It is widely accepted that climate change has exacerbated the risk of acute overheating in residential buildings. South and Southeast of England are particularly exposed to increased temperatures due to global warming. Under the current climate, there is a substantial risk of overheating in London, and if climate change adaptation measures are not incorporated into building regulations, design, and retrofit, occupants' exposure to excessive interior temperatures is likely to significantly increase in the future (Pathan et al., 2017) affecting their health and wellbeing. Recent studies show that climate is undergoing noticeable changes primarily as a result of human activities, particularly due to greenhouse gas emissions, which have reached unprecedented levels in recent times; (I.P.C.C., 2014). These have led to an increase in the duration, intensity, and frequency of heat waves worldwide (Perkins et al., 2012). Other studies suggest that outdoor temperatures may rise even greater than initially estimated (Kala et al., 2016). According to the UK Climate Change Projections 2009 (UKCP09), all UK regions are expected to become warmer, especially during the summer period (DEFRA, 2009), increasing the risk of overheating across the UK (Beizaee et al., 2013)(Lomas & Porritt, 2017). Under the medium emissions scenarios, the greatest rise in summer mean temperatures will be in Southern England with up to a 4.2°C average increase by the end of the century (Murphy et al., 2009). It is expected that by the middle of the century, the daytime temperatures in London will exceed 32°C for one-third of June to August (Hall et al., 2009).

There is a strong link between high temperatures and mortality rates. This was reflected in the 2003 and 2006 European heatwaves resulting in substantial damage and disruption to the economy, infrastructure, and transport, as well as a significant increase in the excess heat-related mortality rates especially amongst older people (Fouillet et al., 2006)(Fouillet et al., 2008)(Kovats & Hajat, 2008). In August 2003, over 30,000 excess deaths were recorded across Western Europe during an exceptional heatwave (Kosatsky, 2005), with a total of 2091 cases in the UK, 616 cases of which were in London (Johnson et al., 2005). Moreover, in cities like London, the risk of overheating is further increased due to the Urban Heat Island (UHI) effect (Santamouris et al., 2015). In 2006, London had the highest rates of heat-related mortality during hot weather (Hajat et al., 2007). According to estimates, the proportion of excessive heat-related mortality in outer London, inner London, and central London that can be attributed to the UHI effect during a warm summer in 2006 was approximately 38%, 47%, and 47%, respectively (Milojevic et al., 2011). The situation is expected to deteriorate increasing the heat-related mortality rates three-fold by 2050 (P.H.E., 2015). Preventing heat-related mortality is therefore a major public concern in the UK and Europe ((Menne & Matthies, 2009-12); (P.H.E., 2015); (W.H.O., 2004-12)).

The UK was the first country in the world to introduce a long-term, legally binding climate change mitigation framework. According to the Climate Change Act 2008, UK emissions must decrease by at least 80% by 2050 when compared to 1990 levels (UKGovernment, 2008). Meanwhile, incorporating energy efficiency regulations may lead to more airtight and highly insulated building envelopes that could lead to trapping heat increasing the risk of acute overheating. Currently around 20% of existing homes in the UK experience overheating (Beizaee et al., 2013) (Hulme et al., 2013), (Z.C.H., 2015). Defective retrofit ((Dengel & Swainson, 2012-12-06); (Shrubsole et al., 2014)) and, in particular, inappropriate energy efficiency measures that are not coupled with suitable passive cooling solutions ((Gupta et al., 2015); (Hub, 2016); (Santamouris & Kolokotsa, 2013)) could increase risk of overheating. Frequent overheating may lead to increased use of mechanical cooling systems that in turn result in higher carbon emissions that will contribute further to climate change (Hulme et al., 2013). Passive design solutions (e.g. the use of natural ventilation, thermal mass, solar shading, glazing type/area, and building orientation) are widely recognized as the most environmentally friendly techniques to mitigate the risk of overheating and improve occupants’ thermal comfort, and health, ((Gupta et al., 2015); (Hub, 2016); (Kolokotroni et al., 2010); (Lafuente & Brotas, 2014); (Santamouris & Kolokotsa, 2013); (Santamouris et al., 2007)).

Investigations have demonstrated that dwelling style (Baborska-Narozny et al., 2016)(Firth et al., 2007)(Firth & Wright, 2008)(Lomas & Kane, 2013)(Mavrogianni et al., 2015)(Wright et al., 2005), construction age, and building fabric (Beizaee et al., 2013)(Firth & Wright, 2008), (Hulme et al., 2013);(Mavrogianni et al., 2015) are important factors contributing to indoor overheating. It has been shown that homes built in the 1960s, 1970s, and after 1990 tend to be most at risk of overheating. There is evidence that highly energy-efficient homes, whether newly constructed or modified, may be susceptible to summer overheating, especially those designed to Passivhaus standards (Morgan et al., 2015)(Sameni et al., 2015)(Mitchell & Natarajan, 2019). In order to minimize the negative effects on occupants' health and well-being, the UK government has been advised by the Committee on Climate Change (CCC) Adaptation Sub-Committee that "more action is needed" to limit overheating hazards of buildings ((Committee, 2014),(Committee, 2017)). Although there have been concerns about the risk of overheating in UK terraced homes and apartments for some time ((Housing et al., 2012);(Alliance, 2014), (Z.C.H., 2015)), the issue is still largely underreported in the literature (Gupta & Gregg, 2016).

Improving buildings' energy performance will reduce energy bills and CO 2 emissions of domestic buildings (Owen et al., 2014), (Energy & Change, 2012)); however, there is still a need to assess the effects of building fabric upgrades on the risk of overheating for the current and future climate scenario in order to develop future-proof retrofit strategies to not only improve energy performance but also avoid the 'unintended' effects of such strategies on indoor environments and health and wellbeing of building occupants. To this end, this study evaluates the effects of energy-efficient retrofit strategies (with a focus on building fabric upgrades) on the energy and thermal performances of an endterraced house located in London.

This study is undertaken in three phases, as shown inFigure 1: the first phase identifies the current thermal performance of the base case scenario, using the DesignBuilder software. The second phase includes the creation of different modelling scenarios; for a) the built era ((B.R.E., 2019-09)), b) Approved Document L for improved existing elements in existing dwellings ((Department for Levelling Up, 2023)), and c) Passivhaus standards ((PassiveHouseInstitute, 2015); (Trust, 2023)), as shown in Table 1. Relevant U-values, G-values, and airtightness rates are assigned to each scenario Table 1. The third phase assesses occupants’ thermal comfort for the future climate scenario using CIBSE Weather Files for 2050.

This paper investigated the thermal comfort and risk of overheating in a social housing case study building. Different scenarios were analyzed to assess and select the optimum option to avoid overheating in summer while improving thermal comfort by reducing cold conditions during winter. For the current weather conditions, both in the base case and the as-built scenarios, the living room and the kitchen were in the acceptable range for summer and winter assessments, while for the Part L scenario, three zones (double room, living room, and kitchen) met the requirements. For the Passivhaus scenario, all the zones were within the acceptable ranges except for the main bedroom which experienced some overheating during winter. Generally, for the retrofitted options, the situation improved for all the zones apart from the main bedroom which experienced a warmer environment. For future weather conditions, bedrooms experience a risk of overheating during both summer and winter seasons. The situation significantly improved for the Passivhaus compared to the other scenarios. Yet, the results revealed that only the living room and kitchen met the requirements for both summer and winter assessments, while bedrooms were in the comfortable range for winter assessments (except for the south-facing main bedroom).

In summary, south-facing bedrooms expose a high risk of overheating and should be considered with more cases for retrofitting. The Passivhaus scenario passed the summer test for the current climate but failed for the 2050 scenario. This agrees with the previous findings highlighting bedrooms as the most exposed to overheating for future climate scenarios during summer (Beasley, n.d.). It should be noted that even though the bedrooms failed the test, there were improvements in the total number of discomfort hours in comparison to other case study scenarios. Building orientation, window opening areas, and insulation strategies combined could lead to overheating particularly during summer. Therefore, further adjustments, such as shading and/or lower G-values, should be considered to reduce the heat gains through the windows for the south-oriented rooms. Finally, more investigation is required to assess the effect of other factors including occupant behavior, natural and mechanical ventilation strategies, thermal mass, construction methods and materials, and other passive design strategies on thermal comfort in buildings.

This document is an output from a research project, Healthy Energy Efficient Dwellings (HEED), funded by the UK Research and Innovation (UKRI), Medical Research Council (MRC) [Grant number: MR/Y503186/1]. Also, the authors would like to thank the Newham Council, Hyde Housing Association, and iOpt Limited for supporting the project and providing access to the properties.

The abstract of this paper was presented at the Environmental Design, Material Science, and Engineering Technologies (EDMSET) Conference -1st Edition, which was held on the 22^nd -24^th of April 2024.