A detailed model is developed for predicting the ventilation rates of the indoor, conditioned zone of a house and the attic zone. The complete set of algorithms is presented in a form for direct incorporation in a two zone ventilation model. One of the important predictions from this model is the leakage flow rate between the indoor and attic zones. Ventilation rates are predicted from a steady state mass flow rate balance for each zone where all individual flow rates through leakage sites are based on a power law expression for flow rate versus pressure difference. The envelope leakage includes distributed leakage associated with background leakage, localized leakage associated with vents and flues, and active fan ventilation. The predicted ventilation rates agree quite well with field measurements of ventilation rates in houses and attics with different leakage configurations, without the use of any empirically adjusted parameters or constants.