*2.2. Characterization of the Airtightness of the Samples*

The assessment of the infiltration level of the classrooms under study was performed by a series of airtightness tests (doors and windows closed) in order to obtain their expected average infiltrations rates (Figure 2). These tests consisted of decreasing the room pressure by using a fan, which extracted air until the indoor-outdoor differential pressure was stabilized. It was achieved by balancing the extracted airflow with the entering airflow through the envelope cracks. Then, the depressurization was decreased in steps by lowering the fan speed, in order to obtain the regression curve of the pressure/extracted airflow relation, which showed the entering airflow when the indoor pressure was equal to the atmospheric one.

**Figure 2.** Protocols developed for the characterization of the airtightness of the classrooms.

These tests were performed by using enclosure pressurization-depressurization equipment or "blower door", as specified in the ISO standard 9972: 2015 [6], considering each classroom as a single zone to be analyzed. The specific model used was the Minneapolis Blower Door Model 4/230 V System, which was controlled by the TECTITE Express software.

The higher-pressure difference used to create this regression curve must be at least ± 50 Pa; in this study, it was reached until a ± 70 Pa differential pressure.

When the classroom had a single access point, the pressurization-depressurization test characterized the airflow that can pass through the envelope by sealing the corresponding door and the adjacent classrooms and common area. However, in most of the studied classrooms there were two access points, so it was necessary to perform three measurements in each classroom, changing the location of the blower door and sealing, or not, and the door in which the blower door was disposed. In this way, it was possible to determine the real airflow that entered the classroom during its normal operation. Adjacent classrooms and common area were sealed, too.

This protocol was designed for medium rooms with two access doors like the one under study, and required three different measurements (Figure 2):


where *V*<sup>50</sup> *<sup>P</sup>*<sup>1</sup> is the air leakage rate at 50 Pa in Protocol 1, *V*<sup>50</sup> *<sup>P</sup>*<sup>2</sup> is the air leakage rate at 50 Pa in Protocol 2, *V*<sup>50</sup> *<sup>P</sup>*<sup>3</sup> is the air leakage rate at 50 Pa in Protocol 3.

Infiltration values measured in each of these three ± 50 Pa depressurization test hypotheses, developed in each classroom, were obtained by the following expressions of the British Standard 5925 standard, obtained from a simplification of the "crack flow equation":

$$m\_{50,AT1} = \frac{V\_{50,DoverA} + V\_{50,env}}{V} \tag{1}$$

$$m\_{50,AT2} = \frac{V\_{50,DovB} + V\_{50,env}}{V} \tag{2}$$

$$m\_{50,AT3} = \frac{V\_{50,cw}}{V} \tag{3}$$

$$m\_{50,t} = \frac{V\_{50,D\text{var}A} + V\_{50,D\text{var}B} + V\_{50,c\text{inv}}}{V} \tag{4}$$

$$n\text{50}\_{\text{f}} = n\text{50}\_{\text{f}}\text{p1}\_{\text{i}} + n\text{50}\_{\text{i}}\text{p2}\_{\text{i}} - n\text{50}\_{\text{i}}\text{p3}\_{\text{i}}\tag{5}$$

where *n*50,*AT1* is the infiltration rate at 50 Pa in protocol 1, in h<sup>−</sup>1; *n*50,*AT2* is the infiltration rate at 50 Pa in protocol 2, in h<sup>−</sup>1; *n*50,*AT3* is the infiltration rate at 50 Pa in protocol 3, in h<sup>−</sup>1; *n*50,*<sup>t</sup>* is the infiltration rate at 50 Pa through the envelope and doors of the room, in h<sup>−</sup>1. *V*50,*DoorA* is the air leakage rate at 50 Pa which circulates through door A, in m3/h; *V*50,*DoorB* is the air leakage rate at 50 Pa which circulates through door B, in m3/h; *V*50,*env* is the air leakage rate at 50 Pa which circulates through the envelope, in m3/h; *V* is the internal volume of the room, in m3.
