Ventilation Strategies to Ensure Thermal Comfort for Users in School Buildings: A Critical Review

Academic Editors: Sergio Montelpare and Camilla Lops Received: 27 March 2025 Revised: 5 May 2025 Accepted: 8 May 2025 Published: 13 May 2025 Citation: Aguilar-Carrasco, M.T.; López-Lovillo, R.M.; Suárez, R.; León-Rodríguez, Á.L. Ventilation Strategies to Ensure Thermal Comfort for Users in School Buildings: A Critical Review. Appl. Sci. 2025,15, 5449. https://doi.org/10.3390/ app15105449 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Review Ventilation Strategies to Ensure Thermal Comfort for Users in School Buildings: A Critical Review María Teresa Aguilar-Carrasco 1, Remedios María López-Lovillo 1, Rafael Suárez 2,* and Ángel Luis León-Rodríguez 2 1Departamento de Construcciones Arquitectónicas I, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, Av. de Reina Mercedes 2, 41012 Seville, Spain; [email protected] (M.T.A.-C.); [email protected] (R.M.L.-L.) 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; [email protected] *Correspondence: rsuar[email protected] Abstract: People spend most of their time indoors, where air quality is crucial to health. In this context, this study conducts a critical review of ventilation strategies in schools to ensure air quality, as well as to guarantee students’ thermal comfort. Based on a bibliographical review, strategies from previous studies are identified and evaluated in order to determine their advantages. After a detailed search, a total of 19 articles were selected, which provides a thorough analysis of the ventilation strategies in school buildings considering thermal comfort. The identified strategies were categorized into natural, mechanical, and hybrid types. The results reveal a prevalence of natural ventilation, which accounts for over 50% of strategies in all climates. Mechanical ventilation is less common and is applicable to around 30% of cases. Hybrid strategies, combining natural and mechanical ventilation, are the least used and only appear in oceanic climates, with a usage rate of 20%. Most studies highlight the lack of air conditioning in many schools, making adequate thermal stress management through ventilation crucial. The results analyzed clearly show a lack of studies with optimal results whose ventilation strategies can be replicated in other similar educational buildings, ensuring thermal comfort and air quality. Keywords: ventilation; thermal comfort; school; strategy; indoor air quality 1. Introduction In contemporary lifestyles, people spend most of their time inside buildings, where factors such as indoor air quality and thermal comfort become essential for ensuring their health, a fundamental part of life. According to the World Health Organization (WHO), air pollution is ranked among the greatest environmental health risks [ 1 ], with prolonged exposure leading to serious health problems in humans. Numerous studies have quantified the risk of suffering different diseases affecting the respiratory [ 2 ], cardiovascular [ 3 ], immune [ 4 ], and neurological systems [ 5 ]. This also increases the risk of individuals developing chronic diseases [ 6 – 8 ]. Ensuring optimal indoor air quality is therefore key to mitigating these health risks and promoting overall well-being. On the other hand, the lack of thermal comfort resulting from overheating and undercooling inside buildings also poses a serious risk to health [ 9 ], causing severe illnesses [ 10 ], impairing cognitive performance [ 11 ], and leading to a decline in dexterity and manual execution capacities [ 12 ]. Appl. Sci. 2025,15, 5449 https://doi.org/10.3390/app15105449 Appl. Sci. 2025,15, 5449 2 of 24 These effects can be even more severe in children, depending on their stage of development, negatively impacting cognitive development [ 13 ], as well as causing the diseases mentioned earlier. Children spend a considerable part of the day at school, where, in addition to the above, a high concentration of CO 2 in the environment can reduce students’ concentration levels and cognitive performance [14]. Therefore, adequate ventilation is essential in order to ensure indoor air quality and to guarantee the health and proper development of children [ 15 , 16 ]. This has been further highlighted by the COVID-19 pandemic, emphasizing the need to ventilate spaces in order to prevent the transmission of airborne diseases [17,18]. However, the high level of interest generated in this field following the COVID-19 pandemic has sidelined an issue that had been gaining traction prior to COVID-19, that of thermal stress, which has now been relegated to a secondary position. If current rates of anthropogenic activity persist, it is highly likely that climate change will lead to an increase in the planet’s average temperature of 1.5 ◦ C above current levels between 2030 and 2052 and 4.8 ◦ C between 2081 and 2100 [ 19 ], further exacerbating the issue of overheating and its impact on indoor environments [ 20 ]. This, in turn, negatively affects thermal comfort and indoor air quality [ 21 ], especially in warmer southern European climates [ 22 ], increasing thermal stress [ 23 ]. Educational buildings are especially vulnerable to overheating due to significant internal occupancy loads on the one hand, and on the other hand, to the fact that many fail to meet thermal conditioning requirements, as they were built prior to current regulations. In Europe, these matters are regulated by Directive 2010/31/EU (amended by Directive (EU) 2018/844) [ 24 ]. Furthermore, in most schools, ventilation is carried out naturally, helping to reduce levels of CO 2 and other pollutants while also promoting energy consumption reduction [ 25 ]. This natural ventilation misuse is arguably the main cause of discomfort and thermal stress [ 26 ], taking the form of cold in winter and heat in summer, resulting in a rise of up to 0.8 ◦ C on average during the summer. Therefore, not only does air pollution impact negatively on children’s health, but thermal stress also contributes to adverse effects such as concentration loss, fatigue, tiredness, anxiety, and stress [27]. In many cases, this increased need for ventilation has led to studies on how to solve comfort and thermal stress issues being cast aside. Furthermore, in some cases these issues have worsened due to the priority given to window opening with no protocol or control, so that ultimately these remain open throughout the day, to the detriment of thermal comfort [28,29]. Therefore, it is essential to identify ventilation strategies, whether natural or mechanical, to ensure both air quality and user thermal comfort. In this study, indoor air quality is assessed through carbon dioxide (CO 2 ) concentration, which is commonly used as a representative indicator of ventilation effectiveness and overall air quality in occupied spaces [ 30 , 31 ]. Elevated CO 2 levels are associated with insufficient ventilation, and controlling this parameter indirectly helps maintain adequate levels of other indoor pollutants. Furthermore, since proper ventilation is a key strategy to reduce airborne virus transmission, monitoring and limiting CO 2 concentrations also contributes to minimizing this risk [ 32 , 33 ]. As for thermal comfort, it is assessed based on temperature and relative humidity, following the criteria established in the ISO 7730 standard [34]. Reviews jointly addressing these topics—ventilation strategies in schools and thermal comfort—have been sought. However, despite the extent of this problem, few reviews have been found which present a set of ventilation strategies and their results regarding thermal comfort. In the Scopus and WoS databases, only seven reviews have been found discussing both topics combined, compared to 40 reviews found addressing the topics separately; 29 of these focus on ventilation strategies, albeit only guaranteeing CO 2 levels and air quality rather than thermal comfort, while 11 emphasized thermal comfort analysis Appl. Sci. 2025,15, 5449 3 of 24 without establishing or quantifying ventilation strategies, only analyzing the causes. As we have seen previously, this lack of reviews addressing both topics jointly may be due to the current context following the COVID-19 pandemic, where studies have primarily focused on air quality to safeguard human health by preventing disease transmission and exposure to pollutants. In addition to showing us the lack of reviews to conduct an umbrella review study, this initial search and analysis of studies highlights the need for a review of articles addressing these topics to search for ventilation strategies and analyze their results, strengths, limitations, and weaknesses in order to create protocols for use and evaluation in schools. 2. Materials and Methods The purpose of this study is to present a critical analysis of ventilation strategies implemented in school environments and aimed at ensuring students’ thermal comfort. To do so, strategies proposed in previous studies were identified and analyzed to establish their individual associated advantages and benefits. This evaluation was conducted through a review of the literature, implementing detailed search criteria as specified in Section 2.1. These criteria include the selection of relevant keywords and a search for articles by year of publication in two specific databases. Section 2.2 outlines the selection criteria employed: an initial filtering process based on the article title, followed by an analysis of the abstract, and a final detailed examination of the article content. 2.1. Search Criteria The search criteria began with the selection of keywords related to the main objective of the research. The keywords chosen initially were “ventilation” AND “school” AND “thermal comfort” AND “strategy”. To complement the information, similar keywords were proposed to broaden the search range: •“ventilation” OR “indoor air” OR “air renovation” OR “airflow” •“school” OR “university” OR “educational building” OR “classroom” •“thermal comfort” OR “occupant comfort” OR “environmental comfort” OR “indoor comfort” OR “environmental quality” • “strategy” OR “technique” OR “methodology” OR “model” OR “mode” OR “design” Due to the focus on natural ventilation strategies during the COVID-19 pandemic, often neglecting occupants’ comfort, the search was divided into two periods, before and after COVID-19, to reduce potential bias. The intermediate year was fixed at 2021. The pre-COVID-19 period ends in 2021 and goes back ten years (2011–2021). The post-COVID19 period is defined as 2022 to the present. Given that this research focuses on schools, the search criteria were refined to include only educational buildings. Classrooms exhibit distinct characteristics, including higher occupancy densities, fixed and intensive schedules, a more vulnerable population, and predominantly natural ventilation systems with limited or no mechanical ventilation or air conditioning. These factors differentiate educational buildings from other typologies and necessitate tailored approaches. The search was conducted in two different databases, WOS and SCOPUS, which are main databases used in other reviews [ 35 , 36 ]. These databases provide advanced search functionalities that enable the automated generation of combinations between keywords and their synonyms, allowing for a comprehensive and consistent search process. In both cases, the search was performed for the proposed keywords in the title, abstract, and keywords, limiting the date range for each period. These search criteria are shown in Figure 1. Appl. Sci. 2025,15, 5449 4 of 24 Appl. Sci. 2025, 15, x FOR PEER REVIEW 4 of 25 keywords, limiting the date range for each period. These search criteria are shown in Figure 1. Figure 1. Methodology: search criteria. 2.2. Selection Criteria Once the above criteria were established, the search for articles was conducted using the keywords specified. In total, 277 articles were obtained: 186 from the pre-COVID-19 period and 91 from the post-COVID-19 period. Of the 186 pre-COVID-19 articles, 50 were found in the SCOPUS database and 136 in WOS. For the post-COVID-19 articles, 32 were found in SCOPUS and 59 in WOS. Subsequently, an analysis was carried out in order to eliminate duplicate articles from these two databases. After excluding duplicates, 163 articles from the pre-COVID19 period and 74 from the post-COVID-19 period were selected, so that a total of 237 articles were selected according to the initial search criteria. To support the selection process, a co-occurrence map was generated using VOSviewer (Figure 2), which revealed five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple). VOSviewer is developed by Leiden University, located in Leiden, The Netherlands. This mapping allowed for a clearer understanding of the thematic structure within the dataset and was used to guide the identification of the most relevant articles—those situated at the intersection of the main clusters and addressing the four core concepts: ventilation, school, thermal comfort, and strategy. From here, a filtering process began, initially selecting articles based on their titles. The titles of all extracted articles were reviewed, selecting those whose titles include aspects relating to the objectives set out in this review. This selection yielded 56 articles from the pre-COVID-19 block and 24 from the post-COVID-19 block, for a total of 80 articles. From these 80 articles, a new selection was carried out based on the abstracts. All the abstracts of the chosen articles were reviewed, and those which refer to ventilation strategies in schools and include thermal comfort were selected. According to the exclusion Figure 1. Methodology: search criteria. 2.2. Selection Criteria Once the above criteria were established, the search for articles was conducted using the keywords specified. In total, 277 articles were obtained: 186 from the pre-COVID-19 period and 91 from the post-COVID-19 period. Of the 186 pre-COVID-19 articles, 50 were found in the SCOPUS database and 136 in WOS. For the post-COVID-19 articles, 32 were found in SCOPUS and 59 in WOS. Subsequently, an analysis was carried out in order to eliminate duplicate articles from these two databases. After excluding duplicates, 163 articles from the pre-COVID-19 period and 74 from the post-COVID-19 period were selected, so that a total of 237 articles were selected according to the initial search criteria. To support the selection process, a co-occurrence map was generated using VOSviewer (Figure 2), which revealed five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple). VOSviewer is developed by Leiden University, located in Leiden, The Netherlands. This mapping allowed for a clearer understanding of the thematic structure within the dataset and was used to guide the identification of the most relevant articles—those situated at the intersection of the main clusters and addressing the four core concepts: ventilation, school, thermal comfort, and strategy. From here, a filtering process began, initially selecting articles based on their titles. The titles of all extracted articles were reviewed, selecting those whose titles include aspects relating to the objectives set out in this review. This selection yielded 56 articles from the pre-COVID-19 block and 24 from the post-COVID-19 block, for a total of 80 articles. Appl. Sci. 2025,15, 5449 5 of 24 Appl. Sci. 2025, 15, x FOR PEER REVIEW 5 of 25 criteria followed in this part, articles focusing on other topics and failing to address users’ thermal comfort or discuss ventilation strategies were excluded. Articles meeting the exclusion criteria were classified by topic, and several recurring themes were identified. In Table 1, the themes identified and the number of articles associated with each are listed. As can be seen, one of the most recurring themes is the study or modification of the building envelope to improve thermal comfort conditions (46% of articles), since one of the keywords used, “strategy”, also refers to forms of retrofitting. After applying the aforementioned selection and exclusion criteria, 20 pre-COVID-19 and 9 post-COVID-19 articles were selected, for a total of 29 articles. Figure 2. Keyword co-occurrence map generated by VOSviewer with five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple). Table 1. Main topics of excluded articles. Causes of discomfort 6 Changes or study of the building envelope/building retrofitting 22 Energy use 8 New simulation models 2 Air quality analysis 9 User perception 1 Finally, a selection based on the content of the article was conducted. The 29 articles selected by abstract were analyzed, identifying articles that meet the following requirements: • propose and analyze at least one natural and/or mechanical ventilation strategy. Figure 2. Keyword co-occurrence map generated by VOSviewer with five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple). From these 80 articles, a new selection was carried out based on the abstracts. All the abstracts of the chosen articles were reviewed, and those which refer to ventilation strategies in schools and include thermal comfort were selected. According to the exclusion criteria followed in this part, articles focusing on other topics and failing to address users’ thermal comfort or discuss ventilation strategies were excluded. Articles meeting the exclusion criteria were classified by topic, and several recurring themes were identified. In Table 1, the themes identified and the number of articles associated with each are listed. As can be seen, one of the most recurring themes is the study or modification of the building envelope to improve thermal comfort conditions (46% of articles), since one of the keywords used, “strategy”, also refers to forms of retrofitting. After applying the aforementioned selection and exclusion criteria, 20 pre-COVID-19 and 9 post-COVID-19 articles were selected, for a total of 29 articles. Table 1. Main topics of excluded articles. Causes of discomfort 6 Changes or study of the building envelope/building retrofitting 22 Energy use 8 New simulation models 2 Air quality analysis 9 User perception 1 Finally, a selection based on the content of the article was conducted. The 29 articles selected by abstract were analyzed, identifying articles that meet the following requirements: Appl. Sci. 2025,15, 5449 6 of 24 •propose and analyze at least one natural and/or mechanical ventilation strategy. •are conducted in an educational building. •consider thermal comfort through monitoring and/or surveys. After performing these analyses, 27 articles were selected: 17 pre-COVID-19 and 10 post-COVID-19. The number of articles extracted in each selection phase is listed in Table 2and Figure 3. Table 2. Number of articles according to filtering phase. Search First Selection by Title Second Selection by Abstract Final Selection SCOPUS PRE 50 31 14 13 POST 19 12 6 7 WOS PRE 113 25 6 4 POST 55 12 3 3 Total 237 80 29 27 Appl. Sci. 2025, 15, x FOR PEER REVIEW 6 of 25 • are conducted in an educational building. • consider thermal comfort through monitoring and/or surveys. After performing these analyses, 27 articles were selected: 17 pre-COVID-19 and 10 post-COVID-19. The number of articles extracted in each selection phase is listed in Table 2 and Figure 3. Table 2. Number of articles according to filtering phase. Search First Selection by Title Second Selection by Abstract Final Selection SCOPUS PRE 50 31 14 13 POST 19 12 6 7 WOS PRE 113 25 6 4 POST 55 12 3 3 Total 237 80 29 27 Figure 3. Methodology: selection criteria. Figure 3. Methodology: selection criteria. Appl. Sci. 2025,15, 5449 7 of 24 After reviewing the 27 articles selected, an additional 8 were excluded, as they did not provide the necessary information to meet the objectives of this study. Consequently, the analysis focuses on the results from the remaining 19 articles, which examined ventilation strategies in school buildings, while also considering thermal comfort. 3. Results Several studies focus on different ventilation strategies to ensure thermal comfort in school buildings. Table 3summarizes these studies, adding insight on the type of ventilation studied and the key aspects of the main selected articles (year of execution, location and climate, type of building, and season of the year when tests and measurements are carried out). Table 3. Summary of the articles whose ventilation strategies were analyzed in depth. Ref Year Location (Köppen Climatic Classification) Type of Building Season of the Year Ventilation Strategy [37] 2018 Goiânia, Brazil (Aw) University Summer •Natural ventilation with constant window opening •Mechanical ventilation with variable flow according to sensors [38] 2014 La Rochelle, France (Cfb) University Summer Winter •Natural ventilation with random window opening •Natural ventilation with window opening based on sensors •Hybrid ventilation [39] 2019 Nicosia, Cyprus (Bsh) Secondary school Winter •Natural ventilation with scheduled window opening [40] 2020 Nicosia, Cyprus (Bsh) Secondary school Summer Winter •Natural ventilation with scheduled window opening [41] 2019 Bucharest, Romania (Df) Secondary school Summer Winter •Natural ventilation with scheduled window opening [42] 2014 United Kingdom (Cfb) Secondary school Summer •Natural ventilation using windcatcher [43] 2017 Lisbon, Portugal (Csa) Secondary school Annually •Natural ventilation with random window opening •Mechanical ventilation with variable flow according to sensors [44] 2016 Drammen, Norway (Dfb) Educational building Winter Spring •Mechanical ventilation with variable flow according to sensors [45] 2019 Seville, Spain (Csa) Secondary school Summer Winter Spring •Mechanical ventilation with constant flow •Mechanical ventilation with variable flow according to sensors Appl. Sci. 2025,15, 5449 8 of 24 Table 3. Cont. Ref Year Location (Köppen Climatic Classification) Type of Building Season of the Year Ventilation Strategy [46] 2023 Basque Country, Spain (Cfb) University Summer Winter Spring •Natural ventilation with constant window opening •Natural ventilation with window opening based on sensors •Mechanical ventilation with variable flow according to sensors •Hybrid ventilation [47] 2022 Extremadura, Spain (Csa) University Winter •Natural ventilation with constant window opening •Natural ventilation with window opening based on sensors [48] 2024 Graz, Austria (Cfb) University Winter Spring •Natural ventilation with constant window opening •Natural ventilation with scheduled window opening •Mechanical ventilation with constant flow [49] 2011 Ancona, Italy (Cfa) Technical School Winter •Natural ventilation with random window opening •Natural ventilation with window opening based on sensors •Natural ventilation using windcatcher •Mechanical ventilation with constant flow [50] 2019 Cassino, Italy (Csa) Primary School Winter •Natural ventilation with scheduled window opening •Mechanical ventilation with variable flow according to sensors [51] 2020 Jordan (Bsk) University Summer Winter •Natural ventilation using stack ventilation [52] 2023 Southern Sweden (Cfb) Primary School Summer •Mechanical ventilation with constant flow •Hybrid ventilation [53] 2023 United Kingdom (Cfb) Educational building Winter + Spring •Natural ventilation with random window opening •Mechanical ventilation with constant flow •Hybrid ventilation [54] 2023 Spain (Csa) Primary/secondary schools Summer Winter Spring •Natural ventilation with random window opening [55] 2021 Sydney (Cfa) Secondary school Winter Spring •Mechanical ventilation with variable flow according to sensors Appl. Sci. 2025,15, 5449 9 of 24 Below are the articles published by year in Figure 4and by country in Figure 5. As can be seen, the publication trend does not increase as a result of COVID-19, which is consistent with the initial assessment made, stating that, after COVID-19, most of the articles focused on air quality, leaving aside thermal comfort. In terms of the countries where these studies have been carried out, most (84%) have been conducted in Europe, and within this group there is a notable presence of the countries of Spain and Italy. Appl. Sci. 2025, 15, x FOR PEER REVIEW 8 of 25 • Natural ventilation with window opening based on sensors [48] 2024 Graz, Austria (Cfb) University Winter Spring • Natural ventilation with constant window opening • Natural ventilation with scheduled window opening • Mechanical ventilation with constant flow [49] 2011 Ancona, Italy (Cfa) Technical School Winter • Natural ventilation with random window opening • Natural ventilation with window opening based on sensors • Natural ventilation using windcatcher • Mechanical ventilation with constant flow [50] 2019 Cassino, Italy (Csa) Primary School Winter • Natural ventilation with scheduled window opening • Mechanical ventilation with variable flow according to sensors [51] 2020 Jordan (Bsk) University Summer Winter • Natural ventilation using stack ventilation [52] 2023 Southern Sweden (Cfb) Primary School Summer • Mechanical ventilation with constant flow • Hybrid ventilation [53] 2023 United Kingdom (Cfb) Educational building Winter + Spring • Natural ventilation with random window opening • Mechanical ventilation with constant flow • Hybrid ventilation [54] 2023 Spain (Csa) Primary/secondary schools Summer Winter Spring • Natural ventilation with random window opening [55] 2021 Sydney (Cfa) Secondary school Winter Spring • Mechanical ventilation with variable flow according to sensors Below are the articles published by year in Figure 4 and by country in Figure 5. As can be seen, the publication trend does not increase as a result of COVID-19, which is consistent with the initial assessment made, stating that, after COVID-19, most of the articles focused on air quality, leaving aside thermal comfort. In terms of the countries where these studies have been carried out, most (84%) have been conducted in Europe, and within this group there is a notable presence of the countries of Spain and Italy. Figure 4. Distribution of the articles analyzed according to year of publication. Figure 4. Distribution of the articles analyzed according to year of publication. Appl. Sci. 2025, 15, x FOR PEER REVIEW 9 of 25 Figure 5. Classification of articles according to the country in which the study was conducted. Based on the articles selected and analyzed, a series of ventilation strategies is presented in Table 4, covering natural, mechanical, and hybrid ventilation. It should be noted that natural ventilation is defined as ventilation in which both intake and extraction are carried out naturally, while mechanical ventilation is ventilation in which both intake and extraction are carried out mechanically, or in which intake is natural and extraction is mechanical. Finally, hybrid ventilation is ventilation in which natural and mechanical ventilation are in simultaneous operation. This means that mechanical ventilation has its own intake and mechanical extraction, with a controlled airflow and number of renewals, while, in parallel, the windows are opened, either manually by the users or mechanically. This ventilation is also referred to in some of the studies as mixed or mixed-mode. Table 4. Classification of different ventilation strategies. NV 1. User-dependent natural ventilation: operation relies entirely on occupant behavior; users decide when to manually open or close windows, without a specific pattern. V: No need for automation systems or technical maintenance. D: High uncertainty in performance; can be ineffective during extreme weather or when users misjudge the need for ventilation. 1.a. With permanently open windows: windows remain open continuously, regardless of outdoor conditions or occupancy levels. 1.b. With irregular window opening patterns: windows are opened sporadically, based on the user’s perception (e.g., feeling warm or detecting odors). 2. User-dependent natural ventilation according to established window opening patterns: users follow predefined window opening schedules or rules, based on time of day. V: Greater predictability and potentially improved performance if users are educated. D: Still relies on consistent user behavior; may not respond in real time to changing conditions. Figure 5. Classification of articles according to the country in which the study was conducted. Based on the articles selected and analyzed, a series of ventilation strategies is presented in Table 4, covering natural, mechanical, and hybrid ventilation. It should be noted that natural ventilation is defined as ventilation in which both intake and extraction are carried out naturally, while mechanical ventilation is ventilation in which both intake and extraction are carried out mechanically, or in which intake is natural and extraction is mechanical. Finally, hybrid ventilation is ventilation in which natural and mechanical ventilation are in simultaneous operation. This means that mechanical ventilation has its own intake and mechanical extraction, with a controlled airflow and number of renewals, while, in parallel, the windows are opened, either manually by the users or mechanically. This ventilation is also referred to in some of the studies as mixed or mixed-mode. Appl. Sci. 2025,15, 5449 16 of 24 Table 9. Natural ventilation protocols. Ventilation Strategy Ref. Number of Different Protocols Detailed Protocols Occupancy Monitored (M)/Simulated (S) Monitored Days Temp Humidity CO 2 Season Cross Ventilation Air Conditioning Different Window Opening Times Are Tested Night Ventilation Different Classes Studied Different Buildings Studied Orientation Yes No Yes No Yes No 1. Userdependent a [ 37 ] 2 Summer · · · · Southeast 25 M 3 · · [ 46 ] 1 Spring · · · Northwest 36 S 1 · · · [ 47 ] 17 Winter · · · · North 5 to 40 M 18 · · · [ 48 ] 2Winter · · · Southwest 20 S 1 · · Spring · · · Southwest 20 S 1 · b [ 38 ] 2Summer · · · South 30 M 20 · · · Winter · · · South 30 M 20 · · · [ 43 ] 2 Annually · · · NorthSouth 25 M 550 · · [ 49 ] 1 Winter · · · N/D 21 M 21 · [ 53 ] 24 Winter + Spring · · · · · · N/D 27 to 33 M 81 · · · [ 54 ] 1 Summer · · · · · N/D 11 to 28 M 150 · · · 1 Winter · · · · · N/D 11 to 28 M 150 · · · 1 Spring · · · · · N/D 11 to 28 M 150 · · · 2. User-dependent according to established window opening patterns [ 39 ] 7 Winter · · · · · · · North + South 23 to 25 M 6/96 · · · [ 40 ] 7Summer · · · · · · · North + South 23 to 25 M 4 · · Winter · · · · · North + South 23 to 25 M 7 · · [ 41 ] 8Summer · · · · N/D 13 to 26 S N/D · · · Winter · · · · N/D 13 to 26 S N/D · · · [ 48 ] 2Winter · · · · Southwest 20 S 1 · · Spring · · · · Southwest 20 S 1 · [ 50 ] 2 Winter · · · · East 25 to 27 M 60 · · 3. Userindependent with parameter control through sensors CO2 [ 46 ] 1 Spring · · · Northwest 36 S 1 · · · [ 47 ] 1 Winter · · · North 33 M 18 · · · [ 49 ] 1 Winter · · · N/D 21 M 21 · · Temp. [ 38 ] 2Summer · · · South 30 M 21 · · · Winter · · · South 30 M 20 · · · [ 46 ] 3 Summer · · · Northwest 36 S 1 · · · Winter · · · Northwest 36 S 1 · · · Spring · · · Northwest 36 S 1 · · · 4. User-independent using windcatcher [ 42 ] 1 Summer · · · N/D 32 S N/D [ 49 ] 1 Winter · · · N/D 21 M 21 · [ 51 ] 8Summer · · · North 48 S 31 · · · Winter · · · North 48 S 28 · · · Appl. Sci. 2025,15, 5449 17 of 24 Table 10. Mechanical ventilation protocols. Ventilation Strategy Ref. Number of Different Protocols Detailed protocols Occupancy Monitored (M)/Simulated (S) Monitored Days Temp Humidity CO 2 Season Air Conditioning Airflow Heat Recovery Night Ventilation Different Classes Studied Different Buildings Studied Orientation Yes No Yes No Yes No 5. Mechanical ventilation with constant airflow [ 45 ] 6 Summer ·6.5 ACH/6.1 ACH · · Southwest 21 S 15 Winter ·6.5 ACH/6.1 ACH · · Southwest 21 S 15 Spring ·6.5 ACH/6.1 ACH · · Southwest 21 S 15 [ 48 ] 4Winter ·2.9–4.45 L/s (m2)· · · Southwest 20 S 1 · · Spring ·2.9–4.45 L/s (m2)· · · Southwest 20 S 1 · [ 49 ] 1 Winter ·134 m3/h ·N/D 21 M 21 · [ 52 ] 1 Summer ·N/D ·N/D N/D M 31 · [ 53 ] 2 Winter + Spring ·N/D · · N/D 30 M 81 ··· 6. Mechanical ventilation with variable airflow [ 37 ] 1 Summer · · · Southeast 25 M 3 · · [ 43 ] 2 Year · · · · North/South 25 M 550 · · [ 44 ] 3Winter · · · N/D N/D M 1 · · Spring · · · N/D N/D M 1 · · [ 45 ] 3 Summer · · · Southwest 21 S 15 Winter · · · Southwest 21 S 15 Spring · · · Southwest 21 S 15 [ 46 ] 1 Spring · · · Northeast 36 S 1 · · [ 50 ] 1 Winter · · · East 25–27 M 60 · · [ 55 ] 1 Winter + Spring · · Northeast 7–25 M 60 ··· Table 11. Hybrid ventilation protocols. Ventilation Strategy Ref. Number of Different Protocols Detailed protocols Occupancy Monitored (M)/Simulated (S) Monitored Days Temp Humidity CO 2 Natural Cross Ventilation Air Conditioning Mechanical Ventilation Airflow (m3/h) Heat Recovery Night Ventilation Different Classes Studied Different Buildings Studied Orientation Season Yes No Yes No Yes No Yes No 7. Hybrid ventilation [ 38 ] 4Summer · · 250 · · · · · South 30 M 20 · · · Winter · · · 250 · · · · South 30 M 32 · · · [ 46 ] 3 Summer · · 250 · · Northwest 36 S 1 · · · Winter · · 250 · · Northwest 36 S 1 · · · Spring · · 250 · · Northwest 36 S 1 · · · [ 52 ] 1 Summer · · N/D · · N/D N/D M 31 · [ 53 ] 4Winter + Spring · · N/D · · · · N/D 31 M 81 · · · Appl. Sci. 2025,15, 5449 18 of 24 4. Discussion This article investigates different ventilation strategies in educational centers and their ability to simultaneously guarantee air quality and hygrothermal comfort conditions. The 19 articles selected were used as the starting point for an analysis of the conditions monitored in different seasons and climates for each of the ventilation strategies considered. The protocols used in each study have been analyzed and a detailed study carried out for the measurements and conditions in place. The results obtained show the main parameters conditioning thermal comfort and air quality–temperature, humidity, and CO2. Due to the volume of data obtained from the available literature, establishing an equivalent comparison in the different climatic zones is a difficult task. This is not only because of the lack of measurement data, but also because the differences in protocols (spot measurements versus continuous measurements, measurements only during occupied time versus measurements throughout the day) hamper comparison of the different results. Furthermore, hygrothermal sensitivity levels vary in the different climatic zones. However, a trend can be identified between the different climates and ventilation strategies. Therefore, this study does not aim to establish an optimal protocol and ventilation conditions to ensure thermal comfort and air quality but to create a guide which can be used for the purposes of reference and comparison in any future studies. In addition to the results presented, this review reveals key trends and patterns that warrant further reflection. Natural ventilation is by far the most widely implemented strategy across all climates, likely due to its low cost and ease of implementation. However, the high dependence on user behavior—especially in strategies without predefined window opening schedules—makes it an unreliable solution for ensuring consistent indoor environmental quality. Mechanical ventilation, on the other hand, though less frequently studied, offers greater control but often lacks thermal treatment or heat recovery elements, which limits its effectiveness in colder climates. Hybrid ventilation strategies, although least represented in the literature, have shown promising results in achieving both acceptable CO 2 levels and thermal comfort, particularly when sensor-based control is involved. These findings suggest that automation and mixed-mode systems could be a direction for future development, especially in regions where outdoor conditions vary widely or where user behavior cannot be easily regulated. The results of the measured variables—temperature, humidity, and carbon dioxide— are shown in Tables 10 and 11, detailing average, maximum, and minimum values. Only winter and summer results are analyzed, as these are the most extreme seasons in terms of thermal comfort. As the tables show, not all studies measured all variables. In Table 10, summer results, temperature values of between 23 and 26 ◦ C, humidity between 45% and 60%, and CO 2 levels below 800 ppm are highlighted in green following the comfort parameters of the UNE EN ISO 7730:2006 [ 56 ] standard and air quality parameters according to UNE EN 16798-1 [ 57 ]. In Table 11, winter results, temperature values of between 20 and 24 ◦ C, humidity between 40% and 50%, and CO 2 levels below 800 ppm are highlighted in green. As observed in Table 12, there are few strategies that achieve optimal temperature and CO 2 values to ensure thermal comfort and air quality. It should be stressed that the lack of data collection in many studies complicates the analysis of the different strategies. There are three cases worth highlighting, as they achieve an average temperature of between 23 and 25 ◦ C and average CO 2 values below 800 ppm. These three strategies were implemented in the same study by Dhalluin et al. [ 38 ]. Suitable values were achieved with user-controlled natural ventilation without any opening pattern, with natural ventilation with window opening based on temperature sensor values, and with hybrid ventilation. It should be noted that no air conditioning systems were used to achieve optimal temperature values in any of these cases. However, due to the significant lack of data, we cannot assert that Appl. Sci. 2025,15, 5449 19 of 24 the remaining strategies are unsuitable. It is therefore necessary to conduct further studies with better control of the various variables which quantify thermal comfort and air quality. Furthermore, Table 13 shows how the situation in winter is similar to that in summer. Firstly, the lack of data encountered again in the bibliography complicates comparison of results and strategies. In this case, it is observed that almost no scenario achieves both adequate thermal comfort and air quality simultaneously. This is only found in the study by Cablé et al. [ 44 ] through a mechanical ventilation strategy with variable flow based on sensor measurements. In some scenarios where CO 2 values below 800 ppm are achieved, the temperature is compromised, with average values below 20 ◦ C, even reaching extremely low values of 11 ◦ C in some cases. Conversely, in scenarios where suitable average temperature values are achieved, this is at the expense of CO 2 values, which can reach up to 1400 ppm in some cases. Therefore, as in the previous case, it is concluded that further studies are necessary to increase the number of strategies studied and the measurements taken. A major limitation found in the reviewed literature is the absence of standardized monitoring and reporting protocols. Many studies fail to specify measurement durations, occupancy levels, or whether the data correspond to peak or average conditions. This lack of uniformity significantly restricts the potential for comparative or meta-analytical approaches. Moreover, very few studies report on all three key variables (temperature, humidity, and CO 2 ), making it difficult to assess strategies holistically. The observed performance gaps, where either thermal comfort or air quality is sacrificed for the other, highlight the complexity of designing effective passive or semi-passive ventilation solutions. Finally, this review does not aim to define a universal strategy, but rather to provide a comparative and critical framework for future research. It underlines the need for more robust experimental designs, broader climate coverage, and better integration of comfort and health criteria. More studies are needed, particularly in underrepresented climate zones and on underexplored strategies like hybrid systems. Incorporating real-time data collection, user behavior modelling, and post-pandemic considerations such as increased air renewal rates will also be essential in shaping future guidelines and ensuring healthy, comfortable learning environments. Appl. Sci. 2025,15, 5449 20 of 24 Table 12. Results obtained from the measurements according to the different strategies in summer. The values shaded in green are those that meet the requirements established by regulations regarding CO2, temperature, and humidity. Ventilation Strategy Climate Ref. Cooling Occupancy Monitored (M)/Simulated (S) Monitored Days T Average (◦C) T Max (◦C) T Min (◦C) H Average (%) H Max (%) H Min (%) CO 2 Average (ppm) CO2Max (ppm) CO2Min (ppm) 1a Tropical [37] Yes 25 M 3 29.4 25.25 74 49 1b Oceanic [38] No 30 M 20 24.4 27.3 20.9 47 73 26 800 1800 300 Mediterranean [54] No 11 to 28 M 150 28.18 36.44 22.29 50.2 71.7 26.1 593 4015 341 2Semi-arid [40] No 23 to 25 M 4 31.6 33.7 28.5 32 50 19 Continental [41] No 13 to 26 S N/D 33.9 31.1 72 57 1300 875 3b Oceanic [38] No 30 M 20 23.5 28.4 19.7 50 76.5 30 700 1420 380 Oceanic [46] No 36 S 1 26.3 29.1 23.9 70 88 58 1079 2299 400 4Oceanic [42] No 32 S N/D Semi-arid [51] No 48 S 31 33.75 64.5 23.75 459 5Mediterranean [45] No 21 S 15 Oceanic [52] No N/D M 31 23 6Tropical [37] Yes 25 M 3 29 25.3 71 40 Mediterranean [45] No 21 S 15 7 Oceanic [38] No 30 M 20 24.95 24.35 24 43.5 60.75 28.75 500 965 220 Oceanic [46] No 36 S 1 26 28.6 23.8 41 45 38 688 1167 400 Oceanic [52] No N/D M 31 22.2 Table 13. Results obtained from the measurements according to the different strategies in winter. The values shaded in green are those that meet the requirements established by regulations regarding CO2, temperature, and humidity. Ventilation Strategy Climate Ref. Heating Occupancy Monitored (M)/Simulated (S) Monitored Days T Average (◦C) T Max (◦C) T Min (◦C) H Average (%) H Max (%) H Min (%) CO 2 Average (ppm) CO2Max (ppm) CO2Min (ppm) 1a Mediterranean [47] No 5 to 40 M 18 16.1 17.0 15.4 56.5 59.5 54.6 539.5 607.8 479.4 Oceanic [48] Yes 20 S 1 17 15 770 1b Oceanic [38] No 30 M 20 20.5 23.9 15.5 53 78 26.5 1800 3000 400 Tropical [49] No 21 M 21 23.16 24.82 21.96 Mediterranean [54] No 11 to 28 M 150 21.24 33.41 12.52 47 69.2 23.7 1194 4950 348 2 Semi-arid [39] Yes 23 to 25 M 6 20.6 22.3 18.2 58 71 45 1219 2632 419 Semi-arid [40] No 23 to 25 M 7 19.3 21.2 16.7 59 68 50 Continental [41] Yes 13 to 26 S N/D 21.9 17.1 36 18 1200 788 Oceanic [48] Yes 20 S 1 21 10 1450 Mediterranean [50] No 25 to 27 M 60 23.3 24.2 21.0 1408 1943 645 3a Mediterranean [47] No 33 M 18 11.09 13.1 8.2 44.22 48.6 39.9 606.99 808 494 Tropical [49] No 21 M 21 23.41 23.91 22.19 920.95 3b Oceanic [38] Yes 30 M 20 20.4 24.8 11 55 76 24.5 1100 2600 400 Oceanic [46] No 36 S 1 20 23.2 16.4 35 49 27 1091 2228 400 4Tropical [49] No 21 M 21 21.61 23.71 21.62 Semi-arid [51] No 48 S 28 27.5 64 25.25 459 5 Mediterranean [45] No 21 S 15 Oceanic [48] Yes 20 S 1 20 950 Tropical [49] No 21 M 21 23.74 24.91 22.04 6 Continental [44] No N/D M 1 23.3 637 Mediterranean [45] No 21 S 15 Mediterranean [50] No 25 to 27 M 60 21.3 22.3 20.4 1002 1072 541 7Oceanic [38] Yes 30 M 32 20.3 26.45 13.2 39.25 58 21.25 775 1850 400 Oceanic [46] No 36 S 1 17.9 20.7 15.1 64 72 58 729 1158 400 Appl. Sci. 2025,15, 5449 21 of 24 5. Conclusions This study has presented a critical review which analyzed the ventilation strategies most commonly used in schools to both guarantee indoor air quality and ensure the thermal comfort of students. After an initial selection of 277 articles and a final selection of 19, a total of nine strategies were identified in five different climates according to Köppen’s classification, grouped by natural, mechanical, or hybrid ventilation. This highlighted the low number of studies which focus on ensuring both thermal comfort and air quality in schools. It is also important to highlight the geographical concentration of the analyzed studies, with most of them being located in Europe. Most of the 80 studies analyzed initially address only one of these aspects, neglecting the other, particularly thermal comfort. Most studies focus on ensuring air quality even when this means sacrificing thermal comfort. This trend has been particularly pronounced since the COVID-19 pandemic, when the focus of studies shifted to air quality in order to prevent airborne disease transmission, irrespective of the thermal comfort of the occupants. Therefore, a clear lack of studies has been identified in this area. Within natural ventilation, the most employed strategies in studies considering air quality and thermal comfort are: user-dependent natural ventilation (with permanently open windows or with irregular window opening patterns); user-dependent natural ventilation according to established window opening patterns; user-independent natural ventilation with parameter control through sensors (CO 2 or temperature sensors); and userindependent natural ventilation using a windcatcher and stack ventilation. Mechanical ventilation strategies can be summarized as ventilation with constant airflow or ventilation with variable airflow, with the latter based on CO 2 or temperature measurements. There are no additional categories of mixed ventilation strategies. Natural ventilation strategies are the most used in all climates: 73% in the continental climate, 68% in the Mediterranean climate, 66% in the oceanic climate, 100% in the semi-arid climate, and 63% in the tropical climate. However, only in 17% of the cases analyzed in the selected works are optimal average temperature and air quality conditions achieved with these strategies in summer. None are achieved in winter, highlighting a clear need for the implementation of hybrid or mixed systems in school spaces. Regarding the analysis of results obtained with the different strategies, it must first be noted that the lack of data prevents us from determining whether some strategies are better than others for the simultaneous promotion of thermal comfort and air quality. After a comparison of the strategies that have sufficient data on temperature and CO 2 , it is concluded that there are almost no studies where both comfort conditions and CO 2 values are ensured simultaneously; in most cases, one of the two variables is compromised. Only two studies, one in summer and one in winter, have been found to show suitable values for both parameters. It is also worth noting a greater wealth of data in winter, which may seem surprising, as some schools are heated by radiators but rarely have cooling. Therefore, summer could be more critical due to the high temperatures in some of the countries studied, including Spain, Italy, and Portugal. However, in the rest of European countries, winter is the most critical season, as they have milder summers. These issues warrant further investigation in future studies, as they represent a relevant area of interest with direct implications for indoor air quality (IAQ) and thermal comfort (TC). Therefore, it is essential to conduct new studies testing different ventilation strategies in educational buildings to simultaneously ensure thermal comfort and air quality. This review serves as a guide to the protocols used in the various studies analyzed. Appl. Sci. 2025,15, 5449 22 of 24 Author Contributions: Conceptualization, R.S. and Á.L.L.-R.; methodology, M.T.A.-C. and R.M.L.-L.; formal analysis, M.T.A.-C. and R.M.L.-L.; investigation, M.T.A.-C. and R.M.L.-L.; resources, M.T.A.-C. and R.M.L.-L.; writing—original draft preparation, M.T.A.-C. and R.M.L.-L.; writing—review and editing, M.T.A.-C. and R.M.L.-L.; visualization, M.T.A.-C. and R.M.L.-L.; supervision, R.S. and Á.L.L.-R. ; project administration, R.S. and Á.L.L.-R. ; funding acquisition, R.S. and Á.L.L.-R. All authors have read and agreed to the published version of the manuscript. 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