Most new homes are leaky. In the typical new home, significant volumes of air enter through cracks near the basement rim joists and exit through ceiling holes on the building’s top floor. These air leaks waste tremendous amount of energy.
In recent years, after years of prodding by building scientists, code officials have finally taken a few stabs at addressing air leakage. (For more information on recent code changes related to building airtightness, see New Air Sealing Requirements in the 2009 International Residential Code and An Overview of the 2012 Energy Code.)
Of course, some builders have focused on energy efficiency for years, and many of these builders own a blower door. If you have your own blower door, you have probably learned by trial and error which cracks matter most.