that environmental retrofit should result in a reduction of carbon emissions by at least 60% in order to stabilise atmospheric carbon concentration and mitigate climate change (Fawcett, 2014; Johnston et al., 2005). Worldwide retrofit schemes such as RetrofitWorks, EnerPHit and the EU’s Renovation Wave, use varying metrics to define low carbon retrofit, but their universally adopted focus has been on end-point performance targets (Fawcett, 2014). This fabric-first approach to retrofit prioritises improvements to the building fabric through: increased thermal insulation and airtightness; improving the efficiency of systems such as heating, lighting and electrical appliances; and the installation of renewables such as photovoltaics (Institute for Sustainability & UCL Energy Institute, 2012). The whole-house systems approach to retrofit further considers the interaction between the occupant, the building site, climate, and other elements or components of a building (Institute for Sustainability & UCL Energy Institute, 2012). In this way, the building becomes an energy system with interdependent parts that strongly affect one another, and energy performance is considered a result of the whole system activity. Economic Retrofit From an economic perspective, retrofit costs are one-off expenses that negatively impact homeowners and landlords, but reduce energy costs for occupants over the long run. Investment in housing retrofit, ultimately a form of asset enhancing, produces an energy premium attached to the property. In the case of the rental market, retrofit expenses create a split incentive whereby the landlord incurs the costs but the energy savings are enjoyed by the tenant (Fuerst et al., 2020). The existence of energy premiums has been widely researched across various housing markets following Rosen’s hedonic pricing model. In the UK, the findings of Fuerst et al. (2015) showed the positive effect of energy efficiency over price among home-buyers, with a price increase of about 5% for dwellings rated A/B compared to those rated D. Cerin et al. (2014) offered similar results for Sweden. In the Netherlands, Brounen and Kok (2011), also identified a 3.7% premium for dwellings with A, B or C ratings using a similar technique. Property premiums offer landlords and owners the possibility to capitalise on their  retrofit investment through rent increases or the sale of the property. While property premiums are a way to reconcile          split incentives between landlord and renter, value increases pose questions about long-term affordability of retrofitted units, particularly, as real an expected energy savings post-retrofit have been challenging to reconcile (van den Brom et al., 2019). Social Retrofit A socio-technical approach to retrofit elaborates on the importance of the occupant. To meet the current needs of inhabitants, retrofit must be socially contextualized and comprehended as a result of cultural practices, collective evolution of know-how, regulations, institutionalized procedures, social norms, technologies and products (Bartiaux et al., 2014). This perspective argues that housing is not a technical construction that can be improved in an economically profitable manner without acknowledging that it’s an entity intertwined in people’s lives, in which social and personal meaning are embedded. Consequently, energy efficiency and carbon reduction cannot be seen as a merely technical issue. We should understand and consider the relationship that people have developed in their dwellings, through their everyday routines and habits and their long-term domestic activities (Tjørring & Gausset, 2018). Retrofit strategies and initiatives tend to adhere to a ‘rational choice’ consultation model that encourages individuals to reduce their energy consumption by focusing on the economic savings and environmental benefits through incentive programs, voluntary action and market mechanisms (Karvonen, 2013). This is often criticized as an insufficient and individualist approach, which fails to achieve more widespread systemic changes needed to address the environmental and social challenges of our times (Maller et al., 2012). However, it is important to acknowledge the housing stock as a cultural asset that is embedded in the fabric of everyday lifestyles, communities, and livelihoods (Ravetz, 2008). The rational choice perspective does not consider the different ways that occupants inhabit their homes, how they perceive their consumption, in what ways they interact with the built environment, for what reasons they want to retrofit their houses and which ways make more sense for them, concerning the local context. A community-based approach to domestic retrofit emphasizes the importance of a recursive learning process among experts and occupants to facilitate the co-evolution of the built environment and the communities (Karvonen, 2013). Involving the occupants in the retrofit process and understanding them as “carriers” of social norms, of established routines and know-how, new forms of intervention  can emerge that are experimental, flexible and customized to particular locales (Bartiaux et al., 2014). There is an understanding that reconfiguring socio-technical systems on a broad scale will require the participation of occupants to foment empowerment, ownership, and the collective control of the domestic retrofit (Moloney et al., 2010).

Buildings are responsible for approximately 40% of energy consumption and 36% of greenhouse gas emissions in the EU (European Commission, 2021). Energy retrofit is also referred to as building energy retrofit, low carbon retrofit, energy efficiency retrofit and energy renovation; all terms related to the upgrading of existing buildings energy performance to achieve high levels of energy efficiency. Energy retrofit significantly reduces energy use and energy demand (Femenías et al., 2018; Outcault et al., 2022), tackles fuel (energy) poverty, and lowers carbon emissions (Karvonen, 2013). It is widely acknowledged that building energy retrofit should result in a reduction of carbon emissions by at least 60% compared with pre-retrofit emissions, in order to stabilise atmospheric carbon concentration and mitigate climate change (Fawcett, 2014; Outcault et al., 2022). Energy retrofit can also improve comfort, convenience, and aesthetics (Karvonen, 2013). There are two main approaches to deep energy retrofit, fabric-first and whole-house systems. The fabric-first approach prioritises upgrades to the building envelope through four main technical improvements: increased airtightness; increased thermal insulation; improving the efficiency of systems such as heating, lighting, and electrical appliances; and installation of renewables such as photovoltaics (Institute for Sustainability & UCL Energy Institute, 2012). The whole-house systems approach to retrofit further considers the interaction between the climate, building site, occupant, and other components of a building (Institute for Sustainability & UCL Energy Institute, 2012). In this way, the building becomes an energy system with interdependent parts that strongly affect one another, and energy performance is considered a result of the whole system activity. Energy retrofit can be deep, over-time, or partial (Femenías et al., 2018). Deep energy retrofit is considered a onetime event that utilises all available energy saving technologies at that time to reduce energy consumption by 60% - 90% (Fawcett, 2014; Femenías et al., 2018). Over-time retrofit spreads the deep retrofit process out over a strategic period of time, allowing for the integration of future technologies (Femenías et al., 2018). Partial retrofit can also involve several interventions over time but is particularly appropriate to protect architectural works with a high cultural value, retrofitting with the least-invasive energy efficiency measures (Femenías et al., 2018). Energy retrofit of existing social housing tends to be driven by cost, use of eco-friendly products, and energy savings (Sojkova et al., 2019). Energy savings are particularly important in colder climates where households require greater energy loads for space heating and thermal comfort and are therefore at risk of fuel poverty (Sojkova et al., 2019; Zahiri & Elsharkawy, 2018). Similarly, extremely warm climates requiring high energy loads for air conditioning in the summer can contribute to fuel poverty and will benefit from energy retrofit (Tabata & Tsai, 2020). Femenías et al’s (2018) extensive literature review on property owners’ attitudes to energy efficiency argues that retrofit is typically motivated by other needs, referred to by Outcault et al (2022) as ‘non-energy impacts’ (NEIs). While lists of NEIs are inconsistent in the literature, categories related to “weatherization retrofit” refer to comfort, health, safety, and indoor air quality (Outcault et al., 2022). Worldwide retrofit schemes such as RetrofitWorks and EnerPHit use varying metrics to define low carbon retrofit, but their universally adopted focus has been on end-point performance targets, which do not include changes to energy using behaviour and practice (Fawcett, 2014). An example of an end-point performance target is Passivhaus’ refurbishment standard (EnerPHit), which requires a heating demand below 25 kWh/(m²a) in cool-temperate climate zones; zones are categorised according to the Passive House Planning Package (PHPP) (Passive House Institute, 2016).  

The in-depth study of energy poverty as a social phenomenon commenced in the late 19th century through the works of British social researchers Booth and Rowntree (O’Connor, 2016). This era was characterised by significant social and economic transformation, and these scholars were troubled by the living conditions and welfare of impoverished urban populations, who were residing in congested and unsanitary environments. Throughout the 20th century, poverty in policy contexts became quite narrowly defined as a lack of income. However, it was another social concern in the UK that led to the development of concepts like ‘fuel poverty’ or ‘energy poverty’ a century after Booth and Rowntree.[i] Following the 1973 oil crisis, the Child Poverty Action Group took the initiative to address how increasing energy costs were affecting low-income households in the UK (Johnson & Rowland, 1976). As essentials like heating, electricity, and fuels became necessary for maintaining a decent standard of living in modern British society, this advocacy group pushed for government financial support. Later, Bradshaw and Hutton (1983) introduced a narrower definition of energy poverty: “the inability to afford adequate heat in the home”. Since then, studies on energy poverty have typically excluded motor fuels, as they fall under transport poverty, a related but separate area of study (Mattioli et al., 2017). Energy poverty, as defined by Bouzarovski and Petrova (2015, p. 33), refers to "the inability to secure or afford sufficient domestic energy services that allow for participation in society." Although the precise boundaries of relevant domestic energy usage are still debated, this definition expands beyond mere heating as it encompasses energy used for cooling, which is particularly relevant in warmer climates (Thomson et al., 2019). Moreover, it enables a socially and culturally dependent understanding of what it means to participate in society (Middlemiss et al., 2019). On 13 September 2023, the European Union (2023) officially defined energy poverty as “a household’s lack of access to essential energy services, where such services provide basic levels and decent standards of living and health, including adequate heating, hot water, cooling, lighting, and energy to power appliances, in the relevant national context, existing national social policy and other relevant national policies, caused by a combination of factors, including at least non-affordability, insufficient disposable income, high energy expenditure and poor energy efficiency of homes”. The doctoral thesis and subsequent book by Brenda Boardman, Fuel Poverty: From Cold Homes to Affordable Warmth (1991), marked a significant breakthrough in energy poverty research. She emphasised the detrimental impact of energy-inefficient housing on health and quality of life. In the decades that followed, substantial literature confirmed her qualitative findings (Thomson et al., 2017). Notably, studies have demonstrated the adverse effects of living in energy poverty on physical health (Liddell & Morris, 2010), mental health (Liddell & Guiney, 2015), stress levels (Longhurst & Hargreaves, 2019), social isolation (Harrington et al., 2005), and absenteeism (Howden-Chapman et al., 2007). Boardman’s work introduced an indicator that has remained influential to this date, although it was not the first attempt to operationalise the concept of fuel poverty (Isherwood & Hancock, 1979). Her ‘2M’ indicator categorises a household as energy poor if it needs to allocate twice the median share of its budget for energy expenses to heat its home adequately. Boardman calculated this threshold to be 10% at that time. Due to its simplicity and ease of comprehension, many governments directly adopted this 10% threshold without considering specific contextual circumstances. Since the early nineties, numerous attempts have been made to develop alternative indicators. Highly influential ones include ‘Low Income High Cost’ (LIHC) by John Hills (2012), ‘Low Income Low Energy Efficiency’ (LILEE) that subsequently became the official British indicator (BEIS, 2022), and a 'hidden' energy poverty indicator by (Meyer et al., 2018). Critiques of these indicators focus, amongst other things, on their simplicity and perceived 'technocratic' approach (Croon et al., 2023; Middlemiss, 2017). This marked the beginning of significant government commitment, initially in the UK and later in other countries to address energy poverty. Although certain forms of cold weather payments had already been introduced by the UK's Conservative administrations, it was under the successive governments of Blair and Brown, following the publication of Boardman's work, that programmes such as the Winter Fuel Payment and Warm Home Discount were implemented (Koh et al., 2012). The UK examples highlight bipartisan support for addressing energy poverty, with both the Conservatives and Labour backing these efforts. This policy objective has also gained momentum in various legislative contexts, leading the EU to incorporate energy poverty alleviation as a fundamental pillar of the European Green Deal and a specific goal of its landmark Social Climate Fund (European Commission, 2021). Over the last three decades, public interest in energy poverty as a 'wicked' problem has surged, particularly during the recent energy crisis. This crisis began in 2021 when energy markets tightened due to a post-pandemic economic rebound, and it worsened dramatically after Russia's invasion of Ukraine in February 2022 (IEA, 2023). Extensive research on the impact of this price surge on energy poverty levels has been carried out throughout Europe and globally (Guan et al., 2023; Simshauser, 2023). Consequently, energy poverty has become a significant focal point in discussions related to the 'just transition,' especially within the realm of energy justice, as it serves as a valuable concept for targeting policies towards a specific vulnerable group in this context (Carrosio & De Vidovich, 2023).     [i] ‘Fuel poverty' and 'energy poverty' are used interchangeably, with the former being more common in the UK and the latter in mainland Europe (Bouzarovski & Petrova, 2015). Previously, scholars in the UK used 'energy poverty' to denote a lack of access to energy and 'fuel poverty' when affordability was the concern (Li et al., 2014). However, this distinction is no longer maintained.

Decarbonisation, a term which echoes through the corridors of academia, politics, practical applications, and stands at the forefront of contemporary discussions on sustainability. Intricately intertwined with concepts such as net-zero and climate neutrality, it represents a pivotal shift in our approach to environmental sustainability. In its essence, decarbonisation signifies the systematic reduction of carbon dioxide intensity, a crucial endeavour in the battle against climate change (Zachmann et al., 2021). This overview delves into the multifaceted concept of decarbonisation within the context of the European Union. Beginning with a broad perspective, we examine its implications at the macro level before homing in on the complexities of decarbonisation within the realm of building structures. Finally, we explore the literature insights, presenting key strategies that pave the way toward achieving a decarbonised building sector. From a broad perspective, decarbonisation is an overarching concept that aims to achieve climate neutrality (Zachmann et al., 2021, p.13). Climate neutrality means achieving a state of equilibrium between greenhouse gas emissions and their removal from the atmosphere, preventing any net increase in atmospheric CO2 concentration (IEA, 2022). From an energy decarbonisation perspective, however, in a document provided by the Economic, Scientific and Quality of Life Policy Department at the request of the Industry, Research and Energy (ITRE) Committee, Zachmann et al. (2021) explain that energy systems require a fundamental shift in the way societies provide, transport and consume energy (Zachmann et al., 2021). In the construct of decarbonisation, as outlined by the Intergovernmental Panel on Climate Change (IPCC), the focus lies on strategic directives aimed at reducing the carbon content of energy sources, fuels, products and services (Arvizu et al., 2011; Edenhofer et al., 2011). This complex process involves the transition from carbon-intensive behaviours, such as fossil fuel use, to low-carbon or carbon-neutral alternatives. The main goal of decarbonisation, therefore, is to reduce emissions of greenhouse gases such as CO2 and methane, which are closely linked to the growing threats of climate change (Edenhofer et al., 2011). Hoeller et al. (2023) explain that decarbonisation efforts within the Organisation for Economic Co-operation and Development (OECD) focus on harmonising economic growth, energy production and consumption with climate objectives to mitigate the adverse effects of climate change while promoting sustainable development (Hoeller et al., 2023). From a pragmatic perspective, however, according to the OECD Policy Paper 31: A framework to decarbonise the economy, published in 2022,  progress on economic decarbonisation remains suboptimal. This raises the urgent need for a multi-dimensional framework that is not only cost-effective but also inclusive and comprehensive in its strategy for decarbonisation (D’Arcangelo et al., 2022). D’Arcangelo et al. (2023) add that such framework should include several steps such as setting clear climate targets, measuring progress and identifying areas for action, delineating policy frameworks, mapping existing policies, creating enabling conditions, facilitating a smooth transition for individuals, and actively engaging the public. From an academic perspective, Weller and Tierney (2018) provide an explanation of decarbonisation, defining it as a twofold concept. Firstly, it involves reducing the intensity of fossil fuel use for energy production. Secondly, it emphasises the role of policy in mitigating the negative externalities associated with such use. They argue that decarbonisation is a politically charged policy area that needs to be 'just', while also serving a means to revitalise local economies (Weller & Tierney, 2018). Kyriacou and Burke (2020) expand on this definition, highlighting decarbonisation as the transition from a high-carbon to a low-carbon energy system. This transition is driven by the need to mitigate climate change without compromising energy security. Boute (2021), on the other hand, emphasises the long-term structural reduction of CO2 emissions as the core strategy of decarbonisation. Boute adds that the effectiveness of decarbonisation must be measured in terms of a unit of energy consumed across all activities. In the economic context, the Oxford Institute for Energy Studies concludes that decarbonisation aims to reduce the carbon intensity of an economy. This reduction is quantified as the ratio of CO2 emissions to gross domestic product (Henderson & Sen, 2021). Addressing methodological concerns, Buettner (2022) added that decarbonisation is often misused as a generic term. Moreover, Buettner highlights the diverse levels at which decarbonisation occurs, ranging from carbon neutrality (focused on reducing CO2 emissions), to climate neutrality (aiming to reduce CO2, non-fluorinated greenhouse gases, and fluorinated greenhouse gases) and, finally, to environmental neutrality (which reduces all substances negatively impacting the environment and health) (Buettner, 2022). The debate on the decarbonisation of the construction sector revolves around similar issues. The report on Decarbonising Buildings in Cities and Regions, published by the OECD in 2022, defines the concept as reducing energy consumption by improving envelope insulation, installing high performance equipment, and scaling up the use of renewable sources to meet the energy demands (OECD, P24). Another definition comes from a working paper by the OECD Economics Department, Hoeller et al. (2023) contend, it is necessary to consider direct emissions from household fossil fuel combustion and indirect emissions from the generation of electricity and district heating used by households (Hoeller et al., 2023). The comprehensive study “Decarbonising Buildings” published by the Climate Action Tracker (CAT) in 2022, defines the term as transforming the building sector to achieve net zero emissions by 2050. Achieving this goal requires various technological solutions and behavioural changes to decarbonise heating and cooling, such as energy-efficient building envelopes, heat pumps and on-site renewables (CAT, 2022). Gratiot et al. (2023) consider decarbonisation as the process of reducing or eliminating CO2 emissions that contribute to climate change from a building’s energy sources. This involves systematically shifting buildings from carbon-intensive energy sources (e.g., gas, oil and coal) to low-carbon or carbon-neutral alternatives (e.g., solar, wind and geothermal). This process includes improving the energy efficiency of buildings through better insulation, lighting and appliances (Gratiot et al., 2023). Blanco et al. (2021) consider the decarbonisation of buildings and operation of buildings. This includes enhancing the energy efficiency of buildings and minimizing embodied carbon from building materials and construction activities of greenhouse gas emissions from the construction and operation of buildings. Achieving a decarbonised building sector is a multifaceted endeavour that demands extensive efforts in several key areas, such as energy sources, building envelope, building policy and transformation funds. The objective of the energy transition is to shift from reliance on fossil fuels to clean or renewable energy sources, primarily used for heating and cooling, such as heat pumps, district heating, hydrogen (Jones, 2021). Decarbonising the building envelope, on the other hand, involves improving the energy efficiency of buildings through better insulation, lighting and appliances. It also necessitates minimising embodied carbon from building materials and construction activities (CAT, 2022; D’Arcangelo et al., 2022). Incorporating effective policies into building construction is crucial. This includes adopting of performance standards and building codes that regulate the energy use and emissions of both new and existing buildings. These regulations directly impact the extent and pace of decarbonisation (CAT, 2022; Jones, 2021). Additionally, it is essential to establish a clear vision and climate targets for the buildings sector and operationalise them with a comprehensive policy mix that encompass emissions pricing, standards, regulations and complementary measures (Jones, 2021). The most significant challenge lies in financing the transition to a decarbonised sector. Therefore, it is imperative to mobilise finance on a large scale and collaborate with industry stakeholders. This collaboration is vital to facilitate the transition, overcome barriers, and manage the costs associated with deploying low- or zero-carbon technologies (D’Arcangelo et al., 2022). In summary, the overarching concept of decarbonisation primarily targets the reduction of carbon dioxide in economic and industrial activities, with a focus on energy production and distribution systems. At the building level, the emphasis lies in integrating low-carbon or carbon-neutral systems to minimise both direct and indirect emissions. Nevertheless, the literature examined indicates that other societal aspects, including social and behavioural factors, have not been thoroughly researched. This gap in knowledge could challenge the realisation of the goal of carbon neutrality by 2050 and underscores the need for further studies in these areas.

As the name suggests, Post-Occupancy Evaluation (POE) is the process of assessing the performance of a building once it has been occupied. It is often conflated and falls under the umbrella of Building Performance Evaluation (BPE) (Boissonneault & Peters, 2023; Preiser, 2005; Stevenson, 2018). Other definitions refer to POE as any activity intended to assess how buildings perform and the level of satisfaction of their users, ranging from simple survey questionnaires to indoor environmental quality (IEQ) measurements, which makes its scope very broad (Li et al., 2018). Nevertheless, in the case of POE, the focus should be on the occupants’ experience of the building and the impact of spaces on their behaviour and well-being (Watson, 2003 in Sanni-Anibire et al., 2016). It is commonly suggested that POE should be conducted at least a year after the handover and occupation of the building so that users can experience and test it under different weather conditions (RIBA et al., 2016). In the context of housing, housing providers, developers and architecture practices can benefit from enquiring what makes a good design from the occupants’ point of view. A systematic and rigorous POE combined with periodic user experience surveys can be very beneficial as it helps to improve relationships with tenants and provide a better picture of the quality of the housing stock. Thus, POEs do not only help to balance the scale between the social, economic and environmental aspects of buildings but also revitalise the role of research in the whole life cycle of projects. Despite its potential benefits for the various stakeholders engaged in the production of the built environment, POE is not a widespread practice in the sector. There is a notable absence of literature and research on the subject (Durosaiye et al., 2019; Hadjri & Crozier, 2009). However, since the 2010s, there has been a growing academic interest in POE, as evidenced by the increasing number of scientific publications, including studies related to post-occupancy evaluation (Li et al., 2018). There is a consensus in literature that learning from experience, whether from unintended consequences of ill-considered design or from successful projects, through the active involvement of occupants and users of buildings is a pathway for innovation. POE is commonly considered as an activity that requires long-term commitment and can be time and resource-consuming. This is a limitation that can be explained by the short-term logic of the construction sector and the fleeting commitment of developers, especially private and profit-driven, to the communities and clients they work with. In the same vein, the question of who is responsible for commissioning and conducting a POE represents the biggest barrier to its widespread implementation in the sector (Cooper, 2001). Concerns are inextricably linked to the cost and scope of the assessment, the equipment and professionals involved, and the possibility of being held accountable for flaws that might be exposed by the activity. Discussions around the importance of inspecting buildings after completion to assess their environmental performance have gained momentum in recent decades as a consequence of the evidenced climate crisis and the significant share of carbon emissions attributable to activities related to the built environment (according to UNEP (2022), 37 per cent of CO2 emissions in 2021). Nonetheless, the emergence of POE as a concept for the built environment dates back to the 1960s in the USA, where it was originally used to assess institutional facilities and fell mainly within the remit of facility managers (Preiser, 1995). Later, the PROBE (Post Occupancy Review of Building Engineering) research conducted between 1995 to 2002 on 23 non-residential case studies in the UK helped spread the concept among the whole gamut of professionals involved in the design and construction of buildings (Bordass et al., 2001; Cohen et al., 2001). With respect to design and housing, the work of Marcus and Sarkissian (1986) in Housing as if People Mattered is worth mentioning. In this book, the authors have outlined a set of design guidelines derived from evidence gathered through POEs. Their research was conducted with the aim of comprehending people's preferences and dislikes about their neighbourhoods and homes, utilizing a people-centred perspective that delves into " the quality of housing environments from a social standpoint, as defined by residents" (p.5). Their approach to POE is grounded in viewing housing as a process rather than a mere product. They propose rethinking the relationship between the designer and inhabitant, extending beyond the completion of buildings. This perspective aligns with that of Brand (1995), who views buildings as intricate systems governed by the 'Shearing layers of change', a concept developed from Duffy's proposal (Duffy & Hannay, 1992). Accordingly, buildings are understood as layered structures in which time plays a pivotal role in the way they interact with each other and with the user. As Duffy stated, quoted in Brand (1995, p.12): “The unit of analysis for us isn’t the building, it’s the use of the building through time. Time is the essence of the real design problem.” This renders it necessary to go back to the building once finished and continue doing so throughout its lifecycle. The levels of POE The literature distinguishes between three ‘levels of effort’ at which POE can be conducted, which differ mainly in terms of the thoroughness and purpose of the assessment: indicative, investigative, and diagnostic (Hadjri & Crozier, 2009; Preiser, 1995; Sanni-Anibire et al., 2016). These levels vary in methods and the degree of engagement of researchers and participants, and encompassing the phases of planning, conducting and applying. They can be described briefly as follows: Indicative: This level provides a general assessment of the most important positive and negative aspects of the building from the users' point of view. It involves a brief data collection period, characterised by walk-throughs, interviews and survey questionnaires with occupants. It is not exhaustive and may reveal more complex problems that need to be addressed with an investigative or diagnostic POE. It can be completed in a few hours or days. Investigative: If a relevant problem identified in an indicative POE requires further research, an investigative POE is carried out. This second level implies a more robust amount of data to be collected, the use of more specialised methods, and possibly the disruption of occupants' routines and building use due to the prolonged engagement in the research endeavour. It can take weeks to months to complete. Diagnostic: This level is characterised by its approach which is both longitudinal and cross-sectional. It may involve one or more buildings and a research process that may take months to a year or more to complete. It is more akin to research conducted by specialised institutions or scholars. The scope can be very specific but also have sector-wide implications. Possible applications of the information gathered through POE A more recent review of the literature on POE studies highlights the variegated range of purposes behind it: impact of indoor environmental quality on occupants, design and well-being, testing of technologies, informing future decision-making or feedforward, and impact of building standards and green rating systems, to name a few (Boissonneault & Peters, 2023; Li et al., 2018). The breadth of applications and rationale for conducting POE studies show that it is a powerful tool for assessing a wide array of issues in the built and living environment, and partly explain the interest it holds for researchers. However, the industry is still lagging behind, which hinders the dissemination and further implementation of the findings and results.  More collaboration between academia and industry is therefore crucial as the great impact lies in applying POE and BPE as a structural part of the sector’s practice. Moreover, since POE primarily relies on fieldwork and the collection of empirical data, a more comprehensive assessment that incorporates mixed methods and a systematic approach can yield greater benefits for the entire building production chain. The collected feedback, analysis and resulting conclusions can create learning loops within organisations and bring about real changes in the lives of current and future building users. Therefore, a robust POE should be accompanied by the implementation of the concomitant action plan to address the problems identified. For this purpose, a theory of change approach can be helpful. In this sense, POE can become a very effective facility management tool (Preiser, 1995). Some examples of the varied uses of data provided by robust POE and BPE include the creation of databases for informed decision-making, benchmarking and integration into BIM protocols or GIS-powered tools. In this sense, generating data that can be compared and benchmarked is critical to the long-term impact and value for money of undertaking the activity. It is therefore imperative to recognise POE for its benefits rather than viewing it as a liability or a mere nice-to-have feature. On the other hand, POE inherently involves a wide range of disciplines within the built environment, including design, engineering, psychology, policy and finance, among others. This multidisciplinary aspect can be leveraged to promote transdisciplinary research to help better understand the relationships between buildings and people It delves into the impact of these relationships, considering human behaviour and well-being. This perspective is often referred to as the building performance-people performance paradigm, as denominated by other POE researchers (Boissonneault & Peters, 2023). Architectural geographers, for instance, have explored the various meanings and emotions ascribed by inhabitants to buildings, particularly council estates in the UK, through actor-network theory-informed research (Jacobs et al., 2008, 2016; Lees, 2001; Lees & Baxter, 2011). Similarly, the work of organisations such as the Quality of Life Foundation encompassed in the Quality of Life Framework (Morgan & Salih, 2023; URBED, 2021), has highlighted the link between the places where we live and its impact on our quality of life through systematic POEs conducted in collaboration with social housing providers and local authorities. Amid the climate emergency and the pressing need to curtail carbon emissions, there is now a need for the sector to innovate and mitigate the impact of building construction and operation. It has been argued that sustainability cannot be achieved only by adopting energy-efficient technologies or by promoting certifications such as LEED, Passivhaus, or assessment protocols such as BREEAM (Building Research Establishment Environmental Assessment Methodology). As discussed earlier, conducting these assessments is an effective tool to mitigate and solve the discrepancy between the expected energy performance of the designed building vis à vis that of its real-life counterpart, the so-called performance gap. POE can be used to ascertain the social performance gap by including qualitative and well-being-related indicators (Brown, 2018). In this way, buildings are evaluated not only in terms of their ability to comply with building regulations and environmental goals, but also in meeting social objectives in order to provide greater sustainability and affordability, particularly in housing.

The concept ‘prebound effect’ refers to the phenomenon where actual energy consumption in buildings is significantly lower than the calculated energy consumption needed to maintain comfortable indoor temperatures (Sunikka-Blank & Galvin, 2012). This is typically observed in older, less energy-efficient homes, where occupants often under-heat their homes due to financial constraints, leading to lower actual energy usage than predicted by models (Galvin & Sunikka-Blank, 2016). This contrasts with the more commonly used term ‘rebound effect’, which occurs when energy consumption increases following efficiency improvements, offsetting some of the anticipated savings (Teli et al., 2015). The difference between the two is clearly visualised in Figure 1, developed by Sunikka-Blank and Galvin (2012). Energy savings discrepancies The prebound effect leads to significant discrepancies in energy savings estimates for retrofitting projects. Standard models often overestimate energy savings because they do not account for the lower baseline consumption caused by the prebound effect. This discrepancy is crucial because it implies that thermal retrofits may not yield the anticipated reductions in energy use and carbon emissions, which are often overstated in policy and financial analyses (Galvin, 2023). A detailed explanation of the performance gap is available in the vocabulary entry by Furman (2024). For instance, in Hungary, research shows that energy consumption predictions are often based on technical data alone, ignoring actual under-consumption behaviours, leading to overestimates in expected savings (Gróf et al., 2022). Similarly, in Italy, studies have highlighted that prebound effects cause a gap between theoretical energy models and actual energy usage, complicating the accurate forecasting of energy savings from retrofits (Giuliani et al., 2016). The study by Van den Brom et al. (2019) also supports this, showing that building characteristics and occupant behavior significantly influence energy consumption, with discrepancies between expected and actual savings due to prebound effects being notable in both the Netherlands and Denmark. Additionally, Gróf et al. (2022) emphasised that the prebound effect is influenced by the financial status of households, with households having limited financial means more likely to under-heat their homes, resulting in a greater prebound effect and further complicating the prediction of energy savings. Inherently linked to energy poverty Therefore, the prebound effect is closely linked to energy poverty, as it often reflects the behaviour of households that cannot afford to heat their homes adequately. Households experiencing fuel poverty are more likely to exhibit high prebound effects due to their efforts to save money on heating (Galvin & Sunikka-Blank, 2016). For instance, social housing tenants in the UK showed significant prebound effects, leading to under-heated homes and reduced energy consumption prior to retrofitting (Teli et al., 2015). Addressing the prebound effect involves recognising and mitigating the financial constraints that lead to under-consumption, thus improving both the accuracy of energy savings predictions and the living conditions of households experiencing energy poverty (Galvin, 2024a, 2024b). Moreover, hidden energy poverty is a critical aspect of this issue. Households often are not categorised as energy poor based on expenditure-based indicators because they do not dare to put their heating on due to financial constraints (Cong et al., 2022; Eisfeld & Seebauer, 2022). These households minimise their heating to the point where their energy expenditures are deceptively low, masking the severity of their situation. However, more recent indicators that focus on energy efficiency rather than high costs would identify them as energy poor (Betto et al., 2020). Addressing hidden energy poverty requires a shift in assessment criteria, also focusing on the energy efficiency of homes rather than just the high cost of energy bills. This approach ensures that households struggling with inadequate heating due to financial constraints are accurately identified and supported (Croon et al., 2023).

Increasing the thermal properties of the building envelope is a passive strategy to reduce energy loss and ensure significant reductions in energy demand (Grecchi, 2022). Van den Brom et al (2019) define thermal renovation as “renovation measures that are taken to reduce energy consumption used for thermal comfort”, and group thermal insulation, airtightness and efficient electrical system into a single category. Accordingly, deep ‘thermal’ renovation occurs when significant improvement in at least three building components bring thermal performance to a level equal to or higher than the current building regulation standards (van den Brom et al., 2019). These building components include roof insulation, floor insulation, façade insulation, window improvements, heating system, domestic hot water system, and ventilation system (van den Brom et al., 2019). Other authors (Institute for Sustainability & UCL Energy Institute, 2012; Sojkova et al., 2019; Traynor, 2019) divide electrical systems into a further category for clearer practical application. The concept of airtightness is revered for saving energy, avoiding structural damage, contributing to thermal comfort (Bastian et al., 2022), and is key to reducing heat loss through ventilation (Roberts, 2008). Draught proofing involves draught-stripping, replacing leaky windows and closing off unused chimneys (Roberts, 2008). The location of an airtight layer should be identified, and all penetrations through it minimised, sealed, and recorded (Traynor, 2019). This airtight layer can be airtight board, a plastered wall, or a membrane with appropriate tape at all junctions such as window openings (Traynor, 2019). Triple-glazed windows in combination with any frame material are the most efficient glazing system at reducing primary energy cost and CO₂ emissions (Sojkova et al., 2019). All air pockets should be sealed to prevent draughts and thermal bridging. Thermal bridging should be eliminated wherever possible, although a comprehensive thermal reduction with low internal surface temperatures can prevent physical problems such as moisture and mould (Bastian et al., 2022). There are many forms of insulation to consider during retrofit that considerably contribute to a reduction in heat loss. Filling external cavity walls with insulation can reduce heat loss through walls by up to 40% (Roberts, 2008). Ground floor insulation and roof insulation are also necessary steps in DER (Grecchi, 2022; Roberts, 2008; Traynor, 2019). Ground floor insulation can occur in suspended timber floors between joists or above solid concrete floors (Traynor, 2019). Roof insulation can be added between structural elements, or using a ‘cold’ roof solution, with insulation laid or sprayed over the existing ceiling (Traynor, 2019). Alternatively, green roofs can reduce the amount of heat penetration through roofs, playing a similar role to roof insulation. This is done by absorbing heat into their thermal mass alongside the evaporation of moisture but will require structural upgrades to manage the new load (Roberts, 2008). External wall insulation (EWI) protects the building fabric, improves airtightness and is relatively quick and easy to install (Roberts, 2008). EWI can also help mitigate overheating by absorbing less heat than the original material, while allowing existing thermal mass from solid masonry walls and concrete to be retained within the insulated envelope (Bastian et al., 2022). The two main external insulation systems are ventilated rainscreen systems and rendered insulation systems (Roberts, 2008). EWI is inappropriate for historical building use because it will cover the historical architectural character. Gupta & Gregg’s (2015) preserved the original exterior façade by using internal wall insulation inside the front façade and EWI on all other façades. However, drawbacks to this solution can include the loss of internal floor area, and reduced energy efficiency as notable heat loss can occur where the internal insulated wall meets the external insulated wall (Gupta & Gregg, 2015).