The Neutral Zone in Ventilation is an engineering research bulletin published in April 1926. This comprehensive study explores the fundamental principles of natural ventilation within buildings, focusing on the concept of the neutral zone. The neutral zone is defined as the level in a building where the pressure inside equals the pressure outside, resulting in no airflow at that level. The authors discuss how temperature differences between indoor and outdoor air create pressure differences that drive airflow through openings in a building. By understanding and calculating the position of the neutral zone, one can predict and enhance the ventilation efficiency of any structure. The bulletin includes detailed discussions on pressure zones, head determination, airflow relations, and practical applications of these principles to buildings. It also addresses the use of the neutral zone concept in controlling infiltration and exfiltration and in designing heating systems. The study is supported by experimental data and provides valuable insights and methods for engineers and architects involved in building ventilation design.