Simplified, physical models for calculating infiltration and ventilation in a single zone, usually calculate the air flows from the two natural driving forces (i.e., wind and stack effects) separately, and then use a superposition law to combine them. Similarly superposition laws may be used to ascertain the effects of mechanical systems on infiltration. In this report a general superposition law will be derived for combining wind, stack, and mechanical ventilation systems together. The superposition law will be derived using general principles of leakage distribution and air flow and will not depend on the details of a particular infiltration model. In the process of generating this law, a quantity called leakage distribution angle is developed to quantify the separation of areas of the building envelope which are subject to infiltration and exfiltration. The general superposition law is compared to other proposed superposition laws including those based on measured data, and the general law is shown to have strong explanatory power. Results are generated for typical buildings. The concept of fan addition efficiency is developed to determine the effectiveness of unbalanced (mechanical) ventilation systems at augmenting infiltration .

1 aSherman, Max, H. uhttps://buildings.lbl.gov/publications/superposition-infiltration-modeling