Seeking to realistically model details of room airflows, researchers have recently embeddeddetailed CFD or sub-zone (zonal) models within multi-zone idealizations of building systems. However a number of issues remain to be resolved to assure the success of this approach. Foremost is the question of zone resistance which is ignored in multi-zone models yet implicitly included in CFD and sub-zone models.This paper defines a fundamental means to measure zone resistance and presents the results of CFD studies to evaluate its relative significance for representative geometries. For the cases considered zone resistance was found to be significant. A simplified model is presented that appears to provide a practically accurate estimate of zone resistance based on inlet and outlet geometric ratios.