## 1 Introduction
### Buildings Leakage Evaluation  
The airtightness of buildings is an important factor affecting the leakage airflow in a building [15; 16; 17]. Even if the pressure difference in the building is the same, the leakage depends on the airtightness of the building envelope [18; 19]. Generally, the airtightness of a building is measured using a blower door test. The blower door test is a method of measuring the airtightness of a building by installing a blower door fan in a place characterized with airflows, such as a door or window, to pressurize or depressurize the room to various predefined pressure differences (typically from 10 to 50 Pa), after which the leakage airflow generated by each pressure difference is measured [20]. Figure 1 shows a blowdoor test. This method is recommended by several international standards (e.g., ASTM E779, ISO 9972, and EN 13829), and it is used globally. The leakage airflow in a building increases nonlinearly with the pressure difference, where the leakage airflow Q is related to the pressure difference \(\Delta\)P, which can be expressed as illustrated in Equation (1) [21]:  
\[Q=C(\Delta P)^{n} \tag{1}\]  
where Q is the leakage airflow rate of the building (CMH), \(\Delta P\) is the indoor-outdoor pressure difference (Pa), C is the fluidization coefficient (m\({}^{3}\)/hPa), and n is the fluidization index (-). Here, C and n are variables related to the airtightness of the building, and the index n is the airflow characteristic, with a value between 0.5 and 1. The leakage function of the test bed is derived by substituting the obtained values of C and n into Equation (1) and can be used to calculate the leakage of the building at the indoor-outdoor pressure difference measured by the pressure difference sensor. The pressurization/depressurization test using an air-handling unit (AHU) uses a fan to measure airtightness, similar to the blower door test. The airtightness measurement method, involving the use of the air conditioning system of a building, was introduced in the CAN/CGSB-149.15-96 standard published by the Canadian General Standards Board (CGSB) in 1999 [22]. This method measures the airtightness of the building envelope by pressurizing or depressurizing the building through the fans of the air conditioning system. When pressurizing the building using the air handler like the one in Figure 2, the leakage is calculated from the supply fan, and the infiltration is calculated from the return fan when depressurizing [23]. The results of the positive pressure method using air handlers are expressed in terms of the pressure difference, as expressed in Equation (1), similar to the lower door test. The blower door test is suitable for measuring leakage airflow in a single zone, but this method requires the use of many blower fans or large fans in complex buildings or buildings with multiple zones. In contrast, the air handling unit method does not require devices, such as blower fans, making it useful for evaluating the airtightness of large buildings. A previous study that measured the airtightness several times through CCN building air handlers reported that the method involving the use of air handlers can measure the moisture in the actual building envelope excluding the moisture between floors, compared to the blower test, which does not exclude interfloor moisture and measures the interfloor moisture [24,25]. [PERSON] [26] suggested that combining the two methods (air handler and blower door) can help in overcoming the problems of using air handler fan pressurization alone, especially in large or high-rise buildings. There have been preliminary studies to predict leakage based on the airtightness performance of buildings in the United States and Europe [27,28]. For example, [PERSON] [29] collected the leakage data of more than 100,000 U.S. homes and developed a regression model from this database.