Assessing Overheating Risks in Moderately Insulated Irish Social Housing: Analysis of Building Energy Ratings and Indoor Temperature Profiles

Abstract

As buildings become more energy-efficient in cold climates, the unintended consequence of increased overheating risk during warmer seasons necessitates attention. In this context, there is an absence of research addressing the assessment of overheating risks in residential buildings in Ireland. This study assesses data from a sample of 1100 social housing units in Dublin, the majority of which have a Building Energy Rating (BER) of C, representing moderately insulated dwellings. Using indoor temperature data and outdoor climate reports for 2022, the research evaluates overheating risks based on both static and adaptive criteria in the living room zone of dwellings. The static methods used include the Chartered Institution of Building Services Engineers (CIBSE) Guide A and the Passivhaus Institute standard, while adaptive methods follow CIBSE TM59. The findings reveal discrepancies in overheating risk assessments: overall, 4% surpass thresholds under CIBSE Guide A. In contrast, 41% of dwellings exceeded overheating thresholds under the Passivhaus standard during the May to September 2022. Adaptive criteria, however, indicated minimal overheating instances, at 0.4%. These results highlight how different assessment methodologies influence overheating risk conclusions. The impact of this study is two-fold. First it further strengthens existing literature which questions the appropriateness of static methods. Secondly, it shows that the risk of overheating in moderately insulated buildings in this sample set is minimal.

1. Introduction

Climate change is poised to significantly impact global weather patterns, with the increased frequency and intensity of heatwaves posing considerable health risks and exacerbating rates of morbidity and mortality [1]. Elevated ambient temperatures in urban areas are linked to numerous health risks, including a rise in premature mortality among vulnerable populations such as the elderly [2]. The European Environment Agency (EEA) highlights that for each 1 °C increase in temperature, mortality risk rises between 0.2% and 5.5% [3,4]. During the heatwave of July 2022, excess mortality in the European Union surged to +16%, compared to +7% in the preceding months [4]. Projections suggest that by the 2080s, Europe could face an annual increase of 60,000 to 165,000 premature deaths due to heat-related impacts, with Southern Europe bearing the brunt of this crisis [3]. These escalating risks underscore the urgency of addressing overheating in residential buildings.

In response to climate change, building designs have increasingly emphasized energy efficiency, particularly in cold climates, by enhancing airtightness and insulation to minimize heat loss. However, these advancements have inadvertently increased the risk of overheating during warmer seasons. This unintended consequence is further compounded by rising global temperatures and solar radiation levels, as projected by the Intergovernmental Panel on Climate Change (IPCC) [5]. Specifically in Ireland, warmer summers are anticipated to result in higher indoor temperatures in residential buildings, posing challenges to thermal comfort and occupant well-being [6]. This highlights the conflict between energy-efficient building designs and the growing challenge of thermal comfort, where practices designed to mitigate winter heating demands inadvertently exacerbate overheating risks during the warmer months.

Efforts to mitigate climate change have largely centred on reducing carbon emissions and improving the energy efficiency of buildings [6,7,8]. However, the impact of a changing climate on building performance—particularly overheating risks—has received comparatively less attention. This oversight is concerning given the increasing frequency and intensity of heatwaves [3] and the rise in tropical nights, where nighttime temperatures remain uncomfortably high [9]. Such extreme conditions, currently sporadic, are projected to become the norm by 2060 under medium emissions scenarios [10], highlighting the need for forward-thinking approaches to building design that also account for overheating risks. These developments underscore the importance of shifting the focus from merely energy-efficient buildings to those that also account for overheating, ensuring both energy performance and occupant health.

Future projections indicate that peak and average summer temperatures across European capitals could rise by up to 10 °C by 2080, intensifying the overheating challenge [6]. The accumulation of internal and external heat gains, driven by urbanization and climate change, is expected to exacerbate this issue [6]. These changes highlight the urgent need to assess and address the risks of overheating in residential buildings, particularly in Ireland, where research on this topic remains limited. These findings suggest that as climate change continues to alter global and regional temperature patterns, there is an urgent need for proactive measures in the design and assessment of buildings to ensure resilience against rising temperatures.

This study investigates the overheating risks in residential buildings in Ireland by examining the indoor temperature profiles of 1100 social housing units in Dublin. By integrating static and adaptive criteria with Building Energy Ratings (BER), the research identifies dwellings at risk and explores correlations between BER and overheating vulnerability. By combining conventional static thresholds with adaptive criteria, this study aims to provide a comprehensive approach to evaluating overheating risks. The findings contribute to a nuanced understanding of how energy-efficient building practices interact with emerging climatic realities, emphasizing the critical need for adaptive retrofit strategies to ensure both thermal comfort and occupant safety in a warming world.

1.1. Policy Relevance of Research

According to the IPCC, they predict a significant rise in global temperatures and solar radiation in the coming decades, with unusually high summer temperatures becoming the norm by 2060 under medium emissions scenarios [5]. Projections from the UK Climate Programmed (UKCP) further highlight a potential increase in global radiation levels in the UK by 20% to 23% under a high-emissions scenario, with heatwaves—currently occurring once per decade—expected to become annual events by 2040 in both the UK and Ireland [11]. Ireland is no exception to these trends, as projections indicate an increase in average summer temperatures, which will exacerbate internal temperatures in residential buildings [6]. Moreover, temperatures observed during current heatwaves may represent cooler-than-average summer conditions by 2060 [10]. This issue is further intensified by the push toward energy-efficient construction and retrofitting, where airtightness and insulation improvements, while effective in reducing heating demands, inadvertently increase the risk of overheating. These projections highlight the growing need for national and international policy frameworks that prioritize both energy efficiency and thermal comfort, particularly in light of the rapidly changing climate and the increasing frequency of extreme weather events.

Overheating in buildings has wide-ranging negative implications for indoor comfort, occupant health, and energy efficiency. Morbidity and mortality rates linked to overheating have been steadily rising across Europe in recent decades [6,12]. In this context, Ireland and the UK, which are currently classified under a Temperate Oceanic Climate (Cfb) according to the Köppen–Geiger climate classification system, are experiencing shifts toward a warmer climate. The prevailing classification of warm temperate (C), fully humid (f), and warm summers (b) is anticipated to transition to a Cfb climate, characterized by warm temperate (C), fully humid (f), and hot summers (a), due to the impacts of climate change [13,14]. In the Irish context, where Cfb conditions are expected to evolve toward hotter summers by 2100 [14], the risk of overheating is particularly concerning. Evidence from countries with comparable climates, such as Denmark, Sweden, the Netherlands, and the UK, highlights the vulnerability of passive houses and nearly Zero Energy Buildings (nZEBs) to overheating [15]. The projected shift from temperate to warmer climates significantly amplifies the risks of overheating in Ireland, necessitating urgent action to understand and mitigate these risks within the context of building designs and regulations.

These projections further underscore the necessity of implementing mandatory overheating assessments and establishing legislative standards for new residential buildings to mitigate the long-term impacts of climate change on dwellings. Such measures will ensure resilience over the lifecycle of residential structures and safeguard occupant comfort and health. The absence of clear regulatory frameworks for addressing overheating in Ireland presents a critical gap that this study aims to address, ensuring that both new and retrofitted buildings can withstand the challenges posed by climate change.

1.2. Aim of the Study

The study seeks to address gaps identified in the literature by conducting an assessment of overheating risks across residential buildings in Ireland. In this regard, this study aims to:

• Evaluate the indoor temperature profiles of a large number of social housing units in Ireland to assess overheating risks during the summer months.
• Assess the impact of different overheating criteria, namely: Static criteria Guide A and the Passivhaus Institute standard, along with adaptive criteria CIBSE TM59.

2. Literature Review

Overheating, defined as excessive indoor heat that negatively impacts the health, comfort, and productivity of occupants, has become a pressing issue in building design and operation. [16]. This issue has garnered attention from European public health authorities, who have raised concerns about rising mortality rates due to heat-related events, urging the implementation of preventive measures [17]. Overheating is driven by a complex interplay of factors, including building characteristics, environmental and urban climate conditions, and architectural design [18]. Despite advances in understanding these factors, accurately calculating overheating remains a significant challenge.

The transition toward highly insulated, airtight buildings designed to meet nZEB and net-zero energy standards has amplified overheating risks. While these designs excel in reducing heat loss in colder climates, they unintentionally increase heat retention during warmer seasons, particularly under current and future climate scenarios [19,20,21]. Studies across Southern, Eastern, Western, and Northern Europe have consistently reported increased overheating risks in such buildings.

For example, McGill et al. [22] explored the prevalence of overheating in low-energy and new-build housing through a meta-analysis of indoor temperature data from 60 dwellings in the UK. The finding revealed that 57% of bedrooms and 75% of living rooms exceeded the CIBSE threshold of 25 °C for more than 5% of annual occupied hours, indicating an overheating problem in modern housing according to this method. Additionally, 30% of living rooms exceeded the adaptive comfort threshold of 3% occupied hours with a temperature increase of ≥1 K.

To address these challenges, the “Resilient Cooling Design Guidelines” by the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) [23], developed in collaboration with the International Energy Agency’s Energy in Buildings and Communities (IEA-EBC) program [24], provide comprehensive strategies for mitigating heat stress. These guidelines emphasize designing cooling systems that are energy-efficient and resilient to challenges like heatwaves and power outages, highlighting the importance of moving beyond conventional thermal comfort considerations.

Heiselberg et al. [25] conducted a multicriteria analysis assessing the overheating risks associated with energy renovations. Their study identified that certain retrofit measures, such as floor insulation and enhanced airtightness, could inadvertently elevate overheating risks. This finding underscores the importance of integrating ventilation and shading solutions into retrofit projects.

Recent research by Davies et al. [26] further explores passive cooling strategies through building fabric optimization in UK residential buildings. Their study found that standard construction with median U-values was more effective against overheating than Passivhaus designs. While cities like Manchester, Glasgow, and Belfast are unlikely to experience significant overheating even under future climate scenarios, London faces extreme overheating risks by 2080, emphasizing the need for adaptive design strategies.

Building on these observations, Mourkos et al. [27] provided a comprehensive overview of current methods for assessing overheating, highlighting the need to refine and standardize calculation approaches to ensure their reliability across different building types and climates. This aligns with the broader recognition of enhancing methodologies to better address overheating risks, particularly as energy efficiency standards evolve.

Mitchell and Natarajan [28] highlight that highly insulated dwellings, particularly those built to the Passivhaus standard, are at increased risk of overheating. McLeod et al. [10] also confirmed that super-insulated dwellings were already at risk of overheating in the UK and Northern Europe. Further supporting this concern, Figueiredo et al. [29] found significant summer overheating in a Passivhaus in Portugal, with indoor temperatures exceeding 25 °C for approximately 18% of the time during the summer, surpassing the Passivhaus overheating threshold, which permits this temperature excess for only 10% of the year.

Morgan et al. [30], monitored 26 Passivhaus-compliant new-build homes, identifying high insulation levels and occupant behavior as key overheating contributors. Notably, 25% of rooms experienced overheating for three to nine months, with 60% of rooms reaching temperatures of 29.5 °C in bedrooms and 28.3 °C in living rooms in July. Supporting these findings, Toledo et al. [31], in their post-occupancy evaluation, further emphasized the significance of ventilation, solar control, and architectural elements in influencing overheating, particularly in retrofitted buildings. This aligns with Morgan et al.’s assertion that future designs must integrate environmental and behavioral factors to mitigate overheating risks effectively.

Zahiri and Gupta [32], further highlighted the risks associated with highly insulated UK social housing by examining 24 dwellings retrofitted with ground source heat pumps (GSHPs). They found that approximately half of the living rooms and bedrooms experienced overheating according to CIBSE TM59 criteria, during the peak summer months of 2022, particularly in top-floor flats and southwest and south-facing bungalows. These studies highlighted the need for design adaptations to address overheating risks.

Finegan et al. [33] conducted an analysis of passive house dwellings using Passive House Planning Package (PHPP) simulations and found that these simulations consistently underestimated the extent of overheating. When the threshold was set to 25 °C, the simulation predicted no overheating, but actual measurements indicated a 2.64% frequency of overheating. When the threshold was lowered to 24 °C, the simulation predicted 2.1% overheating, while measurements showed a 4.67% overheating frequency. These findings underscore the discrepancies between simulated and actual overheating occurrences, pointing to underestimations in simulation tools and highlighting the need for complimentary experimental methods to address the growing challenges posed by overheating.

Building on these observations, Mulville and Stravoravdis [34] simulated a typical UK case study by using the UK national calculation method. Their research highlights significant limitations in current overheating assessment approaches, emphasizing the need for more robust and accurate methodologies to better address overheating risks in buildings.

The complexity of overheating calculation methods is compounded by the lack of standardized approaches across Europe. In this context, the IEA, through Annex 80 on Resilient Cooling in Buildings, has reviewed existing standards and regulations related to overheating calculation methods, criteria, and indicators [35]. Preliminary findings highlight inconsistencies among the methods and the absence of a unified and consistent approach to overheating calculations. Hamdy et al. [36] similarly highlighted significant disparities between countries, underscoring the need for standardized methods tailored to diverse European climates and occupant behavior. Given Europe’s diverse climates and varying thermal adaptation behavior [37], these differences are somewhat expected.

A recent study by Attia et al. [6] analysed 26 countries and identified Switzerland, Spain, Estonia, Germany, the UK, and France as leading countries in overheating evaluation but noted the absence of clear standards for mixed-mode residential buildings.

2.1. Criteria of Overheating Analysis

The literature on overheating is extensive and presents mixed conclusions across different regions, building typologies, and methodologies. The criteria for deriving overheating risk are of particular interest in this study.

The static criteria such as CIBSE Guide A [38,39] and the Passivhaus Institute [40] approach overheating assessment through thresholds based on fixed temperature limits and occupancy percentages. These criteria rely solely on natural temperature regulation, without artificial heating or cooling systems [41].

The CIBSE Guide A static criteria refer to a fixed definition of overheating where the internal operative temperature of living rooms should not exceed 25 °C for more than 5% of annual occupied hours and 28 °C for more than 1% of annual occupied hours. Also, the internal operative temperature of bedrooms should not exceed 24 °C for more than 5% of annual occupied hours and 26 °C for more than 1% of annual occupied hours [38,39].

The Passivhaus Institute, on the other hand, considers temperatures above 25 °C for more than 10% of the year as indicative of overheating [40].

Similarly, CIBSE TM59 [42], tracing its development through CIBSE TM52 [43], as well as European and British Standards BS EN 16798-1 [44] and 16798-2 [45] and their predecessor BS EN 15251 [46], provides operative temperature thresholds for both living rooms and bedrooms. It defines overheating as the condition when temperatures exceed specified limits for a certain percentage of occupied hours [47].

However, these static criteria have limitations, particularly in their ability to accurately predict occupant satisfaction with summer thermal comfort [48]. There is ongoing debate about the applicability of adaptive principles, especially in bedrooms, where the implications of such principles for sleep quality and overall health are critical [49,50]. Additionally, the static criteria fail to account for the severity of overheating, as they focus solely on the percentage of hours above a threshold, without considering the extent or duration of discomfort [51,52].

Adaptive criteria, such as those proposed in CIBSE TM52 [43] and CIBSE TM59 [42], aim to address these shortcomings by incorporating external climate conditions and human adaptability. CIBSE TM52 evaluates overheating risk based on outdoor temperatures and the ability of occupants to adapt to their environment, considering factors like seasonal temperature fluctuations [43,53]. In accordance with CIBSE TM52 and CIBSE TM59, it is recommended that overheating thresholds for the assessment of overheating in naturally ventilated buildings be set based on overheating categories spanning from category I (vulnerable group) to category IV as follows [22,33,41,54]:

• Category I: a category of building that houses vulnerable occupants such as the elderly, sick, and infants. Nursing homes, crèches, and hospitals are all considered to fall under this category [22,33,41,54].

Upper limit T_MAX (°C) = 0.33 T_RM + 18.8 + 2

(1)

Lower limit T_MAX (°C) = 0.33 T_RM + 18.8 − 2

(2)

• Category II: used for new buildings generally and used as the default option. Category II is considered to apply to all new dwellings that are not designed to accommodate vulnerable occupants [22,33,41,54].

Upper limit T_MAX (°C) = 0.33 T_RM + 18.8 + 3

(3)

Lower limit T_MAX (°C) = 0.33 T_RM + 18.8 − 3

(4)

• Category III: for existing buildings only (not be used for the purposes of the TM59 methodology) [22,33,41,54].

Upper limit T_MAX (°C) = 0.33 T_RM + 18.8 + 4

(5)

Lower limit T_MAX (°C) = 0.33 T_RM + 18.8 − 4

(6)

• Category IV: Refers to conditions where values fall outside the specified criteria and are only acceptable for short periods [22,33,41,54].

Upper limit T_MAX (°C) = 0.33 T_RM + 18.8 + 5

(7)

Lower limit T_MAX (°C) = 0.33 T_RM + 18.8 − 5

(8)

CIBSE TM59 builds on these adaptive principles and offers a more granular approach by considering the performance of individual dwelling zones rather than assuming a uniform temperature across the entire building [41]. This zoning approach is aligned with Standards BS EN 16798-1 [44] and 16798-2 [45], which emphasize the importance of assessing room-level performance for a more accurate evaluation of overheating risks [55,56].

While the adaptive approach better accounts for occupant behavior and outdoor conditions, it assumes that all residents are capable of adapting to thermal variations in their environment. This assumption can be problematic in situations where occupants may have limited control over their indoor environment, such as during sleep [57]. For instance, the World Health Organization (WHO) recommends a safe and healthy indoor temperature range of 18 °C to 24 °C, with temperatures above 24 °C posing health risks, in living rooms and particularly sleeping areas [58,59]. The National Health Sustainability Office (HSE) recommends maintaining a comfortable indoor temperature range of between 18 °C and 23 °C. Additionally, the Heatwave Plan for England sets 26 °C as the upper limit for cool areas, with an ideal room temperature of around 20 °C [60]. CIBSE TM59 further underscores the importance of maintaining bedroom temperatures below 26 °C to avoid adversely affecting sleep quality and occupant health [61,62].

By integrating these established temperature guidelines with an understanding of adaptive strategies, it becomes evident that both static and dynamic factors play a key role in addressing thermal comfort and overheating risks.

3. Methodology

This section outlines the methodology employed in this study to assess overheating risk in residential buildings. The procedural framework is illustrated in Figure 1. The primary focus of this research is the analysis and assessment of overheating risk within a large sample set of residential buildings in Ireland.

3.1. Data Collection

This study utilizes an extensive collection of 1100 pre-retrofits and retrofit homes in Dublin. The dataset consists of the indoor temperature profile in 8 different zones over 2022 and the BER Rating of dwellings. The indoor temperature measurements within the zones of the dwellings were captured using temperature sensors developed by the smart technology company Climote [63]. These sensors recorded temperature readings at hourly intervals. The geographical distribution of the sampled dwellings is illustrated in Figure 2, while Figure 3 presents representative images of the dwellings based on their locations.

To ensure seasonal consistency, the full dataset in 2022 related to dwellings was filtered to include only the summer period (1 May to 30 September 2022). Since some dwellings did not have complete indoor temperature data during the specific period, from the initial dataset of 1100 dwellings, 943 dwellings with consistent and reliable data were selected for the overheating risk assessment. These dwellings contain a total of 949 individual monitored zones, distributed across eight different categories, including 900 living rooms, as well as bedrooms, kitchens, heating zones, and others. Since some dwellings contain multiple monitored zones, the total number of zones exceeds the number of dwellings.

Given that living rooms constitute the majority of recorded zones and that significant variability exists among other zone types, direct comparisons based on percentage distribution were deemed impractical. Therefore, this study focuses on the 900 living rooms as primary occupancy spaces to assess overheating risk. All the data analyses were conducted using Microsoft Excel, Power BI (V 2.136.1202.0), and Python (V 3.9.20).

3.2. Indoor Temperature Pattern

Indoor temperature data from a total of 900 living room zones were analysed for the period between 1 May and 30 September of 2022. The data were processed to identify and exclude instances of sensor failure, indicated by a “0” value. These failures were subsequently removed to clean the dataset and ensure data integrity. The indoor temperature pattern of living rooms in the case studies was analyzed on an hourly and monthly basis throughout the summer months.

3.3. Outdoor Temperature Pattern

Meteorological data related to outdoor temperatures were obtained from Dublin airport weather stations. These data were collected using hourly and daily weather reports provided by Met Éireann for the period from May to September 2022.

3.4. Assessment of Overheating Risk

Both static and adaptive approaches were used to thoroughly investigate the prevalence of overheating in the monitored dwellings.

3.4.1. Static Criteria: CIBSE Guide A and Passivhaus Institute

The CIBSE Guide A and Passivhaus Institute were used as static criteria for overheating risk assessment, as thoroughly explained in Section Criteria of Overheating Analysis, and presented in Figure 4.

In this study, both static criteria were evaluated seasonally from May to September 2022. This approach highlights the importance of seasonal analysis, particularly during warmer months, to accurately assess and address overheating risks in dwellings. This methodological choice aligns with the existing literature (e.g., [65,66]) that emphasizes the need for seasonal performance evaluation to ensure occupant comfort and energy efficiency.

3.4.2. Adaptive Criteria: CIBSE TM59

The CIBSE TM 59 criteria were used as adaptive criteria for evaluating the overheating of naturally and mechanically ventilated residential buildings during summertime. This is based on CIBSE TM 52 and CIBSE Guide A.

These criteria are based on CIBSE TM52 and CIBSE Guide A, specifically focusing on the evaluation of overheating risk through the difference between indoor operative temperature (T_OP) and the upper temperature limit for Category III buildings (T_MAX), as described in EN16798-1. The upper limit temperature represents the absolute maximum daily temperature for a room [67].

The overheating risk is determined by the temperature difference (∆T) as follows [28,32,54]:

∆T = T_OP − T_MAX

(9)

To calculate the value of ∆T required to evaluate overheating risk, where the indoor operative temperature (°C), T_OP, is calculated as the average of the indoor air temperature indoor air temperature (T_A), and the mean radiant temperature (T_MRT) [43] as shown in the following equation [28,32,54]:

T_OP = (T_A + T_MRT)/2

(10)

In this study, due to measurement limitations (e.g., the difficulty of directly measuring radiant temperature due to the numbers and scale of the sample set), only T_A was used. As supported by the literature (e.g., [68]), when indoor air velocity is low (typically below 0.1 m/s), the T_A and T_MRT can be assumed to be similar and T_A alone can be a reasonable indicator of thermal comfort [54,68]. Consequently, T_A and T_MRT are considered as identical in this study and T_A was used as a proxy for T_OP, as detailed in studies [69,70].

T_MAX, the upper limit temperature for Category III buildings, is calculated using an exponentially weighted running mean of the daily outdoor mean temperature (°C) T_RM with the following equation [28,32,54]:

T_MAX = 0.33T_RM + 22.8

(11)

where T_RM is the exponentially weighted running mean of the outdoor temperature, defined as [28,32,54]:

T_RM = (T_OD-1 + (0.8 × T_OD-2) + (0.6 × T_OD-3) + (0.5 × T_OD-4) + (0.4 × T_OD-5) + (0.3 × T_OD-6) + (0.2 × T_OD-7))/3.8

(12)

where T_OD-nth_days (°C) is the daily mean outdoor temperature of the n-th day before; T_OD−1 is the daily mean external temperature for the previous day; T_OD-2 is the daily mean external temperature for the day before, and so on.

TM59 uses the first criterion for overheating from TM52, which defines the Hours of Exceedance (H_e), representing the duration of overheating, as follows [28,32,54]:

He = ∑h∀∆T ≥ 1 °C

(13)

The summation is performed over all occupied hours (h) as defined for the type of building. TM59 refines this criterion for domestic application. According to Criterion 1A for living rooms, the occupied hours are set from 9 am to 10 pm. H_e should not exceed 3% of occupied hours for the months of May to September inclusive, based on criterion 1A. The flowchart of adaptive criteria process is presented in Figure 5.

4. Results

This section presents the results of the analysis and provides some insights into indoor temperature trends within living room zones, BER distribution, and their correlation with temperature. Additionally, it examines the impact of these parameters on overheating risk evaluations based on established criteria in residential dwellings.

4.1. BER Rating of All Dwellings in the Sample Set

The BER data for Irish homes are compared here with the data used in this study. The BER is Ireland’s version of the Energy Performance Certificate (EPC) and the Irish database represents approximately 50% of existing buildings. It is not perfect [71] but is used here for comparison purposes. The Irish building stock is reasonably well represented by the data presented here, which are primarily for C-rated (moderately insulated) homes. The comparison is shown in

Table 1. The number and percentage across BER distributions between the dwellings in the sample set in this study and dwellings in the Irish BER Database.

	Sample Set BER			Irish BER Database
no	BER	Number	Percentage (%)	BER	Number	Percentage (%)
1	A	1	<1	A	44,638	6
2	B	27	3	B	115,566	15
3	C	383	41	C	314,831	40
4	D	102	11	D	170,619	22
5	E	41	4	E	74,980	9
6	F	11	1	F	32,182	4
7	G	3	<1	G	37,573	5
8	No code listed	375	40
Total		943	100		790,389	100

.

4.2. Indoor Temperature Pattern

The indoor temperature patterns of 900 living rooms out of 943 dwellings were evaluated. Temperature variations were analysed on both an hourly and monthly basis throughout the period from May to September 2022. The findings are illustrated in Figure 6 and Figure 7.

Figure 6 highlights diurnal temperature fluctuations over 24 h for the 900 living rooms. As shown in Figure 7a, monthly indoor temperature patterns varied throughout the study period. Additionally, Figure 7b presents a histogram indicating that indoor temperatures most frequently fell within the range of 20–22 °C. These findings align with the existing literature, which report that the summer living room temperatures in Ireland and the UK typically range between 17 and 26 °C [33,54,72,73,74].

4.3. Correlations Between Indoor Temperature and BER

The correlation between indoor temperatures in living room zones and BER ratings was analysed, with the results illustrated in Figure 8 during 24 h from May to September 2022. The average indoor temperatures of living rooms across different BER ratings were as follows: 22.46 °C (A-rated), 21.42 °C (B-rated), 21.28 °C (C-rated), 21.23 °C (D-rated), 20.89 °C (E-rated), 20.67 °C (F-rated), 20.25 °C (G-rated), and 20.99 °C (no code listed). The data show nothing of major significance between the different BER groups.

4.4. Outdoor Temperature Pattern

The weather data for this time period of investigation are presented in Figure 9.

Hourly outdoor temperatures recorded from May to September 2022 show a maximum outdoor temperature of 29.1 °C (dashed green line) on 18 July at 12 pm, as illustrated in Figure 9.

Over the entire monitoring period, the average outdoor temperature was 14.3 °C (dashed yellow line), with a standard deviation of 6.6 °C (solid red line), indicating considerable variability.

4.5. Evaluation of Overheating Risk

Aligned with the definitions used in Section 2.1, in this study, according to the static criteria for overheating analysis set out in the CIBSE Guide A and Passivhaus, occupied hours for living rooms were specified as 8 a.m. to 11 p.m. Additionally, in accordance with TM59 Criterion 1A for overheating analysis, the occupied hours for living room zones, as defined in Section 3.4.2, were set as 9 a.m. to 10 p.m.

Evaluation of Overheating Risk Based on Static Criteria and Adaptive Criteria

A summary of the overheating risk according to the static and adaptive criteria is presented in Figure 10.

According to the static criteria, the results highlight the significant difference between CIBSE Guide A and Passivhaus criteria. In the case of CIBSE Guide A, only 4% of living rooms surpassed the threshold, while 41% surpassed the Passivhaus threshold. This difference is likely due to the fact much of the data lay between the two different temperature criteria and was a result of this particular dataset.

According to the CIBSE TM59 adaptive criteria, Criterion 1A, of the 900 living room cases, 26 exceeded the ∆T threshold of 1 °C, with only one dwelling exhibiting an overheating percentage of 3.25%, surpassing the 3% threshold set by Criterion 1A in TM59’s adaptive approach.

There was no significant difference observed across the different BER bands. The sample sets in many of these brackets were too small to draw conclusions.

5. Discussion

This study presents and analyses data from a large sample of 900 dwellings in Ireland, which broadly represents the Irish housing stock in terms of energy performance, but with a greater portion of C-rated dwellings in this sample. The dataset can therefore be considered a reasonable representation of moderately insulated homes. This data set can therefore provide some clues as to whether moderately insulated homes pose any overheating risks.

We assessed the data under three different overheating criteria and found that the risk of overheating was broadly negligible. Under one of the static criteria assessed (Passivhaus), there was a higher portion of dwellings that met this overheating limitation (41%), while the other static criteria (CIBSE Guide A) revealed only 4% of homes posing overheating risks. This highlights the known challenges with static methods and the crude nature of a simple numerator limit and % passing threshold. The latter criteria for overheating are more stringent on the maximum temperature (28 °C compared to 25 °C for the CIBSE Guide A method) but less stringent on the % passing (only 1%). In ways, this shows that that the data assessed here did not show much evidence of extreme temperatures but did show consistently moderately high temperatures. It is difficult to conclude whether this has any significance alone and hence why the adaptive TM59 method was also applied. This method revealed almost zero risk of overheating in the assessed dwellings.

We can therefore conclude with reasonable confidence that the sample set evaluated here showed negligible signs of overheating in moderately insulated homes according to the more refined overheating risks. We also conclude that, as has been reported throughout the literature, the static methods can serve as a useful indicator, but a more detailed approach is needed to ascribe overheating risks with a greater degree of confidence and rigor.

While the results suggest a negligible risk of overheating in moderately insulated homes, it is important to consider the broader context of overheating analysis and the factors influencing variations in reported occurrences. Beyond methodological differences, the literature highlights significant variability in overheating prevalence, which is often attributed to multiple factors. In this study, identifying the exact causes of overheating remained challenging. Multiple factors, including behavioral habits, occupancy patterns, building characteristic, building designs, orientation, floor area, insulation levels, ventilation strategies, and sensor placement, could contribute to the observed trends. This complexity emphasizes the need for a comprehensive and integrated approach to mitigating overheating risks in residential buildings.

Given this complexity, further detailed assessments of other key occupancy zones within the home are necessary to fully understand potential overheating risks. To this end, in addition to analysing living room temperatures, an evaluation of bedroom temperatures was conducted to gain further insights into thermal comfort in sleeping spaces. The analysis of bedroom temperatures revealed similar patterns to those observed in living rooms, albeit with slightly higher average temperatures. The maximum frequency of bedroom temperatures occurred at 22 °C, with C-rated dwellings averaging 22.65 °C. This aligns with the existing literature, which reports that summer bedroom temperatures in Ireland and the UK typically range between 15 °C and 26 °C [33,54,72,73,74,75].

Under the static criteria, only 2.5% of bedrooms surpassed the CIBSE Guide A threshold, while 0.5% surpassed the Passivhaus threshold. Detailed indoor temperature analysis and static criteria overheating analysis of these three bedrooms were presented in a previous paper [64].

When evaluated using the adaptive comfort approach outlined in CIBSE TM59, no exceedances of ∆T ≥ 1 °C were observed over the entire 24-h period in bedrooms under Criterion 1A. However, Criterion 1B, which applies a threshold of 26 °C during sleeping hours (10 p.m. to 7 a.m.), identified one bedroom exceeding this limit for 8 h, accounting for 0.5% of the observed period. This value remained below the 1% overheating threshold. The detailed overheating analysis of these bedrooms was presented in a previous paper [76].

Similar to the living room analysis, the bedroom results also showed negligible overheating risks in moderately insulated homes. However, further research is needed to explore additional factors and refine assessment methods, as discussed in the limitations and future work section.

Limitation and Furthur Work

While this study provides value in that the sample set is particularly large and representative of a large portion of Irish dwellings, there are several limitations. These are listed here:

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The data set, while larger than many, still only covers a group or segment of the Irish building stock. More research should be conducted on similarly rated homes in other parts of the country.

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Although there was an intention to assess the overheating risk in bedrooms, upon filtering and review of the data, the sample size was deemed too small to make any significant conclusion. We therefore propose that this be assessed in future work.

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The data used were not from sensors installed by the authors of this study. Access was provided by the local council. As with all data of this kind, there are risks that some of the data points could be misleading. Given the size of the dataset and following the filtering process, we believe it is large enough to accommodate those potential issues.

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The static methods are a crude assessment of overheating. We acknowledge this but nonetheless found the results interesting and worthy of sharing.

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Ireland has a mild temperate climate and is not expected to experience overheating risks to the same extent as other parts of the world, particularly in moderately insulated homes. Nonetheless, this study contributes by giving a degree of confidence to designers that the risk of overheating in moderately insulated homes in mild temperate climates is likely negligible.

Additionally, several further future areas of work are listed below:

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Future work will assess retrofitting strategies (e.g., shading systems, insulation level, MVHR system, and replacement windows) to mitigate overheating risks in homes.

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Future investigations will focus on modern, efficient air conditioning systems with a short payback period and long service life.

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Furthermore, future studies should monitor some factors, alongside variables, to determine the exact causes of exceedance hours and the risk of overheating in dwellings such as building characteristics, including location, orientation, construction materials, floor area, ventilation strategies, and window configurations.

6. Conclusions

This study investigated the overheating risks in a large sample set of residential buildings in Ireland by examining the indoor temperature profiles of 1100 social housing units in Dublin from May to September 2022. By integrating static and adaptive criteria alongside Building Energy Ratings (BER), the research identifies dwellings at risk and explores correlations between BER and overheating vulnerability.

The evaluation of overheating risks using static criteria and adaptive comfort criteria reveals that the conclusion on overheating prevalence is highly dependent on the assessment method used, reinforcing the limitations of static overheating criteria and underscoring the necessity for more adaptive, context-sensitive evaluation frameworks.

This evaluation also indicates that, while overheating risks are present, they remain relatively low across the studied dwellings.

In summary, this study concludes that the risk of overheating in moderately insulated homes is low and that the method applied can yield different conclusions in certain datasets.