Quantifying airtightness-to-infiltration conversion: A long-term field study under variable temperature differences in university classrooms

We investigated the relationship between natural infiltration rates and airtightness performance in buildings through long-term measurements. The study area is Korea, where the annual temperature difference is 30 °C. The appropriateness of the existing correlation factor, namely the divide-by-20 rule, was examined by comparing the infiltration rates measured using tracer gas methods under different indoor and outdoor temperatures and the airtightness performance measured using blower door tests. Additionally, a variable correlation factor based on climatic conditions was derived and evaluated. The findings revealed very low to moderate airtightness values of 0.2–1.0 h⁻¹ for the test rooms. The infiltration rates varied according to the indoor and outdoor temperature differences, showing a direct correlation between temperature difference and outdoor wind speed. The derived correlation factors ranged from approximately 40–122, exceeding the existing “rule of thumb” value of 20, suggesting that the divide-by-20 rule may overestimate actual infiltration rates. Therefore, using a single correlation factor derived from limited experiments or controlled environmental conditions may lead to substantial errors in estimating the infiltration rate. Based on the climatic conditions in Korea, we recommend applying new correlation factors segmented into indoor and outdoor temperature differences of 5 °C for the cooling and heating seasons. In this study, we emphasized the importance of seasonal variations in the correlation factors in regions with significant annual temperature differences.