**1. Introduction**

Human beings depend on energy for almost all of their daily activities. Energy is not only required to cover basal needs, but also those which allow them to remain comfortable to face climate dynamic variations outdoors [1], even more for vulnerable populations, as children. These variations have been altered by anthropogenic activity, boosting extreme weather conditions related to climate change, or more complex effects, such as urban heat islands [2].

This effect also impacts on indoor air quality, resulting in discomfort and even affecting health [3–5], especially in risk groups, such as the elderly [6,7], children [8] and births [9], but also with a significant incidence in the active population [10,11].

Among the most studied buildings in the field of indoor comfort, schools represent a relevant group. One of the main reasons is the exposure of children to spending a long time under indoor environmental conditions. These children are considered a risk population, and there are also other considerations, such as social or vulnerability aspects, which can influence, so investigating in this regard has become a global priority, as a development objective sustainable by 2030 [12].

Comfort studies in schools have progressed in the last fifty years [13–16] and recently on indoor environments [17]. Recently, many of the studies in educational buildings include more innovative methods: student performance, cognitive processes or disruptive and engaging techniques, such as storytelling [18], gamification [19] or adaptation, and comparison among more traditional ones, as Post-Occupancy Evaluation (POE) [20–22]. Qualitative techniques are sometimes included as part of mixed methods, such as open-ended interviews [23]. Techniques to evaluate subjective aspects of users' perception on comfort have been developed [24], as well as on emotional state [25,26], but often are misnamed as "qualitative" [27–29].

According to the ASHRAE 55 standard [30] and the subsequent ISO 7730 [31], the thermal sensation experienced by human beings is mainly related to the global thermal balance in their body. It depends on physical activity, clothing and on environmental parameters, such as air temperature, average radiant temperature, air velocity and air relative humidity, whose values can be measured or estimated, to calculate the Predicted Mean Vote (PMV) comfort index. However, "nonthermal environmental parameters are not considered, such as air quality, acoustics and illumination or other physical, chemical or biological space contaminants that may affect comfort and health" [30]. Since thermal comfort is considered only a part of environmental comfort, and other environmental factors affect the thermal sensation, productivity, concentration and health of occupants indoors, this research proposes an overall comfort perception study, by a mixed method that allows to deepen the global satisfaction related to environmental comfort of users at class.

Questionnaires are a common practice either in internal environmental comfort research [32], or just in thermal comfort. These surveys are commonly based on ISO 7730 thermal comfort parameters and may include those related to potential local thermal discomfort (by unwanted air flows, temperature asymmetries, etc.). However, in recent years, research shows an increasing trend to complete the perception analysis also including nonthermal environmental factors that affect overall satisfaction and comfort perception, such as illumination, noises, odors, ventilation frequencies, spatial design elements of finishing and so on [33].

The inclusion of physiological indexes was not an objective of this study, due to several reasons: as considered in [32], physiological measurement (skin temperature, blood flow, core temperature, heart rate, etc.) can be correlated with thermal comfort/discomfort, but it consists of an invasive (contact) method that implies to have certain devices and a deep knowledge of the correct measuring method, for avoiding potential measuring mistakes, adding uncertainties to results. Since users' surveys are considered a traditional (contact) measure, the author did not want to interrupt the daily tasks at the classroom. Otherwise, the proposed technique, with its limitations, is understood as more effective and user-friendly, since it is not as invasive as physiological measures and is easier to carry out, with results more engaging and fun for kids, and it deals with behavior-change by debate sessions.

In the Mediterranean climate, thermal comfort studies in schools are scarce compared with tropical climates, however [34], there are many comfort studies in schools that develop research focused on the cold season, since it occupies most of the school period, sometimes also including midseason [15,35]. However, climate change is effectively lengthening the warm season, especially in areas of Southern Europe, where episodes of overheating affect the performance of daily tasks and health inside these buildings [36]. With the slow but constant rise in temperatures, the hot season extends over months that until a few years ago were months that were considered merely warm. This has forced the coincidence of the hot season with the start of classes in schools. In Seville, where "heat" implies exceeding 35 ◦C, the classrooms become spaces with an increasing lack of thermal comfort.

Despite some research, the literature has not taken into account children to express how they feel or perceive emotions when they interact with their built environment [37], or they are considered passive agents subject to the teacher's preferences [38]; recent studies have been demonstrated that the non-adult population is able to offer interesting insights to researchers, and they are in a position to express their thermal sensation and make adjustments to improve the thermal acceptability in the

classroom [34]. Jindal's study carried out in India reported that students demonstrated adaptability to indoor temperature variations if they were able to adjust windows and switch off/on fans [39].

Fostering participative research with engaging techniques allows children motivation and broad-mindedness. The power of these techniques in sustainability, indoor environmental quality and users' global comfort in schools may not only make students aware, but also move their knowledge to homes and share it with their families [40].

These new approaches recently discussed in schools in the Netherlands [41–44] demonstrate that internal comfort in schools results in a wide-spread research topic. However, there are still some gaps that could be studied in greater depth and solved.

The main objective in this study consists of knowing the perception of overall satisfaction and environmental comfort perception in classrooms by secondary school children (12–16 years), using participatory techniques. It had been established a participatory, mixed method, where subjective and objective aspects can contribute to build a coherent discourse on the perception of comfort in the classroom, taking into account the active participation of the users.

We consider occupant perception to be a fundamental aspect of the process of architectural creation and engineering of environmental control. The purpose is therefore to provide mechanisms which can improve the response to the specific requirements of individuals. An improved adjustment to these needs can contribute to the design of more suitable (mechanical and constructive) systems, as well as to better energy use, improving both the practical energy efficiency of buildings and user experience.
