A study of excessive air leakage in the ductwork of a large pharmaceutical plant located in the Southeast United States is executed in order to determine the energy loss associated with the excessive ductwork leakage. Much of the air supplied by the ductwork is delivered to clean rooms. The analysis requires the development of a model that is used to predict the increased energy costs. The model is applied for each 15 minute interval over the entire year (approximately 35,000 data points). The results are broken down into extra energy used to heat, humidify, and also cool/dehumidify the air over each 15 minute period throughout the year. The results are presented which show that the excessive duct leakage results in more than $1,000,000 loss over the life of the system. Also the additional makeup outside air dramatically increases the dust loading on the hepa filters used to clean the air before introduction into the clean rooms.