Airtightness testing has long—since the 1980’s—been used to test high-performance housing. The 2012 version of the International Residential Code requires testing of every new home. Recently there has been a growing trend of testing the airtightness of large buildings as well. This session reviews why one would invest in airtightness testing for a large building, how the testing is done, how the results are interpreted, and how this information can be used.