The document discusses the importance of achieving an air tightness level of 0.6 air changes per hour or less in Passivhaus buildings. It explains that this level of air tightness is necessary to achieve energy efficiency, indoor air quality, durability and comfort. Specific details are provided on how to calculate the target air changes per hour and cubic feet per minute levels based on building volume. The challenges of installing an effective air barrier and maintaining it are discussed along with examples of different air barrier and building assembly details that can achieve the required air tightness.