Are minimum indoor air quality requirements combined with passive retrofit solutions enough to guarantee thermal comfort in southern Spain High Schools?

01002 Are minimum indoor air quality requirements combined with passive retrofit solutions enough to guarantee thermal comfort in southern Spain High Schools? Carmen María Calama-González1,*, Rocío Escandón2, and Rafael Suárez2 1Departamento de Construcciones Arquitectónicas y su Control. Escuela Técnica Superior de Edificación. Universidad Politécnica de Madrid. Avda. Juan de Herrera 4, 28040, Madrid, Spain. 2Instituto Universitario de Arquitectura y Ciencias de la Construcción, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, Av. de Reina Mercedes 2, 41012 Seville, Spain. Abstract. This study addresses the challenging research gap of assessing whether passive energy retrofit solutions are adequate for simultaneously achieving indoor air quality and thermal comfort in educational buildings, given the minimum ventilation rate required by Spanish regulations. A representative educational building is selected as case study to simulate the performance of the existing high school stock in southern Spain. Then, a multi-objective optimisation approach based on NSGA-II genetic algorithms is used for the evaluation of a range of passive energy retrofit strategies, with the aim of identifying the most effective. The findings indicate that, even though the minimum ventilation rate ensure indoor air quality, it is ineffective to maintain thermal comfort. This study highlights a clear overheating problem, especially in summer, that is aggravated when incorporating insulation to the envelope, given the high internal loads and the difficulty of heat dissipation of the envelope. It is concluded that optimising ventilation according to the seasonal period and applying higher ventilation rates than the ones established in the applicable regulations are crucial for reaching both indoor air quality and thermal comfort only considering passive retrofit solutions. 1 Introduction Since the COVID-19 global pandemic, there has been a growing awareness of the importance of maintaining adequate indoor air quality (IAQ) and reducing pollutant exposure to air-borne pathogens [1]. This all has led to an increased focus on this concerning issue by both public authorities and the general population. Educational buildings are of particular concern due to several factors, including their high occupancy levels and internal loads or the extended daily hours spent in these environments by children [2]. Furthermore, the heightened susceptibility and vulnerability to illness observed in children serves to exacerbate this problem [3]. * Corresponding author: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 The assessment of indoor environmental quality remains contingent upon the consideration of thermal comfort as a primary factor [4]. In southern Spain, the lack of thermal conditioning systems, coupled with the typically poor natural ventilation in schools, low-efficiency constructions and inadequate operational strategies, can lead to significant health concerns [5]. Several studies have identified adverse effects of poor IAQ, frequently associated with elevated stress, increased absenteeism or lower academic performance [6]. There is a clear consensus among the scientific community on the importance of natural ventilation as a means of ensuring IAQ, yet no standard proposes a combined IAQ and thermal comfort analysis [7]. For instance, Spanish regulations propose an outdoor ventilation rate based on occupation, indoor CO2 or surface with no consideration of outdoor or indoor temperatures. Moreover, several studies have already demonstrated the negative impact of ventilation on indoor thermal comfort and worn about overheating risks, which may be worsen by excessive buildings’ airtightness and climate change. In their analysis of the thermal performance of a representative secondary school in Nicosia (Cyprus), Heracleous and Michael [8] conclude that, while daytime ventilation maintains adequate IAQ during summer, it fails to ensure thermal comfort conditions for most of the time. These authors advocate for the use of natural night-time ventilation as a means of reducing peak indoor temperatures. Zinzi et al. [9] find that nearly zero energy building retrofit solutions in Italian schools can significantly enhance building performance in the heating season. However, they also report increased overheating risk during the remaining months, with average indoor temperatures exceeding 28°C during most of occupied hours. It is therefore evident that one of the key challenges our society faces is to simultaneously ensure adequate IAQ and thermal comfort in indoor environments, particularly in educational buildings, given the higher children’s health vulnerability. The objective of this research is to address the existing research gap in this field by responding to the following questions: • Are passive energy solutions enough to maintain IAQ and thermal comfort in southern Spain schools, considering minimum ventilation rates required by regulations? • Which passive retrofit strategies are most effective in these educational environments to simultaneously guarantee adequate IAQ and thermal comfort? To do so, a representative educational building of the existing High School stock in southern Spain (Mediterranean area) is selected as case study through a clustering analysis. Under a minimum ventilation scenario in accordance with Spanish regulations, an array of passive energy retrofit solutions, focused on the vertical envelope, are proposed and analysed. The suitability of these strategies is evaluated from a thermal comfort perspective through a multi-objective optimisation analysis, which identifies the most appropriate options. 2 Methodology The methodology applied to the presented research includes the development of several work packages (Fig. 1), which are detailly described in the following subsections. Fig. 1. Workflow and work packages developed. 2.1 Educational building archetype characterization In this stage, a representative educational building which reproduces the typical thermal performance of the High School stock in southern Spain (Csa Mediterranean Köppen’s climate [10]) is selected as case study, after conducting a clustering analysis involving over 870 public secondary schools in the region [11]. The building selected corresponds to the Polytechnic High School located in Seville, which was built around the early 70s, before the implementation of the first thermal energy performance regulation [12]. Two similar classrooms have been analysed in detail (Fig. 2). Each classroom in the building is 6.30 m by 6.60 m, with a ceiling height of 3.15 m, and is separated from the other by a 2 m wide corridor. Regarding the classrooms’ orientation, one of them is facing southeast (SE), while the other one faces northwest (NW). Each classroom is equipped with two 6 mm single glassing windows with 5 cm aluminium frame with no thermal bridge break (UWINDOW = 5.70 W/m²·K) at 1 m from the floor, which results in a window-to-wall ratio of about 31% per classroom. Additionally, all windows feature external PVC roller blinds for solar protection. Regarding the opaque vertical envelope, the façade comprises a 24 cm perforated brick wall, 2 cm of cement mortar, a 2.5 cm unventilated air cavity, a 10 cm hollow brick layer, and 2 cm of gypsum (UFACADE = 1.03 W/m²·K). The partition wall that separates the classrooms from the corridor is a 12 cm hollow brick wall with 2.5 cm of gypsum on both sides (UPARTITION = 1.80 W/m²·K). Fig. 2. Floor plan of the classrooms analysed and example of indoor environment. The occupation in the classrooms is 24 people, including students and teacher. The occupancy profile was determined based on the school’s academic schedule: classrooms are in use from 8:00 to 11:00, followed by a 30-min break, and then from 11:30 to 14:30. The academic year runs from January 9 to June 23 and from September 15 to December 22. No occupation has been considered during weekends or regional and national holidays. As to the infiltrations, values in the rooms are approximately 0.35 ACH. And regarding natural ventilation, it occurs only through the sliding windows when the rooms are occupied. Natural ventilation rates have been set at the minimum value established by the Spanish regulatory framework [13], which considers a minimum of 12.5 l/s per person. This value corresponds to 8.24 ACH, considering the occupation and volume (m3) of the classrooms. Internal loads have also been considered: lighting loads are 5 W/m2 and equipment loads are 30 W/m2 (several personal computers, a huge screen and a projector have been installed in each classroom). The internal loads schedule has been set as equal to the occupancy profile. It is also important to highlight that, even though the school has no mechanical ventilation nor cooling air-conditioning systems, the classroom has two ceiling fans. 2 E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 The assessment of indoor environmental quality remains contingent upon the consideration of thermal comfort as a primary factor [4]. In southern Spain, the lack of thermal conditioning systems, coupled with the typically poor natural ventilation in schools, low-efficiency constructions and inadequate operational strategies, can lead to significant health concerns [5]. Several studies have identified adverse effects of poor IAQ, frequently associated with elevated stress, increased absenteeism or lower academic performance [6]. There is a clear consensus among the scientific community on the importance of natural ventilation as a means of ensuring IAQ, yet no standard proposes a combined IAQ and thermal comfort analysis [7]. For instance, Spanish regulations propose an outdoor ventilation rate based on occupation, indoor CO2 or surface with no consideration of outdoor or indoor temperatures. Moreover, several studies have already demonstrated the negative impact of ventilation on indoor thermal comfort and worn about overheating risks, which may be worsen by excessive buildings’ airtightness and climate change. In their analysis of the thermal performance of a representative secondary school in Nicosia (Cyprus), Heracleous and Michael [8] conclude that, while daytime ventilation maintains adequate IAQ during summer, it fails to ensure thermal comfort conditions for most of the time. These authors advocate for the use of natural night-time ventilation as a means of reducing peak indoor temperatures. Zinzi et al. [9] find that nearly zero energy building retrofit solutions in Italian schools can significantly enhance building performance in the heating season. However, they also report increased overheating risk during the remaining months, with average indoor temperatures exceeding 28°C during most of occupied hours. It is therefore evident that one of the key challenges our society faces is to simultaneously ensure adequate IAQ and thermal comfort in indoor environments, particularly in educational buildings, given the higher children’s health vulnerability. The objective of this research is to address the existing research gap in this field by responding to the following questions: • Are passive energy solutions enough to maintain IAQ and thermal comfort in southern Spain schools, considering minimum ventilation rates required by regulations? • Which passive retrofit strategies are most effective in these educational environments to simultaneously guarantee adequate IAQ and thermal comfort? To do so, a representative educational building of the existing High School stock in southern Spain (Mediterranean area) is selected as case study through a clustering analysis. Under a minimum ventilation scenario in accordance with Spanish regulations, an array of passive energy retrofit solutions, focused on the vertical envelope, are proposed and analysed. The suitability of these strategies is evaluated from a thermal comfort perspective through a multi-objective optimisation analysis, which identifies the most appropriate options. 2 Methodology The methodology applied to the presented research includes the development of several work packages (Fig. 1), which are detailly described in the following subsections. Fig. 1. Workflow and work packages developed. 2.1 Educational building archetype characterization In this stage, a representative educational building which reproduces the typical thermal performance of the High School stock in southern Spain (Csa Mediterranean Köppen’s climate [10]) is selected as case study, after conducting a clustering analysis involving over 870 public secondary schools in the region [11]. The building selected corresponds to the Polytechnic High School located in Seville, which was built around the early 70s, before the implementation of the first thermal energy performance regulation [12]. Two similar classrooms have been analysed in detail (Fig. 2). Each classroom in the building is 6.30 m by 6.60 m, with a ceiling height of 3.15 m, and is separated from the other by a 2 m wide corridor. Regarding the classrooms’ orientation, one of them is facing southeast (SE), while the other one faces northwest (NW). Each classroom is equipped with two 6 mm single glassing windows with 5 cm aluminium frame with no thermal bridge break (UWINDOW = 5.70 W/m²·K) at 1 m from the floor, which results in a window-to-wall ratio of about 31% per classroom. Additionally, all windows feature external PVC roller blinds for solar protection. Regarding the opaque vertical envelope, the façade comprises a 24 cm perforated brick wall, 2 cm of cement mortar, a 2.5 cm unventilated air cavity, a 10 cm hollow brick layer, and 2 cm of gypsum (UFACADE = 1.03 W/m²·K). The partition wall that separates the classrooms from the corridor is a 12 cm hollow brick wall with 2.5 cm of gypsum on both sides (UPARTITION = 1.80 W/m²·K). Fig. 2. Floor plan of the classrooms analysed and example of indoor environment. The occupation in the classrooms is 24 people, including students and teacher. The occupancy profile was determined based on the school’s academic schedule: classrooms are in use from 8:00 to 11:00, followed by a 30-min break, and then from 11:30 to 14:30. The academic year runs from January 9 to June 23 and from September 15 to December 22. No occupation has been considered during weekends or regional and national holidays. As to the infiltrations, values in the rooms are approximately 0.35 ACH. And regarding natural ventilation, it occurs only through the sliding windows when the rooms are occupied. Natural ventilation rates have been set at the minimum value established by the Spanish regulatory framework [13], which considers a minimum of 12.5 l/s per person. This value corresponds to 8.24 ACH, considering the occupation and volume (m3) of the classrooms. Internal loads have also been considered: lighting loads are 5 W/m2 and equipment loads are 30 W/m2 (several personal computers, a huge screen and a projector have been installed in each classroom). The internal loads schedule has been set as equal to the occupancy profile. It is also important to highlight that, even though the school has no mechanical ventilation nor cooling air-conditioning systems, the classroom has two ceiling fans. 3 E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 2.2 Construction of a parameterized building simulation model A building simulation model is constructed in the open-access EnergyPlus simulation engine v.9.1.0, a widely validated tool commonly used by the scientific community, using the morphological, constructive, physical and operational information of the case study. However, due to the limitations of the physical simulation tool, several assumptions are required during the modelling process. Firstly, a simplified simulation model based on the typical and representative space scheme “classroom – corridor – classroom” is developed. Furthermore, the envelope is defined by detailing each construction layer and including their physical properties (density, conductivity, solar absorptance, and/or specific heat), which have been sourced from the Spanish Technical Datasheets and Regulations, so that the specific U-values described in section 2.1 are obtained. Only the vertical envelopes and the partition walls are modelled to accurately represent heat transfer between the classroom, corridor, and adjacent classroom. In other words, the remaining construction surfaces are treated as adiabatic in the simulation model. Moreover, due to software limitations, to simulate a 50% blind aperture level in the EnergyPlus environment and closely match real conditions, both windows and external blinds are modelled in two sections (upper and lower), allowing the blind aperture to be set in the model to 0%, 50% or 100% open. Once the single-case study simulation model is developed, it undergoes a calibration and validation stage based on hourly on-site monitoring data, guaranteeing its feasibility to represent the real thermal and energy performance of the building. The calibration and validation of the model may be found in a previous work [14], where Bayesian calibration techniques were implemented, as in similar studies [15]. Later, a parameterization of the simulation model is carried out using the open-access jEPlus software version 2.1 and EP Macro programming code. The parameterization process enables automatic simulation runs and the exploration of various possible values for the parameterized variables, reducing the need for manual adjustments of the inputs in the model. Additionally, it allows for the simultaneous simulation of different case studies by varying specific variables, such as orientation or physical properties. This is conducted so that the several passive retrofit strategies and simulation conditions defined in the following stage may be easily imported into the model. 2.3 Proposal of passive retrofit strategies In this stage, several passive energy retrofit strategies are defined for improving thermal comfort in the case study. The proposed retrofit strategies are mainly focused on the external vertical envelope, including both opaque and glazed elements. The selected solutions are based on the most commonly retrofit strategies used in public Mediterranean school buildings, widely validated and included in retrofit plans proposed at the political level by public authorities. Moreover, given the low-efficiency construction solutions in southern Spain schools and the high costs of active retrofit solutions, only passive measures were considered. Passive strategies have been defined based on the ranges of thermal transmittance values for typical energy retrofit solutions used in façades and windows in southern Spain. Additionally, as an operational measure, the level of aperture of the solar protection system (external rolling blinds), has been incorporated to assess its influence on improving thermal comfort, given its high operational dependency on the occupants. Table 1 presents a comparison of the values of the building variables defining the simulation model for the specific case study, as well as for the final parameterised simulation model that will be used to optimise the proposed retrofit strategies. It can be observed that for the optimisation analysis North, South, East and West orientations (0, 90, 180 and 270º North) have been considered to determine the influence of building orientation on the retrofit results. Regarding infiltration and ventilation rates, as well as internal loads and schedules, they are described in section 2.1 of this paper. Table 1. Formatting sections, subsections and subsubsections. Variables Specific values for case study Variability range for parameterised model Orientation (º) 35 0, 90, 180, 270 Classroom length / width / height (m) 6.60 / 6.30 / 3.15 6.60 / 6.30 / 3.15 Corridor width (m) 2.00 2.00 U-value: façade / window / partition (W/m²·K) 1.03 / 5.70 / 1.80 0.35 to 1.85 / 1.70 to 5.70 / 1.80 Façade solar absorptance 0.40 0.30 to 0.90 Blinds aperture level 50 % open 50 % open / 100 % open / 50% open in summer & 100% open in winter 2.4 Numerical optimisation of the proposed retrofit strategies A multi-objective analysis is conducted to numerically optimise the proposed energy retrofit solutions under the scenario of the minimum ventilation rate established by Spanish regulations [13]. To do so, the open-access jEPlus+EA version 2.1 tool is used, incorporating NSGA-II genetic algorithms, highly computationally efficient, robust and advantageous for multi-objective tasks. The optimization objective variables correspond to the percentage of annual overheating and undercooling hours calculated from the adaptive thermal comfort model in EN 16798-1:2019 [16]. This model calculates the adaptive comfort temperature based on the running mean dry-bulb outdoor temperature for today (T) and the daily mean dry-bulb outdoor temperature for the previous 1 to 7 days (T1-7) (Equations 1 and 2). Tcomfort = 0.33 x T + 18.8 (1) T = (T1+ 0.8 T2+ 0.6 T3+ 0.5 T4+ 0.4 T5+ 0.3 T6 + 0.2 T7) / 3.8 (2) In this study, the temperature range of the comfort band has been defined with an upper limit of +3 °C and a lower limit of -4 °C. This range corresponds to a maximum predicted percentage of dissatisfaction (PPD) of 10% and a predicted mean vote (PMV) of ±0.5. In order to calculate the percentage of overheating hours, it is necessary to determine the amount of hours in which the upper limit of the adaptive comfort band is exceeded. Similarly, the percentage of undercooling hours is calculated by identifying the proportion of hours in which the lower comfort band limit is not reached. In the optimization problem, both the percentage of annual overheating and undercooling hours are minimised. 3 Analysis and Results Figure 3 presents the results obtained from the optimisation analysis, displaying the data according to the predominant orientation of the classroom. Up to 1,760 simulations were conducted with the jEPlus+EA software, yielding around 8.5% optimal solutions. Calculation 4 E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 2.2 Construction of a parameterized building simulation model A building simulation model is constructed in the open-access EnergyPlus simulation engine v.9.1.0, a widely validated tool commonly used by the scientific community, using the morphological, constructive, physical and operational information of the case study. However, due to the limitations of the physical simulation tool, several assumptions are required during the modelling process. Firstly, a simplified simulation model based on the typical and representative space scheme “classroom – corridor – classroom” is developed. Furthermore, the envelope is defined by detailing each construction layer and including their physical properties (density, conductivity, solar absorptance, and/or specific heat), which have been sourced from the Spanish Technical Datasheets and Regulations, so that the specific U-values described in section 2.1 are obtained. Only the vertical envelopes and the partition walls are modelled to accurately represent heat transfer between the classroom, corridor, and adjacent classroom. In other words, the remaining construction surfaces are treated as adiabatic in the simulation model. Moreover, due to software limitations, to simulate a 50% blind aperture level in the EnergyPlus environment and closely match real conditions, both windows and external blinds are modelled in two sections (upper and lower), allowing the blind aperture to be set in the model to 0%, 50% or 100% open. Once the single-case study simulation model is developed, it undergoes a calibration and validation stage based on hourly on-site monitoring data, guaranteeing its feasibility to represent the real thermal and energy performance of the building. The calibration and validation of the model may be found in a previous work [14], where Bayesian calibration techniques were implemented, as in similar studies [15]. Later, a parameterization of the simulation model is carried out using the open-access jEPlus software version 2.1 and EP Macro programming code. The parameterization process enables automatic simulation runs and the exploration of various possible values for the parameterized variables, reducing the need for manual adjustments of the inputs in the model. Additionally, it allows for the simultaneous simulation of different case studies by varying specific variables, such as orientation or physical properties. This is conducted so that the several passive retrofit strategies and simulation conditions defined in the following stage may be easily imported into the model. 2.3 Proposal of passive retrofit strategies In this stage, several passive energy retrofit strategies are defined for improving thermal comfort in the case study. The proposed retrofit strategies are mainly focused on the external vertical envelope, including both opaque and glazed elements. The selected solutions are based on the most commonly retrofit strategies used in public Mediterranean school buildings, widely validated and included in retrofit plans proposed at the political level by public authorities. Moreover, given the low-efficiency construction solutions in southern Spain schools and the high costs of active retrofit solutions, only passive measures were considered. Passive strategies have been defined based on the ranges of thermal transmittance values for typical energy retrofit solutions used in façades and windows in southern Spain. Additionally, as an operational measure, the level of aperture of the solar protection system (external rolling blinds), has been incorporated to assess its influence on improving thermal comfort, given its high operational dependency on the occupants. Table 1 presents a comparison of the values of the building variables defining the simulation model for the specific case study, as well as for the final parameterised simulation model that will be used to optimise the proposed retrofit strategies. It can be observed that for the optimisation analysis North, South, East and West orientations (0, 90, 180 and 270º North) have been considered to determine the influence of building orientation on the retrofit results. Regarding infiltration and ventilation rates, as well as internal loads and schedules, they are described in section 2.1 of this paper. Table 1. Formatting sections, subsections and subsubsections. Variables Specific values for case study Variability range for parameterised model Orientation (º) 35 0, 90, 180, 270 Classroom length / width / height (m) 6.60 / 6.30 / 3.15 6.60 / 6.30 / 3.15 Corridor width (m) 2.00 2.00 U-value: façade / window / partition (W/m²·K) 1.03 / 5.70 / 1.80 0.35 to 1.85 / 1.70 to 5.70 / 1.80 Façade solar absorptance 0.40 0.30 to 0.90 Blinds aperture level 50 % open 50 % open / 100 % open / 50% open in summer & 100% open in winter 2.4 Numerical optimisation of the proposed retrofit strategies A multi-objective analysis is conducted to numerically optimise the proposed energy retrofit solutions under the scenario of the minimum ventilation rate established by Spanish regulations [13]. To do so, the open-access jEPlus+EA version 2.1 tool is used, incorporating NSGA-II genetic algorithms, highly computationally efficient, robust and advantageous for multi-objective tasks. The optimization objective variables correspond to the percentage of annual overheating and undercooling hours calculated from the adaptive thermal comfort model in EN 16798-1:2019 [16]. This model calculates the adaptive comfort temperature based on the running mean dry-bulb outdoor temperature for today (T) and the daily mean dry-bulb outdoor temperature for the previous 1 to 7 days (T1-7) (Equations 1 and 2). Tcomfort = 0.33 x T + 18.8 (1) T = (T1+ 0.8 T2+ 0.6 T3+ 0.5 T4+ 0.4 T5+ 0.3 T6 + 0.2 T7) / 3.8 (2) In this study, the temperature range of the comfort band has been defined with an upper limit of +3 °C and a lower limit of -4 °C. This range corresponds to a maximum predicted percentage of dissatisfaction (PPD) of 10% and a predicted mean vote (PMV) of ±0.5. In order to calculate the percentage of overheating hours, it is necessary to determine the amount of hours in which the upper limit of the adaptive comfort band is exceeded. Similarly, the percentage of undercooling hours is calculated by identifying the proportion of hours in which the lower comfort band limit is not reached. In the optimization problem, both the percentage of annual overheating and undercooling hours are minimised. 3 Analysis and Results Figure 3 presents the results obtained from the optimisation analysis, displaying the data according to the predominant orientation of the classroom. Up to 1,760 simulations were conducted with the jEPlus+EA software, yielding around 8.5% optimal solutions. Calculation 5 E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 time was around 4 hours, considering a computer with i7-8700 CPU 3.20 GHz of 12 cores and 64 GB RAM. Fig. 3. Optimal retrofit strategies results shown per building orientation. It can be observed that the percentage of undercooling hours in all orientations is below 15%, while overheating hours vary from 44 to 67%, being especially significant in the East and West, as well as in the South, due to higher solar radiation influence. This translates to a monthly mean temperature during occupied hours in winter of 20.7°C in the North, 22.3°C in the South, 21.6°C in East and 21.2 in West. In summer, the monthly mean temperatures registered during occupied hours are 25.6°C in the North, 26.8°C in the South, 28.8°C in the East, and 28.0°C in the West, similar to the ones reported by Zinzi et al. [9]. This clearly demonstrates that there is a significant indoor overheating problem in schools in southern Spain and that, not only simply insulating the envelope or using more-efficient windows does not solve the issue, but it may also worsen it. For the North, East, and West orientations, the U-value of the opaque vertical envelope varies significantly within a range from 0.46 to 1.61 W/m²·K in the optimised solutions. The lowest values (0.56 and 0.46 W/m²·K), which would totally ensure compliance with the maximum heat transfer limits permitted by Spanish regulations in the case of energy retrofit interventions, is especially significant in the East, accounting for 51% of the optimal solutions. Also, it represents 40% of the optimised solutions in the North and 30% in the West orientation. For the South, it is particularly noteworthy that 100% of the optimised solutions consider U-values ranging from 1.26 to 1.61 W/m²·K, in other words, a nonretrofitted facade. This highlights the overheating issue in the south-facing orientation, given the higher direct solar radiation on this façade. Solar absorptance also plays a crucial role in indoor thermal comfort in all orientations, as most of optimised solutions select the lowest value within the calculated range (i.e., 0.3), proving that solar reflectance is key to reduce indoor overheating. Regarding windows, it can be said that generally the optimal solutions involve a significant improvement in the energy efficiency of the original windows (5.70 W/m²·K). However, among all the glazing solutions, the thermal transmittance values range between 1.89-1.98 W/m²·K (corresponding to typical double-glazing windows) in approximately 53.1% of the solutions in the North, 100% in the South, 60.5% in the East, and 68.1% in the West. This means that although the performance of the glazed surface is significantly improved, opting for much more efficient glazing solutions does not notably reduce the problem of indoor overheating. Moreover, given the minimal influence of the frame due to its small thickness in the case study, among the optimal solutions, both aluminium frames with thermal bridge break and three-chamber PVC frames are considered. Finally, in relation to the external solar shading, the most optimal solution for any orientation is generally to set its aperture level to 50% during the occupied period. Considering an optimised operation aperture of 100% open in winter and 50% open in summer, was found to be optimal for only 24% of the simulated cases in the North. 4 Conclusions This paper presents an analysis to determine whether it is possible to maintain indoor thermal comfort conditions in southern Spain schools, while considering the minimum ventilation rate to ensure IAQ according to the Spanish regulation and incorporating passive energy retrofit solutions to improve the façade’s performance. The main results obtained are: - While limiting the natural ventilation rate to the minimum regulatory values is an effective method for ensuring IAQ, it is insufficient for maintaining thermal comfort. - The high internal loads in classrooms have a positive impact on the indoor thermal performance in winter by increasing heat gains and improving thermal comfort. As a result, the percentage of undercooling hours is drastically reduced. Yet, these high loads worsen the performance in summer by decreasing the heat dissipation capacity through the thermal envelope, thus exponentially increasing overheating hours. - Therefore, there is a clear overheating problem where even increasing the insulation of the thermal envelope can lead to greater cases of overheating, not only in summer but also in winter. This is mainly due to the high internal loads in the classrooms and limited envelope heat dissipation. - From the above, it is concluded that it is essential to control ventilation based on the seasonal period, using systems that allow for the optimization of heat exchange with the outside, such as implementing evaporative cooling systems or promoting the need for night ventilation rates in summer to ensure thermal dissipation when exterior conditions are more favourable. - Moreover, future extensive retrofit research will assess in detail the economic costs and policy implications of possible recommended retrofit strategies. The authors wish to acknowledge the financial support provided by Grant (PID2020-117722RB-I00) “Retrofit ventilation strategies for healthy and comfortable schools within a nearly zero-energy building horizon” funded by MICIU/AEI/10.13039/501100011033. C.M. Calama-González: Data curation, 6 E3S Web of Conferences 636, 01002 (2025) https://doi.org/10.1051/e3sconf/202563601002 ICSREE 2025 time was around 4 hours, considering a computer with i7-8700 CPU 3.20 GHz of 12 cores and 64 GB RAM. Fig. 3. Optimal retrofit strategies results shown per building orientation. It can be observed that the percentage of undercooling hours in all orientations is below 15%, while overheating hours vary from 44 to 67%, being especially significant in the East and West, as well as in the South, due to higher solar radiation influence. This translates to a monthly mean temperature during occupied hours in winter of 20.7°C in the North, 22.3°C in the South, 21.6°C in East and 21.2 in West. In summer, the monthly mean temperatures registered during occupied hours are 25.6°C in the North, 26.8°C in the South, 28.8°C in the East, and 28.0°C in the West, similar to the ones reported by Zinzi et al. [9]. This clearly demonstrates that there is a significant indoor overheating problem in schools in southern Spain and that, not only simply insulating the envelope or using more-efficient windows does not solve the issue, but it may also worsen it. For the North, East, and West orientations, the U-value of the opaque vertical envelope varies significantly within a range from 0.46 to 1.61 W/m²·K in the optimised solutions. The lowest values (0.56 and 0.46 W/m²·K), which would totally ensure compliance with the maximum heat transfer limits permitted by Spanish regulations in the case of energy retrofit interventions, is especially significant in the East, accounting for 51% of the optimal solutions. Also, it represents 40% of the optimised solutions in the North and 30% in the West orientation. For the South, it is particularly noteworthy that 100% of the optimised solutions consider U-values ranging from 1.26 to 1.61 W/m²·K, in other words, a nonretrofitted facade. This highlights the overheating issue in the south-facing orientation, given the higher direct solar radiation on this façade. Solar absorptance also plays a crucial role in indoor thermal comfort in all orientations, as most of optimised solutions select the lowest value within the calculated range (i.e., 0.3), proving that solar reflectance is key to reduce indoor overheating. Regarding windows, it can be said that generally the optimal solutions involve a significant improvement in the energy efficiency of the original windows (5.70 W/m²·K). However, among all the glazing solutions, the thermal transmittance values range between 1.89-1.98 W/m²·K (corresponding to typical double-glazing windows) in approximately 53.1% of the solutions in the North, 100% in the South, 60.5% in the East, and 68.1% in the West. This means that although the performance of the glazed surface is significantly improved, opting for much more efficient glazing solutions does not notably reduce the problem of indoor overheating. Moreover, given the minimal influence of the frame due to its small thickness in the case study, among the optimal solutions, both aluminium frames with thermal bridge break and three-chamber PVC frames are considered. Finally, in relation to the external solar shading, the most optimal solution for any orientation is generally to set its aperture level to 50% during the occupied period. Considering an optimised operation aperture of 100% open in winter and 50% open in summer, was found to be optimal for only 24% of the simulated cases in the North. 4 Conclusions This paper presents an analysis to determine whether it is possible to maintain indoor thermal comfort conditions in southern Spain schools, while considering the minimum ventilation rate to ensure IAQ according to the Spanish regulation and incorporating passive energy retrofit solutions to improve the façade’s performance. The main results obtained are: - While limiting the natural ventilation rate to the minimum regulatory values is an effective method for ensuring IAQ, it is insufficient for maintaining thermal comfort. - The high internal loads in classrooms have a positive impact on the indoor thermal performance in winter by increasing heat gains and improving thermal comfort. As a result, the percentage of undercooling hours is drastically reduced. Yet, these high loads worsen the performance in summer by decreasing the heat dissipation capacity through the thermal envelope, thus exponentially increasing overheating hours. - Therefore, there is a clear overheating problem where even increasing the insulation of the thermal envelope can lead to greater cases of overheating, not only in summer but also in winter. This is mainly due to the high internal loads in the classrooms and limited envelope heat dissipation. - From the above, it is concluded that it is essential to control ventilation based on the seasonal period, using systems that allow for the optimization of heat exchange with the outside, such as implementing evaporative cooling systems or promoting the need for night ventilation rates in summer to ensure thermal dissipation when exterior conditions are more favourable. - Moreover, future extensive retrofit research will assess in detail the economic costs and policy implications of possible recommended retrofit strategies. The authors wish to acknowledge the financial support provided by Grant (PID2020-117722RB-I00) “Retrofit ventilation strategies for healthy and comfortable schools within a nearly zero-energy building horizon” funded by MICIU/AEI/10.13039/501100011033. C.M. 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