## 2 Materials
### Monitoring  
Data on the outdoor environmental conditions and the classrooms' indoor environmental conditions were monitored over a two-year period. Simultaneously, data on the number of students attending each class and the class grade being taught in the classroom were gathered in order to derive a measure of classroom occupancy. For the two-year study period, daily records for more than 20 different classes from 5 th to 12 th grade were analyzed, for a total of more than 500 students. Because the number of students per class remained fairly constant (approximately 20 students per class) regardless of grade, we used the metabolic CO\({}_{2}\) production as the measure of classroom occupancy to distinguish between classes of younger (5 th grade) and older (12 th grade) students. Table 2 summarizes the sensor characteristics and the details of occupancy data gathering. Figure 2: Photograph of a classroom. Based on the gathered data, the upper plot in Figure 3 presents time series of the monitored indoor physical variables for one of the monitored classrooms: daily mean values of metabolic CO\({}_{2}\) production rate and indoor air temperature (\(\overline{S}_{c}\) and \(\overline{T}_{\text{in}}\), respectively). Additionally, the daily mean values of outdoor air temperature and wind speed (\(\overline{T}_{\text{out}}\) and \(\overline{WS}\), respectively) are presented in the upper plot. The time series for outdoor variables are continuous, while for the indoor variables we excluded weekends as well as Bank, Christmas, February half-term (Carnival), Easter, and summer holidays. The lower plot in Figure 3 adds the monitored indoor physical variable, namely, the daily median CO\({}_{2}\) concentration (\(\overline{c}\)), along with the outdoor variables, namely, the daily mean outdoor relative humidity (\(\overline{RH}_{\text{ma}}\); note that to avoid clutter, the moving average (ma) of seven data points is presented for this variable) and daily sums for precipitation and total solar energy (\(\sum R\) and \(\sum E_{\text{sol}}\), respectively)  
The outdoor temperatures in Figure 3 (upper plot) confirm Lisbon's temperate climate (Csa in Koppen scale [71]). The interquartile range of daily outdoor temperatures (outdoor relative humidity) varies between 13.0 and 20.5 \({}^{\circ}\)C (65 and 83%), with a median of 16.4 \({}^{\circ}\)C (74%). The median daily solar energy (daily total) equals 15.8 MJ/m\({}^{2}\). Daily precipitation is low, with the median daily total being 0.0 mm. The daily wind speed remains stable throughout the year, fluctuating around a median value of 2.2 m/s. Confirming previous studies on naturally ventilated classrooms [2; 3; 10; 11; 12], Figure 3 (lower plot) shows that large daily classroom CO\({}_{2}\) concentrations (\(\overline{c}\) reaching 2000 ppm) are frequent during the wintertime when classrooms are in use. Given the fairly stable occupancy with stable values of the daily metabolic CO\({}_{2}\) production rate \(\overline{S}_{c}\), and given the low air permeability of the window frames (see Table 1), the observed CO\({}_{2}\) concentration pattern is associated with the occupants' behavior, i.e., window (blind) opening/closing patterns. Indeed, in the absence of heating or cooling systems, window and blind opening/closing is how the indoor environment of these classrooms is controlled.