Airtightness measurement of large buildings by using multi-zonal techniques: a case study

Nowadays the improvement of building airtightness is an essential condition to achieve high energy performance of buildings. Therefore, there is a need to precisely describe and quantify buildings infiltrations. 
In France, building airtightness is determined by fan pressurization, as described in the international standard ISO 9972. Appliqued to large buildings however, several technical questions arise, such as the ability to generate a pressure difference of at least 50Pa, or to achieve a uniform indoor pressure for the whole building. For these reasons, it is suggested to divide large buildings into smaller elements. This approach, often referred as multi-zonal, was applied here to a case study. According to ISO 9972 standard, two different techniques can be used: providing an identical pressure in adjacent zones, or including inter-zonal air flows in the calculation of the airtightness of a room. Both techniques were applied here to a 32 000 m3 building. 
The building was made of offices, classrooms and four large halls devoted to research. For offices and classrooms, an airtightness level n50 lower than 1.47 h-1 has to be achieved. For the halls however, it should be lower than 2.89 h-1. Consequently, both part had to be measured separately. In addition, the four halls could not be measured simultaneously as they were not contiguous. 
A first step consisted in assessing inter-zonal airflows. Each part of the building should be isolated from the other, on an airflow point of view. However, many important leakages were detected: at the junction between the ceiling and separation walls, between the exterior wall and separation walls, at doorways where doorsills or seals were missing, and through the separation walls because of the crossing with beams, sheathes, and networks ducts. Next, the building airtightness was measured according to several multi-zonal methods. First, halls were measured one by one, then each hall was measured while the adjacent halls were simultaneously depressurized, and finally the four halls were measured simultaneously, with depressurized adjacent offices and classrooms. Offices and classrooms were tested with and without depressurization of adjacent halls. 
The results showed that inter-zonal airflows were non-negligible and significantly influenced the measured airtightness: the n50 value was overestimated and ranged from about 70% to 100%. On average, a higher difference (+92%) was obtained for the halls while it reached as much as 80% for offices and classrooms. 
According to ISO 9972 standard, all multi-zonal measurement methods used in this study are allowed to determine the airtightness of this building. The huge difference between results is a problem, especially when airtightness is measured for compliance with a specification of a building code or standard, in the context of calculation of energy performance of buildings.