Energy efficiency is the name of the game in Passive House construction. Air leaks decrease energy efficiency and increase costs in heating and cooling. The tighter the envelope, the better the energy efficiency. One way to identify air leaks is by performing a blower door test. This depressurizes a house and exaggerates the air leaks making it easier to identify the problem areas. 
Pictured here are the results for the Passive House we are building in the Vinegar Flats area after window install. The number on the right is our test result. Anything under 4.0 is great for the current standard in new construction. Close to zero, like this, is awesome.
For more info on blower door testing visit www.greenbuildingadvisor.com Blower Door Basics page.