Over the past decade, there has been an increasing effort to reduce leakage and air infiltration in new residential construction in order to improve energy efficiency. For example, California’s Title 24 Residential Compliance Manual For California’s 2005 Energy Efficiency Standards, which went into effect in October 2005, recognizes that air infiltration (leakage) is a “major component of heating and cooling loads,” and section 3 addresses measures to reduce infiltration, such as sealing around penetrations, installing air-retarding wrap, and meeting window performance standards. While reducing infiltration has a positive effect on energy efficiency, a lack of adequate ventilation can lead to poor indoor air quality. Airtight residences need to balance the desire for energy conservation with the need for high quality indoor air. Doing this requires an understanding of the factors which affect air quality in homes.