## 2 Materials and Methods
### Methodology
#### 2.3.2 Natural-Ventilation Protocols  
After an intense debate in academia, described in Section 1, the worldwide scientific community established natural ventilation as the most reliable source of improvement of the IAQ and the consequence reduction in the airborne transmission of several infections such as COVID-19. In the case of the Andalucia region (Spain), the regional Government prescribed the need to provide windows which open fully to guarantee natural ventilation. However, this was a temporary solution that causes additional heating/cooling demands, which must be addressed at some point. To solve this issue, in a second step, the authors proved a natural ventilation protocol that seeks the minimum air renovation to guarantee a maximum CO2 concentration of 800 ppm. Ideally this will help to maximize the comfort periods while reducing the heating/cooling energy demand. This was called the COVID-19 natural ventilation protocol, as it mainly addressed the infection issue. The protocol described ideal periods to provide constant air renovation through manual window operation, but without any control of its effectivity. As an improvement of these early natural ventilation protocols, this paper suggests different approaches, based on the following:  
* CO2 concentration that can be obtained through CO2 sensors. They are cheap and easy to find and allow any worker in an educational building to have a real-time IAQ assessment. Even more, they can be connected remotely to cloud control via an app, so anyone can access the IAQ data report. In _DesignBuilderTM_ the CO2 is obtained through virtual sensors, as described in Section 3.1. Figure 4: Classroom layout (a) and north 3D axonometric view of classroom (b). * The opening factor described as a WW\({}_{\text{eq}}\)R, which can be manually or mechanically operated on site, following any of the suggested algorithm rules. It is also possible to model it in _DesignBuilderTM_ through EMS scripting, as described in Section 3.1. The Steady Natural-Ventilation Protocol  
The SNVP can increase the natural ventilation while being easily implemented in the building. It describes an ideal steady-state opening WWR for all the operational time, according to the classroom configuration. See Figure 5 for details. For scenarios where the outdoor CO\({}_{2}\) is circa 400 ppm, the SNVP can be described as follows:  
* The WW\({}_{\text{eq}}\)R is calculated via _DesignBuilderTM_ simulations to maintain the CO\({}_{2}\) concentration under 800 ppm. * The WW\({}_{\text{eq}}\)R obtained is then applied on-site to the case study in scenario 1. If the CO\({}_{2}\) concentration is below 800 ppm during this time, then the objective is accomplished and the WW\({}_{\text{eq}}\)R can be maintained while the rest of the conditions remain the same. * On the contrary, if the CO\({}_{2}\) concentration is not below 800 ppm, then the WW\({}_{\text{eq}}\)R must be revised in several steps, until it finally decreases to under 800 ppm. The Dynamic Natural-Ventilation Protocol  
The DNVP has been designed as an improvement on the SNVP to increase air renovation control while minimizing the negative effect of thermal loads in extreme seasons. The ease of the DNVP will provide the opportunity for a successful implementation of natural ventilation policies in those school buildings where complex management systems are not available, for some reason. The DNVP can be seen in Figure 6. For scenarios where the outdoor CO\({}_{2}\) is circa 400 ppm, the summary of the DNVP can be described as follows:  
* In the first step, if the CO\({}_{2}\) concentration is above 800 ppm, then windows must be fully opened to maintain the CO\({}_{2}\) as low as possible. * In the next step, when CO\({}_{2}\) is below 800 ppm, the windows on both sides of the room will be opened according to a rising scale of WW\({}_{\text{eq}}\)R. The WW\({}_{\text{eq}}\)R is the average number of the WWR in the case of cross ventilation with different window sizes in the windward and leeward facade.