Of the three duct design procedures recommended by the 1997 ASHRAE Handbook--Fundamentals only the T-method incorporates life-cycle cost. This method requires many computations, which makes non-cost optimizing techniques more attractive to designers. A new least life-cycle cost, three-stage procedure for air duct system design is discussed in this article. The Initial Duct Sizing, Pressure Augmentation and Size Augmentation (S) method has the potential to reduce ducting system life-cycle costs and the time it takes to design them.