**2. Methodology**

#### *2.1. Site and Building*

The case of study is a primary public school in Valladolid (Castilla y León, Spain). The building is placed in a plot mostly occupied by the playground, and it is next to other public buildings to the north and south sides, a park to the east and a four-lane avenue with medium-high traffic volume to the west (Figure 1). Valladolid has a Continental Mediterranean climate with cold winters and minimum temperatures below zero

(Csb-Temperate, dry and temperate summer). As a consequence, maintaining the indoor temperature efficiently becomes an important factor.

**Figure 1.** Aerial view of the school from Google Earth Pro (2021).

The building, which was built in 1980, has a construction system that was widespread in the construction of public educational buildings built in the same decade: a reinforced concrete structure and vertical envelope composed of brick masonry, a non-ventilated air chamber (insulated only on some occasions) and single-hollow brick as the inner layer. It has a ground floor and two additional floors, with a central corridor and classrooms on each side (oriented to north and south). Each classroom (Figure 2) has a rectangular floor area of 60 m<sup>2</sup> and is 2.85 m in height. The classrooms have two doors to the corridor, with a panel of adjustable methacrylate slats over them, and four tilt-and-turn PVC windows with integrated shutters to the exterior (the original exterior windows were recently replaced).

**Figure 2.** 360◦ view from a tested classroom.

The building is naturally ventilated, and the heating system has a central boiler and aluminium radiators, with uninsulated pipes through the classrooms.

The usual class schedule is Monday through Friday from 9:00 a.m. to 2:00 p.m., with an intermediate break time of 30 min.

#### *2.2. Test Design*

Different parameters that define IAQ in classrooms were compared in different ventilation scenarios. In the first phase (pre-COVID-19), IAQ parameters before the pandemic under no ventilation protocol were collected. Next, scenarios A and B, which emerged from the protocol prescribed by the public administration in Castilla y León [21], were assessed. Finally, alternative scenarios (C, D and D') were defined according to the guidelines proposed by the Harvard T.H. Chan School of Public Health and CSIC [22–24]. At the same time, hygrothermal comfort conditions for the different ventilation configurations were analysed, considering their viability in a post-COVID time.

For each test, measurements were carried out simultaneously in two facing representative classrooms (Figure 3), with the aim of evaluating the impact of cross-ventilation. In this regard, cross-ventilation was determined from the data monitored in the corridor from the simultaneous opening of doors and windows in facing classrooms. This resulted in a significant increase in the ventilation flow by the pressure gradient between the windward and leeward façades.

**Figure 3.** Scenario locations and classrooms within the building.

All tests were carried out during the cold season. The data collection for the pre-COVID-19 scenario was carried out for 5 school days (Monday–Friday), 3 days (Wednesday– Friday) for scenarios A and B and 5 school days (Monday–Friday) for scenarios C, D and D' (Table 3). During all the test phases the occupation of the classrooms was constant.


**Table 3.** Organisation of classrooms and test protocols.

Other sensors took measures outdoors and in the corridor, respectively.


In all cases, the opening scheme could be modified at the discretion of the teachers and such circumstances were registered.

#### *2.3. IAQ Monitoring*

The sensors used were AirQualityEgg, which measure several IAQ conditions, specifically: air temperature, relative humidity (RH), CO2, TVOC, PM10, PM2.5 and PM1.0. The systematic measurement error (bias) of the different sensors integrated were:


Measurements were taken every five minutes from the previous hour, before the academic activity, up to one hour after its completion. The position of the measurement devices within the classroom was determined by previous tests, to limit distortions produced by the occupancy, academic material, blackboards, windows and doors. Sensors were kept between 1 and 2 m away from the area where the students were, and at least 1 m away from other possible disturbance sources. Regarding height, negligible variations of

less than 3% in CO2 and pollutant levels were observed between the breathing plane of the students in a sitting position (1.20 m) and 1 m above (2.20 m above the floor level).

#### *2.4. IAQ Limit Values*

The limits established for the different pollutants were determined based on the criteria established by different regulations and guidelines:


## **3. Results**

The results obtained in this study are structured in three phases. Firstly, the data obtained during the winter season in 2020, in a pre-COVID-19 situation, are shown. Afterwards, Phases 1 and 2, with the data of the different ventilation protocols evaluated, during the winter season in 2021 and in a COVID-19 scenario, are presented (Table 4).

In the scenario pre-COVID-19, the monitoring results obtained in the classrooms (Level 2◦C, 1◦A, 2◦A and Pre-school) revealed very poor values, achieving a maximum CO2 concentration gradient indoors of 4025 ppm, more than six times above the limit set for RITE [17]. Taking into account the CO2 concentration, its level was out of the normative range between 81% and 93% of the school time. The TVOC concentration was out of the range between 3% and 50% of the time, with a maximum value of 767 ppm reached in Level 2◦A. In all cases, the level of PM was adequate under the maximum recommended levels. The ventilation was quite low, and cross-ventilation was close to zero. The rare opening of windows entailed that the only air change occurred through air infiltration. The lack of ventilation also caused the increase in temperature during the school day and poor IAQ.

Furthermore, cross-ventilation was an active strategy within scenarios C and D. In these scenarios, the classrooms maintained an average comfort temperature throughout the day, but there were significant periods with out-of-range conditions. In all cases, the mean CO2 concentration level during the school day was within the adequate range. However, there were short periods in which CO2 levels were above the maximum recommended values, namely for scenarios A (7.10%) and D (6.23%). This implies that the classroom had inadequate ventilation rates for 20 min per school day. PM was not a problem in any case. Instead, TVOC levels were over the recommended level of 500 ppm in certain moments, in a small percentage.

Finally, it is worth mentioning that during the performance of the tests, the conditions of the outdoor environment were continuously monitored (Table 5). The average outdoor temperature and the temperature differential between the hour before the start of the activity and the average temperature during the day were practically the same. Of particular importance are the CO2 values outdoors since they serve as a reference in most of the regulations based on the concentration gradient between the inside and the outside.




**Table 5.** Outdoor conditions for each test Phase.

#### *Inquiry to the Teachers*

After the tests, a brief survey was sent to the ten teachers of the classrooms studied, with two questions related to their experience during the process:


They could also make additional comments or clarifications (Table 6).


