The ASHRAE 1478-RP Measuring Air-Tightness of Mid and High-Rise Non-Residential Buildings project is intended to fill a knowledge gap in the airtightness database for recently constructed midrise buildings. The project measured the air-tightness of non-residential buildings(except for two that were mixed use residential and commercial) four-stories or greater built since 2000. Tested buildings were located in climate zones 2 through 7 of the IECCi Climate Zone Map. Data were collected from 16 buildings up to fourteen stories high. The dataset includes buildings with no particular attention to making the building air-tight, buildings where some attention was given to air-tightness, and buildings where extensive attention was paid to air-tightness. A subset of the buildings (10) were designed to be LEED Silver and higher. Major air leakage sites were identified in the course of testing. A fan pressure testing protocol for large buildings based on ASTM E779 was developed by the project team. A number of issues in using E779 to test large buildings were identified, discussed, and addressed. Building air-tightness was reported in cfm per square foot of above grade enclosure at a 75 Pascal induced enclosure pressure difference, and cfm per square foot of complete enclosure (including slab and below grade conditioned space walls). The results range from 0.057 cfm/ft²@ 75 Pa to 0.80 cfm/ft²@75 Pa of complete enclosure. Factors that are associated with the most air-tight enclosures include air-barrier continuity detailed in construction documents, buildings where the enclosure was commissioned by a consultant, and precast concrete panel construction. Air leakage through HVAC related penetrations was measured in a subset of the buildings. Damper air leakage turned out to be a significant portion of the total enclosure air leakage in some of the buildings. The significance of air leakage of HVAC systems is reviewed in relation to building air tightness.