Reduction of energy usage in mechanically ventilated buildings, without violating minimum dilution and pressurization requirements, demands greater outdoor air control precision. Precision can be improved by avoidance of unwanted and unnecessary uncertainties from multiple design or operating assumptions. Indirect methods of outdoor air rate determination with multiple uncertainties propagate control errors. An undiscovered total uncertainty in control may prevent the satisfaction of minimum ventilation requirements, energy constraints and the concurrent requirements of ASHRAE Standards 62.1, 90.1 and 189.1. Direct methods of outdoor air intake rate and space population determination can minimize control uncertainties and improve air system repeatability for improved comfort and energy control. Design considerations and appropriate component selections provide the keys to a successful application of direct methods.