Focus on the air tightness of buildings has so far mainly been on small domestic houses, such as single-family houses. This has not only reduced the energy demand for new buildings, but has also improved the quality of the construction. By testing the air tightness, and thereby testing the care of which the building was put together. Contractors have focused on learning by there mistakes, this has led to the fact that air tightness in domestic single family houses is no longer a problem. In regard to large buildings (>1500 m2), air leakage testing are not performed during construction, because buildings have to be divided into smaller volumes in order to test with the conventional equipment. In practice this has shown to be very difficult, i.e. in buildings with spacious rooms like atriums. As an alternative a method, which is new in Denmark, where several smaller air moving units (or blowerdoors) are combined to one big virtual blowerdoor, making it possible to test the total air tightness of large buildings. Two tests runs with this method on different buildings 3500 m2 and 9900 m2 (65000 m3) showed that the overall air leakage of the building could be determined and air leaks located in the same way as for small buildings. This paper describes the method and its practical application on new and old buildings; how the method can be used to improve the air tightness of new buildings and for investigations of air leaks in the existing building stock. The paper will further discus the relevance of having the same demand for air tightness for both large and small buildings.