## 1 Introduction
### Minimum Natural Ventilation to Ensure Indoor Air Quality  
There is a vast body of literature that recognizes how natural ventilation plays a vital role in IAQ, TC and EE [4; 5; 32; 33; 34]. On sites where urban pollution is not an issue, IAQ can be considered as a result of the balance between the indoor pollutants produced against the outdoor renovation rate introduced [32]. The first element of the balance is provided by the number, the type of activity and the age of the occupants, while the second element of the balance depends mostly on natural/mechanical ventilation. In most of the cases, the infiltration through the building's envelope can be avoided due to its lack of relevance in comparison with the other elements [33]. Polluted sites are not considered adequate for air renovation without previous filtration. Natural ventilation is driven by different thermodynamic airflow issues. That airflow is provided due to different air pressures and temperatures or by wind flows [34]. The basic classification describes two kinds of natural ventilation: single or cross [35]. Then, the effectiveness of the natural ventilation depends on the size, the proportions, the shape, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Indicator** & **Protocol 2** & **EN 16798-1** & **ASHRAE 62.1** & **WELL [30]** & **RITE [29]** \\ \hline CO\({}_{2}\) & 800 ppm & 900 ppm & 1000 1 ppm & 900 ppm & 500 ppm \\ \hline \hline \end{tabular} \({}^{\dagger}\) recommend considering assumption [31], 2 hygiene measures against COVID-19 [25].  
\end{table} 
Table 1: Most relevant IAQ indicators by standard. \begin{table}
\begin{tabular}{c c|c c} \hline \hline
**Main Parameters** & \multicolumn{3}{c}{**Thermal Transmittance**} \\ \hline  & **Element** & **Components** & **U (W/(m\({}^{2}\)K))** \\ \hline Area & 49.60 m\({}^{2}\) & Root boards & Asbestos + air cavity & 3.077 \\ Height & 2.85 m & Ceiling & Concrete slab & 1.505 \\ Volume & 141.36 m\({}^{3}\) & Flooring & Concrete slab & 1.505 \\ Occupancy & 26 pupils + teacher & External wall & 2 brick leaves & 1.390 \\ Occupancy ratio & 0428 p/m\({}^{2}\) & Openings & Aluminum frame + 2 panes & 1.960 \\ Area & 49.60 m\({}^{2}\) & Interior partition & 2 brick leaves & 1.425 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Classroom simulation parameters. and the position of the openings and the window-to-wall ratio (WWR) of each facade [36]. Traditionally, the WWR has been extensively used in research due to the fact that it is easy to use and understand, and influences the way of combining different building performance objectives: daylighting, thermal, and acoustic comfort, as well as cooling/heating energy demand [37]. Elements for defining a cross-ventilation system are graphically described in Figure 1 below: occupancy (O), window inlet (Wi), window outlet (Wo), and room size in depth (D), length (L), and height (H). According to the literature, cross ventilation works when local winds pass through the Wi in the windward side, cross the room and are expelled out through the Wo in the leeward facade [36; 38]. For most of the airflow studies, the Venturi effect cannot be fully presumed, and an additional computational fluid dynamic must be considered to obtain the assumptions. If the window is the most relevant factor to manage, then there are several options to consider, such as size, wide-to-height ratio, height position, and the number of single/multi windows per facade [39; 40; 41]. As described by these studies, different configurations in the Wi-to-Wo ratio provide an internal pressure rise when the Wi/Wo is bigger than 1. If it is smaller, then the internal pressure decreases, providing better airflow conditions. Therefore, it results in a more effective ventilation configuration for reducing the Wi-to-Wo ratio [36; 39]. For those rooms with a different window size at the windward and leeward facades, an average combined-window area can be defined, as described in Equation 1[36]. The equation includes a summation of areas at each of the opposite sides of the room.