## 2 Methods  
The pressure difference in a building, which causes dampness and leakage, is created by the air conditioning system of the building, and depending on the operating conditions, the room may be at a positive or negative pressure relative to the outside [35; 36]. Pressure control is typically accomplished by adjusting the fan speeds or dampers to maintain the desired pressure relationship as the exhaust airflow changes. ASHRAE recommends maintaining a positive pressure in the room to reduce unintended outdoor air infiltration. Additionally, proper fan control for VAV systems is required to pressurize the building [37; 38]. The airflow during automatic operation with a variable airflow system is shown in Figure 3. In Figure 3, the airflow Q\({}_{\text{SA}}\) and Q\({}_{\text{RA}}\) represent the airflow supplied and returned to the room, respectively, and Q\({}_{\text{P}}\) is the leakage in or out of the room caused by the pressure difference. According to the law of conservation of mass, the sum of the airflow entering the room and the sum of the airflow leaving the room are equal. This theoretically implies that the sum of the supply airflow of the air handler and the immersion volume of the outer wall are equal to the sum of the ventilation airflow and the leakage volume of the building envelope, so the difference between the supply airflow and the ventilation airflow is equal to the airflow entering and leaving through the outer wall. Therefore, the proportional expression for the relationship between the indoor and outdoor pressure difference in a building due to changes in the supply and return airflow of the air conditioning system can be expressed as shown in Equation (2):  
\[\Delta P=a\times\Delta Q+b \tag{2}\]  
where \(\Delta\)P is the indoor-outdoor pressure difference (Pa), \(\Delta\)Q is the difference between supply and return airflow (CMH), and a and b are coefficients that represent the relationship between the difference between supply and return airflow and the indoor-outdoor pressure difference. The relationship between the two values can be determined by analyzing the data collected through measurements. To derive the relationship between the airflow and leakage, a test bed with a manually controllable variable airflow system was selected. The airtightness performance of the test bed was measured, and a virtual airflow meter was derived for airflow measurement. Using this, the relationship between airflow and leakage was derived, and the details are as follows. Step 1: Using the fan pressurization/depressurization to derive the leakage function  
First, the fan pressurization/depressurization method is used to calculate the amount of leakage caused by the pressure difference between indoors and outdoors. This method involves increasing or decreasing the fan speed of the air handler, while maintaining the indoor-outdoor pressure difference and measuring the resulting leakage to derive the leakage function. In this process, the air supply and return airflow are regulated by the fan of the air handler in a VAV system. By evaluating the change in leakage as a function of pressure difference, a functional relationship can be derived to characterize the leakage in a particular space. Step 2: Measuring indoor-outdoor pressure differences by varying supply and return airflow rates  
In this step, you change the fan speed of the air handler and the variable air volume terminal unit opening rate to adjust the supply and return air volumes under different conditions. In each condition, you monitor the pressure difference between the room and the outdoors and record how the room pressure changes with the change in airflow. By doing this, you will determine the relationship between the airflow and the pressure difference. By measuring the pressure difference at different airflow settings, you can more accurately analyze the response of indoor and outdoor pressure to changes in airflow.