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

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.

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 [6,7,8]. 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].

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 CO₂ 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 CO₂ 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 (CO₂) concentration, which is commonly used as a representative indicator of ventilation effectiveness and overall air quality in occupied spaces [30,31]. Elevated CO₂ 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 CO₂ 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 CO₂ levels and air quality rather than thermal comfort, while 11 emphasized thermal comfort analysis 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-COVID-19 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.

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.

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. 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

, 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.

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.
• 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. 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

and Figure 3.

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 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
[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

summarizes 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).

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.

Based on the articles selected and analyzed, a series of ventilation strategies is presented in

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.
			3. User-independent natural ventilation with parameter control through sensors: ventilation is automatically triggered based on indoor environmental parameters. V: Reliable performance; responsive to real-time indoor air quality; reduces reliance on occupant behavior. D: Requires sensor calibration and maintenance; may have higher initial installation costs.
			3.a. CO2 sensors: ventilation activates when indoor CO2 levels exceed a threshold, indicating occupancy and air quality deterioration.
			3.b. Temperature sensors: ventilation responds to indoor temperature levels to maintain thermal comfort.
			4. User-independent natural ventilation based on wind and thermal gradient: utilizes natural forces such as wind pressure and stack effect to drive airflow through building openings or passive devices. V: Energy-free operation; can be highly effective if well-designed. D: Highly climate-dependent; performance varies with external conditions; complex to design for year-round effectiveness.
			4.a. Windcatcher: architectural elements (typically towers) capture wind and channel it into indoor spaces, facilitating cross or downward airflow.
			4.b. Stack ventilation: warm indoor air rises and exits through high-level openings, drawing in cooler outside air through lower openings.
MV			5. Mechanical ventilation with constant airflow: a mechanical system provides a fixed rate of air exchange regardless of occupancy or indoor conditions. V: Predictable air exchange; simple to design and operate. D: Energy-intensive; may lead to over- or under-ventilation depending on actual needs; no adaptability.
			6. Mechanical ventilation with variable airflow based on CO2 or temperature sensors: ventilation rate is modulated in response to sensor data (e.g., CO2 concentration or temperature), providing air only as needed. V: Optimizes energy use; improves indoor air quality dynamically; aligns with demand-controlled principles. D: More complex systems; requires sensor maintenance; potential delays in system response.
HV			7. Hybrid ventilation: hybrid ventilation refers to systems where natural and mechanical ventilation operate concurrently. In this setup, mechanical ventilation functions with its own dedicated air intake and exhaust, providing a controlled airflow and defined air change rate, while at the same time, windows may be opened—either manually by occupants or automatically by a control system. V: Adaptable to a wide range of climates and building uses. D: Higher system complexity.

NV: natural ventilation, MV: mechanical ventilation, HV: hybrid ventilation, V: advantages, D: disadvantages. The shading tone in each row identifies the type of strategy. The tones are grouped into natural ventilation (warm tones), mechanical ventilation (blues), and hybrid ventilation (purples). The tones correspond to those used in Figure 6.

, 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.

Additionally, as shown in

Table 5. Grouping of the climates of the articles into five groups according to Köppen classification.

Climate Grouping	Köppen Climatic Classification	Number of Analyzed Articles
Tropical	Aw, Cfa	3
Semi-arid	Bsh, Bsk	3
Oceanic	Cfb	6
Mediterranean	Csa	5
Continental	Df, Dfb	2

, the various existing climates have been grouped into five categories: tropical, semi-arid, oceanic, Mediterranean, and continental.

Figure 6 presents a list of the types of strategies used according to the climate, referencing the five groups previously shown in Table 5. As shown in Figure 6, natural ventilation is one of the ventilation strategies most widely used in schools, accounting for more than 50% of the total strategies used in all cases, irrespective of climate. In some cases, such as the semi-arid climate, this figure can even reach 100%. An analysis of natural ventilation strategies shows a clear majority of user-dependent strategies, with either no specific window opening patterns or scheduled timings. In all cases, these totals exceed 75%, reaching up to 95.5% or even 100% in Mediterranean and continental climates. In contrast, mechanical ventilation strategies barely reach 30% in most cases. In some climates, such as the oceanic climate, they only account for 15%, and in the semi-arid climate, they are not employed at all. Finally, hybrid ventilation strategies are the least used, and are studied solely in the oceanic climate, where they account for 20%.

The different strategies are analyzed independently below in order to thoroughly examine the various protocols and results. For this purpose, the analysis is divided into three groups––natural, mechanical, and hybrid ventilation––as defined previously.

3.1. Natural Ventilation Strategies

The studies with natural ventilation strategies are shown in

Table 6. References with natural ventilation strategies.

		Air Conditioning		1		2	3		4
Climate	Ventilation	Heating	Cooling	a	b		a	b	a	b
Semi-arid	Cross	Yes	No	-	-	[39] (W)	-	-	-	-
		No	No	-	-	[40] (S)	-	-	-	[51] (S, W)
	Single-sided	Yes	No	-	-	[39] (W)	-	-	-	-
		No	No	-	-	[40] (S, W)	-	-	-	-
Tropical	Cross	Yes	No	-	-	-	-	-	-	-
		No	No	-	-	-	-	-	[49] (W)	-
	Single-sided	No	Yes	[37] (S)	-	-	-	-	-	-
		No	No	[37] (S)	[49] (W)	-	[49] (W)	-	-	-
Oceanic	Cross	Yes	No	-	[53] (W + SP)	-	-	-	-	-
		No	No	-	-	-	-	-	[42] (S)	-
	Single-sided	Yes	No	[48] (W)	[53] (W + SP)	[48] (W)	-	[38] (W)	-	-
		No	No	[46] (SP) [48] (SP)	[38] (S, W)	[48] (SP)	[46] (SP)	[46] (S, W, SP) [38] (S)	-	-
Mediterranean	Cross	Yes	No	-	-	-	-	-	-	-
		No	No	[47] (W)	[43] (Y)	-	[47] (W)	-	-	-
	Single-sided	Yes	No	-	-	-	-	-	-	-
		No	No	-	[54] (S, W, SP)	[50] (W)	-	-	-	-
Continental	Cross	Yes	No	-	-	[41] (W)	-	-	-	-
		No	No	-	-	[41] (S)	-	-	-	-
	Single-sided	Yes	No	-	-	-	-	-	-	-
		No	No	-	-	-	-	-	-	-

and Figure 7. In these, different groups have been created within each strategy in order to analyze the results by climate at a later stage, irrespective of the presence or absence of cross ventilation and of air conditioning. Additionally, the seasons during which the individual studies are conducted (Summer (S), Winter (W), Spring (SP), or Annually (A)) are specified.

In the analyzed articles, the study of cross ventilation predominates in semi-arid, Mediterranean, and continental climates, especially in continental climates where no cases of single-sided ventilation have been analyzed. In contrast, single-sided natural ventilation strategies are predominant in tropical and oceanic climates. Natural ventilation is the most commonly used method in the majority of climates, and in some cases it performs well, as is the case with the Mediterranean climate during periods without extreme temperatures, which is characterized by milder and more favorable temperatures. Nevertheless, in most instances, it must be complemented by active systems. It is observed that, regardless of the climate, most schools and educational buildings implementing natural ventilation strategies do not have air conditioning, whether heating or cooling, making it essential to ensure the control of thermal stress.

3.2. Mechanical Ventilation Strategies

The studies examining mechanical ventilation strategies are shown in

Table 7. References with mechanical ventilation strategies.

	Ventilation		Air Conditioning		Mechanical Ventilation Strategies
Climate	Heat Recovery	Thermal Treatment	Heating	Cooling	5	6
Tropical	No	No	Yes	Yes		[37] (S)
			No	No		[55] (W, SP)
	Yes	No	Yes	No
			No	No	[49] (W)
Oceanic	No	No	Yes	No	[48] (W)
			No	No	[48] (SP) [52] (S)	[46] (SP)
	Yes	Yes	Yes	No	[48] (W) [53] (W + SP)
			No	No	[48] (SP)
Mediterranean	No	No	Yes	No
			No	No		[43] (A)
	Yes	No	Yes	No
			No	No	[45] (S, W, SP)	[45] (S, W, SP)
		Yes	No	No		[50] (W)
Continental	No	No	Yes	No
			No	No		[44] (W, SP)
	Yes	Yes	Yes	No
			No	No

and Figure 8. As before, different groups have been created within each strategy to analyze the results by climate at a later stage, depending on whether the mechanical ventilation has a heat recovery unit or thermal treatment (with or without a battery) and whether there is air conditioning. The seasons during which each study is conducted (Summer (S), Winter (W), Spring (SP), or Annually (A)) are also indicated.

As seen in Figure 8, the use of heat recovery units or batteries in mechanical ventilation systems is only predominant in the Mediterranean climate, reaching 75%. This figure is reduced to 50% in the tropical climate and to 43% in the oceanic climate. No mechanical ventilation strategies with heat recovery or batteries have been studied in the continental climate. As in the case of natural ventilation, in the selected study sample, most schools and educational buildings have no air conditioning, whether heating or cooling.

3.3. Hybrid Ventilation Strategies

As noted at the beginning of the results section, the oceanic climate is the only climate where studies on hybrid ventilation strategies are found. As shown in

Table 8. References with hybrid ventilation strategies.

Climate	Natural Ventilation	Mechanical Ventilation	AC	Hybrid Ventilation Strategies
Oceanic	Cross	Variable	Yes	[38] (S, W)
	Single-side	Constant	Yes	[53] (W + SP)
			No	[46] (S, W, SP) [52] (S)

, there are different combinations: natural cross ventilation with constant flow mechanical ventilation and single-sided natural ventilation with constant and variable flow mechanical ventilation. Therefore, considering the limited number of studies on hybrid strategies and the variety of combinations, no recurring pattern is observed.

3.4. Protocols Used

The different protocols used for each type of ventilation strategy are detailed in

Table 9. Natural ventilation protocols.

Ventilation Strategy		Ref.	Number of Different Protocols	Detailed Protocols											Occupancy	Monitored (M)/Simulated (S)	Monitored Days	Temp	Humidity	CO2
				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. User-dependent	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]	2	Winter		·	·				·			Southwest	20	S	1	·		·
				Spring		·		·			·			Southwest	20	S	1			·
	b	[38]	2	Summer		·		·			·			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]	7	Summer	·	·		·	·	·	·	·		North + South	23 to 25	M	4	·	·
				Winter		·		·	·		·	·		North + South	23 to 25	M	7	·	·
		[41]	8	Summer	·			·	·		·			N/D	13 to 26	S	N/D	·	·	·
				Winter	·		·		·		·			N/D	13 to 26	S	N/D	·	·	·
		[48]	2	Winter		·	·		·		·			Southwest	20	S	1	·		·
				Spring		·		·	·		·			Southwest	20	S	1			·
		[50]	2	Winter		·		·	·		·			East	25 to 27	M	60	·		·
3. User-independent 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]	2	Summer		·		·		·				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]	8	Summer	·			·			·			North	48	S	31	·	·	·
				Winter	·			·			·			North	48	S	28	·	·	·

,

Table 10. Mechanical ventilation protocols.

Ventilation Strategy	Ref.	Number of Different Protocols	Detailed protocols											Occupancy	Monitored (M)/Simulated (S)	Monitored Days	Temp	Humidity	CO2
			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]	4	Winter	·		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]	3	Winter		·			·		·			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	·	·	·

and

Table 11. Hybrid ventilation protocols.

Ventilation Strategy	Ref.	Number of Different Protocols	Detailed protocols													Occupancy	Monitored (M)/Simulated (S)	Monitored Days	Temp	Humidity	CO2
				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]	4	Summer		·		·	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]	4	Winter + Spring		·	·		N/D		·		·	·	·	N/D	31	M	81	·	·	·

.

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 CO₂ 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 Table 10 and Table 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 CO₂ 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 CO₂ levels below 800 ppm are highlighted in green.

As observed in

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 (%)	CO2 Average (ppm)	CO2 Max (ppm)	CO2 Min (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
2	Semi-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
4	Oceanic	[42]	No	32	S	N/D
	Semi-arid	[51]	No	48	S	31		33.75			64.5	23.75		459
5	Mediterranean	[45]	No	21	S	15
	Oceanic	[52]	No	N/D	M	31	23
6	Tropical	[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

, there are few strategies that achieve optimal temperature and CO₂ 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 CO₂ 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 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. 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 (%)	CO2 Average (ppm)	CO2 Max (ppm)	CO2 Min (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
4	Tropical	[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
7	Oceanic	[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

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 CO₂ 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 CO₂ 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 CO₂), 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.

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 (CO₂ or temperature sensors); and user-independent 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 CO₂ 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 CO₂, it is concluded that there are almost no studies where both comfort conditions and CO₂ 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.