It is commonly believed that infiltration is due to poorly weatherstripped doors and windows. Although this is partly true, the fact is 80% or more of infiltration is due to the many imperfections that are designed or built into exterior envelopes. "Will we have enough fresh air to breathe?" is a concern about air-tightening the envelope. ANSI/ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air Quality has taken care of that problem. Doesn't air tightening cause sick buildings? To quote, Joseph Lstiburek, Ph.D., P.Eng., Member ASHRAE,¹ "in order to control air, you must first contain it."